VW van rear brake mechanics, help?

Forbiddenbiker

Forbiddenbiker

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I'm a motorcycle mechanic by profession. But I’ve not been a member of any workshop forums for years, so knowing that there’s lots of knowledgeable mechanics on here… :) :thumbs:

The problem.
Drivers side, rear disc brake will not bite as it should when brake pedal is pressed...approx 75kg when tested on brake machine (very poor)...as opposed to the passenger side at 145kg. (just about acceptable)

Its not the fluid.
Its not the piston jamming.
Is not the sliders.
Its not the pads
Its not the disc

Everything is in good condition and does what it should as far as I can tell without testing it on a brake machine again (Due again in the morning)

What I think is a fault?
The only odd thing for me is the hand brake cable which is knackered and becomes very slow to return to the off position (when hand brake is in the off position)...this means the leaver on the back of the brake calliper stays in the half applied position. (not fully off)

This doesn't make the brake drag or anything ....but I'm wondering if it (The leaver on the back of the calliper) being in the half applied position somehow affects the overall pressure applied to that calliper/piston when using normal braking with the foot pedal ....and in doing so reduces the effective pressure at that calliper?

Does anyone know what I'm talking about??? :)
 
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Flash In The Pan said:
Also what about the brake bias valve?
Click to expand...

Was thinking that, the braking system is most likily diagonal brake i.e front left on the same brake lines as rear right, front right connected to rear left.

The last time i had that problem it was actually uneven wear to the front disc it was badly scored. Well worth checking that theres nothing wrong upfront to, leaking brake lines, and worth getting some to watch the rubber brake lines connecting to the calipers to make sure there not overly expanding on one side or the other.

Not a mechanic myself but my brother is.
 
checked the hoses? as any leak after the bias valve could be an issue, bias valve also a possibility

I don't know the Vdub system specifically
 
Spending a few years on vw's i would guess air in the system or bias valve would deff start with air in the system :thumbs:
 
Hi guys, I've not heard of a bias valve before...is that on the master cylinder...how and what to adjust please?

So the hand brake lever on the back of each rear calliper cylinder is not ringing any bells for anyone then?

The front brakes work 100% btw, over 250kg on both. :thumbs:

...so mechanical I'm getting the impression that the fluid is being pushed equally around...

Its definatly not air in the system....sucked a pint or two of fresh though today with my vacume bleed tool....that was the easy bit.

I'll go read up on the bias valve ...ta chaps.
 
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Well I don't seem to have a bias Valve .....usually located on the rear axil to allow compensation for heavy loads being loaded.

However my modified van doesn't use this method. So that that ones sorted.

It does have a proportion valve though …as part of the master cylinder... From what I've read so far this does work diagonally as suggested.:thumbs: ...however thats not helping because as I say the front brake pressure is fine ...not less on the diagonal with the rear calliper with the weakness problem or anything.


:thinking:

Anyone else. :)
 
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hi there my hubsand is a time served diesel fitter and says: if the hand brake is partly seised the pads will burn what you could do is take one pad out of one side and swap it to the other side also there is a load sensor valve on the back which may not be set correctly which when empty will not brake correctly until fully loaded please give these ago fingers crossed.....:thumbs:
 
Hi Shell ...thanks for that both of you.

The lever being left on though doesn't bind the brakes ...its not quite on enough for that.

--

Its the mechanism inside the calliper I'm unsure of here though ...does the handbrake being on slightly mean the calliper is restricted in supplying full power in some way

...the pads and disc are all in good shape, nothings ever got too hot or binds in any way.



Flash In The Pan said:
Try clamping the flexi on the diagonal front brake, see if that makes any difference.
Click to expand...

Hmm, good idea mate. ... unfortunately it gets tested at a dvla station and I'm not allowed to fiddle... so I'm using a huge torque wrench and swearing to to totally mislead myself so far. ...(flipping pain these big four wheeled things)

I've fixed the handbrake return problem pretty much, (lubed it up and stuck a great spring on it whist I test ) ...but I won't know for sure until I put it on the brake tester .... and I've already failed that test twice now.

I was hopping someone would confirm my suspicions about the lever...

But no one has ...boohoo...
 
If the handbrake lever has been seized, the caliper self adjusting mechanism has probably not been working correctly as the pad/disc has worn.

So when you press the brakes, the drivers side has a long way to travel before contacting the disc, whilst the pasenger side would have much less distance. If the handbrake mechanism is unused for a while, the self adjusting tends to stick.
 
What van and how old? Is this one where the handbrake acts on the disc or is there a mini drum with shoes built into the disk hub?

If as you seem to suggest, the caliper/pistons/etc are OK, I would be looking at the hoses and pipes. Collapsed flexy hose as the first possibility.
 
Many many thanks for your help everyone...

singlespeed said:
If the handbrake lever has been seized, the caliper self adjusting mechanism has probably not been working correctly as the pad/disc has worn.

So when you press the brakes, the drivers side has a long way to travel before contacting the disc, whilst the pasenger side would have much less distance. If the handbrake mechanism is unused for a while, the self adjusting tends to stick.
Click to expand...


Not seized just held back by a slow cable dragging... all fixed now ...still no good.

I understand the mechanism now after speaking to VW (seems to work fine unfortunately)...I've also stripped and reassembled as per VW instructions ...wind the piston right back in, with the hand brake lose ...bleed...and pump up the foot bake...then when all is settled use the handbrake for the first time ...did all that twice ...no change.




who? said:
What van and how old? Is this one where the handbrake acts on the disc or is there a mini drum with shoes built into the disk hub?

If as you seem to suggest, the caliper/pistons/etc are OK, I would be looking at the hoses and pipes. Collapsed flexy hose as the first possibility.
Click to expand...

Its based on a VW Transporter (pick up truck) with a modified back end by a company in Northampton called 'Razor Back'. its 1999. ACV engine 2.5tdi

It has two single sided swing arms on the back with a simple disc brake on each...no bias valve.

The hand brake is split into two cables with a simple bridge type connection....each cable then runs down under either side of the under-body and attaches to the little arm on the back of each caliper.


I am now looking at changing the flexi pipes (all other solid pipe work seems fine) the brake fluid flow is very good however. ...spits out a good two feet fron the nipple with moderate/normal foot pedal pressure being applied.

I wish I could test the pressure at the nipple but I can't think of a way...

Will a twisted or slow pipe (crushed, swelling) really cause such a huge loss of pressure???...like a 90% losss?
 
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Forbiddenbiker said:
Hmm, good idea mate. ... unfortunately it gets tested at a dvla station and I'm not allowed to fiddle... so I'm using a huge torque wrench and swearing to to totally mislead myself so far. ...(flipping pain these big four wheeled things)
Click to expand...


I didn't mean to clamp it for the MOT :lol: just to test it ;)
 
Yeah I got you Graham ..the problem is I can't feel the difference between 45kg/per and 145kg/per ... anything over 10kg/per is way more power than I can pull/turn anyway ....It has to be a rolling road.

Bummer init. :gag:

I’ve spoken to VW about it so many times now, been interrogated by their bestest mechanics, nothing …even their bewildered now. Lol.

No its not funny. :(
 
Forbiddenbiker said:
I understand the mechanism now after speaking to VW (seems to work fine unfortunately)...I've also stripped and reassembled as per VW instructions ...wind the piston right back in, with the hand brake lose ...bleed...and pump up the foot bake...then when all is settled use the handbrake for the first time ...did all that twice ...no change.
Click to expand...

I get it, that you have wound it back and it seems to adjust back again... However can the piston actualy travel more than it has been. If the piston can't move any more than it has been, pressing the peddle harder would just increase the line pressure, and still see no movement. Take out the inside pad and substitute this with a thin piece of wood. 6mm plywood should be ideal as that would be about the same thickness as the backing plate for one of the pads, but thinner than the whole pad. The wood is only to stop the piston traveling too far and popping out a seal, or damaging the disc. Now reasemble the caliper over the disc, with outer pad and bit of wood. Press the brake... Confirm that the piston has actualy traveled more than is needed when both pads are fitted.

With the pads removed, the caliper should be free to move on the slider mechanism at least enough to cover the thickness of the pads friction material... Though any stickiness would normaly leave one of the pads significantly more worn than the other.
 
:D
I'm still alive, no scratches... woot...shame about the other cars though. :lol:

Nah, not really ...I tried clamping all three ...guess what it brakes. errr but how much it brakes I have little idea, especially considering I'm putting all of the squeeze onto one caliper anyway, so its bound to be working a little better.

Pointless test done, not sure if I gained any ground ...Im off for a cuppa, I've had enough for now.. :gag:
 
Forbiddenbiker said:
I wish I could test the pressure at the nipple but I can't think of a way...

Will a twisted or slow pipe (crushed, swelling) really cause such a huge loss of pressure???...like a 90% losss?
Click to expand...


Could do. If the flexy is delaminating internally you can get a situation where it almost totally blocks in one direction and yet is fine in the other (so it doesnt hold on and cause sticking).
 
singlespeed said:
I get it, that you have wound it back and it seems to adjust back again... However can the piston actualy travel more than it has been. If the piston can't move any more than it has been, pressing the peddle harder would just increase the line pressure, and still see no movement. Take out the inside pad and substitute this with a thin piece of wood. 6mm plywood should be ideal as that would be about the same thicknessas the backing plate for one of the pads, but thinner than the whole pad. The wood is only to stop the piston traveling too far and popping out a seal, or damaging the disc. Now reasemble the caliper over the disc, with outer pad and bit of wood. Press the brake... Confirm that the piston has actualy traveled more than is needed when both pads are fitted.

With the pads removed, the caliper should be free to move on the slider mechanism at least enough to cover the thickness of the pads friction material... Though any stickiness would normaly leave one of the pads significantly more worn than the other.
Click to expand...

Hi Singlespeed, thanks for your help. :)

Yeah the calper sliders and all seem to work perfectly... I am a very experianced motorcyle mechanic so I do understand whats what ...its just a little bigger thats all.

I'm liking your idea about the plywood, I should be able to see by how much it crushes the wood. Although I'm not sure that helps nail the pressure problem, soft wood will compress easy etc

The actual deforming brake seal within the cylinder is approx 4mm wide so I'm guessing 2mm or so would be fine to judge max travel when the brakes are applied? what do you think? ( Edit: I realise you are intending me to test if theres a restriction on the piston, good long travel etc ..excuse.)

You got any general figures for a caliper like mine on how much movement of the piston should occur under normal braking?

...I'll go do that test in a bit.
 
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who? said:
Could do. If the flexy is delaminating internally you can get a situation where it almost totally blocks in one direction and yet is fine in the other (so it doesnt hold on and cause sticking).
Click to expand...

Forbiddenbiker said:
I am now looking at changing the flexi pipes (all other solid pipe work seems fine) the brake fluid flow is very good however. ...spits out a good two feet fron the nipple with moderate/normal foot pedal pressure being applied.
Click to expand...

Possible, but unlikely as theres reasonable fluid flow if it can squirt.

Be carefull pressing the brake peddle to the floor when bleeding the brakes. Some VW master cylinders are easily damaged when the MC is bottomed out without pressure being built up ( because your letting it all out of the nipple). One of the internal seals rolles over its seat and needs a rebuild to sort it.
 
who? said:
Could do. If the flexy is delaminating internally you can get a situation where it almost totally blocks in one direction and yet is fine in the other (so it doesnt hold on and cause sticking).
Click to expand...

Hmm yeah, tis possible init ... could work fine at low pressure but deform at high pressure I pondered.

Non in stock today unfortunatly, 10 days from somewhere... damn. VW reckon nah not a chance of this one, but then they don't know either, lol.

I'm not convinced, but its ordered. :shrug:

:) :thumbs:
 
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singlespeed said:
Be carefull pressing the brake peddle to the floor when bleeding the brakes. Some VW master cylinders are easily damaged when the MC is bottomed out without pressure being built up ( because your letting it all out of the nipple). One of the internal seals rolles over its seat and needs a rebuild to sort it.
Click to expand...


:thumbs: Nice one ta.

What an awful design flaw :gag: ...I may be rebuilding the master if I don't get to the bottom of this anyway...
 
Forbiddenbiker said:
You got any general figures for a caliper like mine on how much movement of the piston should occur under normal braking?
Click to expand...

Caliper pistons should have enough travel to move unobstructed across the full range required from a new set of pads and a new disc, right down to fully worn pads and disc (note that this isn't always pads right down to the metal, but the specified worn figure)

Once the disc and pads are bedded in and adjusted. Usual movement would only be 0.5 - 1mm or so, from pressing the peddle to it being released.
 
Forbiddenbiker said:
:thumbs: Nice one ta.

What an awful design flaw :gag: ...I may be rebuilding the master if I don't get to the bottom of this anyway...
Click to expand...

If your using a pressure or vac bleeder, this shouldn't be an issue.


It could be worth checking if theres any bleed nipples on the master cylinder. Some cylinders have 2 bleed nipples ( one for each circuit ) if your sure the main lines and calipers are air free then this has helped on some golfs.
 
singlespeed said:
Once the disc and pads are bedded in and adjusted. Usual movement would only be 0.5 - 1mm or so, from pressing the peddle to it being released.
Click to expand...


No problems with bleeding no, tis all very easy with a vacuum bleeder....always good to know about the over stroke though.

I've crushed my piece of wood...and the results are ...well I could have munched it right up...serious crushing....way passed 2mm.

So that’s fine and as expected :thumbs:...good test idea though ta Neil,. thanks for the confirm on the 'press' distances too. :thumbs:

..If I could find a piece of wood I knew crushed at 100kg/per ..and another at 110kg/per etc ...now that would be like the rolling road I've pondered. :nuts: ;)
 
Forbiddenbiker said:
No problems with bleeding no, tis all very easy with a vacuum bleeder....always good to know about the over stroke though.

I've crushed my piece of wood...and the results are ...well I could have munched it right up...serious crushing....way passed 2mm.

So that’s fine and as expected :thumbs:...good test idea though ta Neil,. thanks for the confirm on the 'press' distances too. :thumbs:

..If I could find a piece of wood I knew crushed at 100kg/per ..and another at 110kg/per etc ...now that would be like the rolling road I've pondered. :nuts: ;)
Click to expand...

:lol: It wasn't really intended as a wood crushing exercise. It was simply to prove wether or not, the piston was capable of moving further than the point at which the pads would be pressing against the disc. So if your bit of wood was thinner than the pad it replaced( either before or after being crushed), then obviously there’s no significant mechanical reason preventing the piston pressing the pad against the disc, other than the handbrake linkage not being adjusted the same.

Is the handbrake mechanism for both Left and Right callipers sitting in the same position with the handbrake off? On the Mk4 golf, you can measure between the calliper body and the linkage with a set of feeler blades to set/adjust the balance. So not sure of the exact callipers and cables on the T4, it may be similar.

Just had a thought :thinking: reminded from a ride in a friends newly acquired MOT failure :eek:. If the discs are vented, but have been on for a while. check to see if the disc is being crushed... Yes, you did read that correctly, the vanes between the two faces of the disc has rusted away, meaning that the two faces of the disc could be pushed together. Not very good when you want to shove the pads against them.

Are they the same size callipers Left/Right. Has one or both been replaced since the last mot? If the piston size is different, then the force will be different also. Unlikely but still possible if someone has been on a scrap yard rummage.

Was there an imbalance across the front brakes? Probably with the NSF being lower force than the OSF. Being as the duff OSR calliper would be linked to the NSF, this would possibly indicate air in the circuit or poor linkage adjustment, hence the check above... The braking force is lower because when you press the brake peddle, this acts on the master cylinder which is actually two pistons on a tandem shaft, so one can only move as far as the other... If the movement is compressing some air in one of the circuits then the pressure and resulting brake force will be lower in that circuit. Try bleeding again. First off at the MC if there’s a couple of bleed points... Chances are theres sod all room to get to them, so a 7mm 1/4 drive deep socket on a t-bar could be useful. Try the old method of pressing the brake, then cracking the bleed nipples, but just be gentle and don't pump the peddle trying to build up pressure like a foot pump. You need to pull through enough fluid to bleed the MC, ABS unit, lines and calipers.


Hopefully I won't be asking the same sort of questions next week. I should be fitting a big brake upgrade to the Audi if all the bits have arrived. 284mm vented discs on the rear and 325mm with Porsche callipers for the front :love:
 
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Had a similar sort of problem a few years ago on an old Landy. Ended up replacing the entire brake system from master cylinder to wheel cylinders including solid and flexible pipework. I figured that brakes were more important than saving a few quid so did the lot. Never managed to sort out exactly what the problem was but I suspect that one of the wheel cylinders wasn't playing nicely even though they all seemed to work fine under test conditions.
 
Yea Nod, I'm going to replace the caliper on that side I think... I'm kinda holding off doing so because I like to think I'd know why I'm replaceing it, rather than just throwing new parts at it... I'm a mechanic not a bloody fitter, IYKWIM.
:gag: :lol: :bang: :thumbs:

singlespeed said:
It wasn't really intended as a wood crushing exercise. It was simply to prove wether or not, the piston was capable of moving further than the point at which the pads would be pressing against the disc. So if your bit of wood was thinner than the pad it replaced( either before or after being crushed), then obviously there’s no significant mechanical reason preventing the piston pressing the pad against the disc, other than the handbrake linkage not being adjusted the same.

:D Aye, works just fine that bit Neil.

Is the handbrake mechanism for both Left and Right callipers sitting in the same position with the handbrake off? On the Mk4 golf, you can measure between the calliper body and the linkage with a set of feeler blades to set/adjust the balance. So not sure of the exact callipers and cables on the T4, it may be similar.

Yep, same for both sides its a very simple two into one cable mechanism.
The pressure is less on that side when using the hand brake, as it is when using the foot brake... But that just confirms the problem is at the caliper end for me.

Just had a thought reminded from a ride in a friends newly acquired MOT failure . If the discs are vented, but have been on for a while. check to see if the disc is being crushed... Yes, you did read that correctly, the vanes between the two faces of the disc has rusted away, meaning that the two faces of the disc could be pushed together. Not very good when you want to shove the pads against them.

Hmm, like that idea ...cast Iron discs but I guess a tad of flex is just possible under load. ...

I'll check it out. ...just back from a week in Wales as you know.


Are they the same size callipers Left/Right. Has one or both been replaced since the last mot? If the piston size is different, then the force will be different also. Unlikely but still possible if someone has been on a scrap yard rummage.

Yeah, exactly the same.

Was there an imbalance across the front brakes? Probably with the NSF being lower force than the OSF. Being as the duff OSR calliper would be linked to the NSF, this would possibly indicate air in the circuit or poor linkage adjustment, hence the check above... The braking force is lower because when you press the brake peddle, this acts on the master cylinder which is actually two pistons on a tandem shaft, so one can only move as far as the other... If the movement is compressing some air in one of the circuits then the pressure and resulting brake force will be lower in that circuit. Try bleeding again. First off at the MC if there’s a couple of bleed points... Chances are theres sod all room to get to them, so a 7mm 1/4 drive deep socket on a t-bar could be useful. Try the old method of pressing the brake, then cracking the bleed nipples, but just be gentle and don't pump the peddle trying to build up pressure like a foot pump. You need to pull through enough fluid to bleed the MC, ABS unit, lines and calipers.

No imbalance across the front as I've said... they're diagonal linked so that doesn't help either. ..No abs, just the most basic of systems over all I've discovered... all fully bled, tis easy stuff.


Hopefully I won't be asking the same sort of questions next week. I should be fitting a big brake upgrade to the Audi if all the bits have arrived. 284mm vented discs on the rear and 325mm with Porsche callipers for the front
Click to expand...

Hopefully not no Neil, lol, sounds like a nice project though .... Just got to buy super sticky tires to keep up with it all now then ...and some racing dampers to keep the wheels on the ground. ...yeehaa..
 
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