Front brakes binding

[marcus]

It's not a stupid question and this thread has become quite long, so the history is probably not so obvious...

Yes they were binding before the disc and pad swap - however, they weren't binding before the original MOT test. The timeline was:

1) Brakes working fine with old pads and discs
2) MOT brake test
3) Brakes binding
4) Pad and disc swap
5) Brakes not binding
6) MOT brake test
7) Brakes binding intermittently then totally seized

[bensspares]

ok,theres a underlying problem, i have never come across this,maybe someone else has the answer.

it has to be the servo/mastercyl thats all i can deduce,unless the linkage across the bulkhead is jamming somehow.

the cheapest action will be to buy a working second hand unit and swop it.

goodluck

[marcus]

Not wanting to plunge directly into taking apart the master cylinder and servo, I contented myself with taking the calipers off and dissassembling those first, thinking that I might as well, due dilligence and all that...

Well, the pistons and the bores look ok, no obvious rust or scoring, but there was quite a lot of brown sludge behind the piston, which conceivably could have been preventing the piston from retracting fully - there was pretty much an equal amount of it both sides, so that may be the problem.

I'll clean the calipers and put them back together, though will still check the master cylinder, since it too may be sludged up...

Anyone have any idea how much a caliper seal kit should cost? £15.90 +VAT per side from brakepartsuperstore.org.uk seems a bit high to me, given there's only the sealing ring and the dust cap, plus the two rubbers for the guide bolts.

[ChrisPears]

You don't have Vented discs and pads for a car with non vented do you?

I had the same sort of problem but it was because the linkage between the pedal to the bar crossing the engine bay was badly adjusted...4mm out of spec. Was never a problem until I switched to the 16v servo.

Check that the hoses aren't kinked or twisted, could possibly (maybe) restrict the return of fluid.

[marcus]

No, the discs I took off were vented, the only difference is the new discs are also grooved but they are the same thickness (bar a couple of mm of wear), and the pads are for vented discs, the difference in pad thickness is a few mm which is clearly down to wear - I've had the car seven years and the pads hadn't been changed in that time.

Interesting that you mention the linkage because I did take up some slack when I changed the rear drums - but I don't believe that's the problem because there was previously too much play in the pedal, and after the adjustment I wasn't getting the seizing until I took it for MOT and they stamped on the brakes.

I think part of the problem is the sheer volume of gunge that was in the front calipers, which will have been too thick to pass through the tiny aperture leading to the flexible hose, and will hence have been an obstacle to the pistons retracting fully, coupled with the increased thickness of new pads and disc vs old pads and discs.

Am currently rebuilding the front calipers (as I write, one of them is sitting in an electrolytic de-rusting bath in the garden, though my automatic battery charger doesn't seem too happy about it...).

However, I also don't believe that's the whole story, so I will be disassembling the master cylinder and potentially the servo - re the latter, the vacuum hose has hardened and there is a bit of a kink in it, so conceivably, it's holding on when it shouldn't, so is an obvious candidate for replacement.

Good point re the hoses, unfortunately it doesn't seem to be the case, though it's not hard to see that a hose that is kinked would allow fluid to the caliper under the high pressure from the master cylinder but wouldn't let it back out under the relatively low suction from the release of the pedal.

[PeteGLi]

, the vacuum hose has hardened and there is a bit of a kink in it,

If by this, you mean the vacuum hose between the inlet manifold and the brake servo, then it is supposed to be hard - it is made of hard molded plastic, shrunk onto the one way valves at manufacture and formed into the shape it is.

The short flexible pieces of rubber hose (about 2" long) at each end, that actually connect this hose to the inlet manifold and the brake servo do, however, tend to get very soft, perish and split and need replacement.

[marcus]

Interesting that the hose is supposed to be rigid, one wonders why, given the engine will move on it's mounts a little, which must put some strain on the hose...

In case anyone is interested, the electrolytic de-rusting does seem to work but you do need an old-fashioned non-automatic charger, the automatic ones freak out and keep turning themselves on and off, which means you don't get a steady flow of current through the electrolyte, plus the buzzing of the relay sounds scary.

Will post some pics of the caliper rebuild on a new thread when I'm done, in case anyone's interested.

Phoned up my local VW dealer to ask about replacement seals (part number 171698471A according to vagcat.com) and they're still available and there are a small handful in stock at the main warehouse - but they're £20.93 inc VAT per side, which I find quite shocking when whole calipers are available from GSF for £34.50 +VAT.

[marcus]

Just looking at vagcat.com again, there is a seemingly identical seal kit with part number 171698471B ie differs just by the letter B rather than A. The B set appears to be associated with calipers for vehicles whose VIN is > 022 075... and mine starts 025 - I doubt it'll be any cheaper though... :(

[marcus]

I rebuilt the calipers, after thorough de-rusting in an electrolytic bath, and they were beautifully shiny and sliding smoothly on their slide bolts, the pistons were running smoothly in their bores... though I did forget to apply a little copper grease to the points where the outer edge of the caliper slides on the carrier (but before the MOT they were as dry as a bone and working ok).

Replaced the front hoses and thoroughly bled the brakes - took it for a quick spin around the block and all seemed well.

Took it for a longer drive today and by the time I'd done five miles the first signs of stickiness had re-appeared and when I parked up you could feel the heat radiating off the discs and the alloys were warm to the touch.

After a few minutes doing an errand, I drove off and by the time I reached my next destination about four miles further on, the brakes were really dragging - to the extent that pulling away from a standstill required more loud pedal and more aggressive clutch control.

After an hour in Tesco, the discs had cooled right off and following the one mile drive back home, the discs were hot but not so hot you couldn't touch them.

The pedal feel was noticeably different when the brakes were sticking ie very little travel and quite hard, when not sticking there was normal movement and less hardness.

So WTF do I do now?! The following could be wrong:

• Wrong discs or pads, though this seems unlikely, because their respective thicknesses are exactly right as per Haynes and the previous ones I took off (allowing for wear)
  The calipers ie the pistons are still sticking in their bores, though this seems unlikely and I'm reluctant to shell out the thick end of £100 'just in case'...
  Overzealous handbrake adjustment? Seems unlikely...
  Oversealous rear brake wedge adjustment? Seems unlikely...
  Overactive servo ie intermittently collapsing hose keeping vacuum in the servo? Again seems unlikely because this problem first arose immediately after the MOT really stressed the brakes
  Defective master cylinder not allowing fluid back in ie sticking? To me this seems to be the most likely culprit, if only because it's the only component other than the servo that hasn't been overhauled (calipers rebuilt, front hoses replaced, discs, pads, drums, shoes, wheel cylinders, springs and misc hardware replaced)

This is driving me crazy!!! I don't want to have to scrap the car but equally I don't want it to become a money pit...

[wasted]

I'd go with master cylinder problems. It's the most likely suspect, given the symptoms you're experiencing (i.e. both sides doing the same thing).

Out of interest, are your rear brakes binding too?

[marcus]

As far as I tell tell the rears are fine - touching the drums through the alloys revealed them to be cold when the discs were hot.

[wasted]

Which is interesting, because if it were something to do with the overall functioning of the master cylinder, you'd expect the same symptom on the rears. Makes me think it's something specific to the front brake circuit. Unfortunately I can't suggest what!

[marcus]

That's a very interesting point - as you say, if the master cylinder were faulty it should affect both front and rear, given the master cylinder has two chambers which serve one front and one rear brake, split diagonally.

The obvious difference between front and rear is that when disc brakes bind they overheat and the metal expands so bite harder, exacerbating the problem, but when drums bind and overheat, the expansion of the drums lessens the bind and hence the heating so in some sense they 'fix' themselves.

Which makes the calipers suspect again, though I can't understand how... they seemed to be in good enough nick and the fact the brakes are binding equally (ie there's no steering pull to one side or the other) is even more confusing.

[marcus]

Following on from PeteGLi's question re the drums, I've just remembered that when the fronts were binding and the pedal was hard, there was almost no resistance on the handbrake lever, suggesting that the rears were either fully or nearly fully engaged too... despite there being no obvious excess heating of the rears...

Which points back to the master cylinder... I feel like I'm going around in circles!

[go-for-it1]

Following on from PeteGLi's question re the drums, I've just remembered that when the fronts were binding and the pedal was hard, there was almost no resistance on the handbrake lever, suggesting that the rears were either fully or nearly fully engaged too... despite there being no obvious excess heating of the rears...

Which points back to the master cylinder... I feel like I'm going around in circles!

So end the circle and change the master cylinder!!