rodgling
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| posted on 23/8/11 at 07:51 PM |
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rear brake problems
I've noticed my rear brakes (and wheels, and driveshafts) get very very hot in fairly gentle normal use. It's probably not a co-incidence
that my fuel economy is nowhere near where it should be. Unfortunately I can't figure out why. The symptoms are:
- only rears affected
- lots of heat (more on drivers side), wheels getting too hot to keep your hand on them
- I can easily push the car when they're hot so they're not binding on
- rears are locking up first
Things I have tried include:
- bleeding the brakes
- adjusting the handbrake (there's about 10 mm of slack when it's in the off position)
- ensuring the pedal can come as far forward as it likes so it's not stopping the master cylinder from opening
- replacing the calipers with brand new ones
- swapping the master cylinder with the clutch master cylinder
- replacing the pads
- grinding back the pads to create more space (there's plenty when I first install them)
- ensuring the discs are central in the calipers
- checking the master cylinders are the right way round (.625 on the front, .7 on the rear)
- cursing a lot
They're Wilwood master cylinders attached to Golf Mk IV calipers.
Any suggestions? This is really pissing me off because it's the last thing to fix before I have a fully functional, healthy car (it's been
a long long road to get to this point as I'm sure you can all understand, and I just want to have a healthy car while there's still some
summer left).
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alexg1965
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| posted on 23/8/11 at 08:43 PM |
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I had a similar problem with my Sierra Cosworth rears which I eventually sorted by replacing the calipers. But you've already done that. Only
things I can think of are that the pins that the calipers slide on aren't sufficiently greased so they release too slowly (if the VW design is
the same) or there is a restriction in the brake pipe somewhere, again preventing the brake fluid pressure from instantly dropping back to zero. It
could be a kink if you've used copper pipe.
Another thought, could it be that when the suspension moves, the handbrake is pulled on because the cable is fixed to the chassis and the rear upright
moves?
[Edited on 23/8/11 by alexg1965]
[Edited on 23/8/11 by alexg1965]
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rodgling
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| posted on 23/8/11 at 09:24 PM |
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Forgot to say, it can't possibly be the handbrake because I tried disconnecting it completely at the calliper end with no improvement.
Don't think it's the calliper pins as these were replaced with the calliper, and in any case the calliper seems to slide freely.
No copper pipes, braided hose from master cylinder to calliper - could be a damaged hose I guess. I will inspect carefully tomorrow to rule that one
out though.
Thanks for the ideas.
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britishtrident
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| posted on 24/8/11 at 06:01 AM |
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I realise you have checked the master cylinder push rods but the symptoms point to the master cylinder piston not coming fully back. Often the
cause of this type of problem is the washer at the end of the master cylinder not allowing the mushroom end of the push rod to come far enough back.
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cliftyhanger
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| posted on 24/8/11 at 07:25 AM |
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No solid pipes, just braided flexy hose??
Ok, that may be normal, and it should reduce the number of joints. However it is not unknown to have brake hoses cause problems, and not visible from
outside either. Usually in rubber, but they can form a little one-way valve. Brakes go on, then take a while to release. Not sure apart for the master
cylinder where you can go you can go.
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big-vee-twin
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| posted on 24/8/11 at 08:53 AM |
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Check your wheel bearings
Duratec Engine is fitted, MS2 Extra V3 is assembled and tested, engine running, car now built. IVA passed 26/02/2016
http://www.triangleltd.com
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DixieTheKid
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| posted on 24/8/11 at 09:09 AM |
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I would agree and say wheel bearings, the heat is not coming down the drive shafts from the diff is it? Highly unlikely but worth asking.
COS IT'S Worth IT
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rodgling
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| posted on 24/8/11 at 09:14 AM |
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The hose sounds worth looking at... not sure what I can do apart from replace it as, like you say, damage may not be visible, but it's worth a
go.
Wheel bearings... both sides? New wheel bearings? There's no obvious play in the wheels so I think that's a long shot. I'll double
check them over tonight.
I think swapping the master cylinder with the clutch has ruled that out.
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rodgling
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| posted on 24/8/11 at 09:25 AM |
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Good point about the diff - I've already checked for this. The diff does get very hot after a longish drive, but on a short drive the brakes
seem to get hot before the diff has time to get hot, with the discs seeming to be the hottest point, so I don't think it can be the diff.
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DixieTheKid
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| posted on 24/8/11 at 09:27 AM |
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Could be just the way it is! If it is just add a bios valve to keep the ass end in check.
COS IT'S Worth IT
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DixieTheKid
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| posted on 24/8/11 at 09:31 AM |
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quote:
Originally posted by rodgling
Good point about the diff - I've already checked for this. The diff does get very hot after a longish drive, but on a short drive the brakes
seem to get hot before the diff has time to get hot, with the discs seeming to be the hottest point, so I don't think it can be the diff.
This may sound silly but, cheap discs can expand something stupid! I had a set on my ERST with 2wd cossy calipers, i put them in the fecking bin after
they warpped. Lesson learnt.........
COS IT'S Worth IT
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rodgling
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| posted on 24/8/11 at 09:59 AM |
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They're mintex discs, should be OK I think. I'll check them over to see that they're straight and the right thickness tonight.
I can't really ignore the problem sadly - locking up the rears first isn't something I can live with, and the rubbish fuel economy
doesn't help either. I'd rather figure out the root cause than put a bias valve in to bodge it.
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adithorp
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| posted on 24/8/11 at 10:33 AM |
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quote:
Originally posted by rodgling
They're mintex discs, should be OK I think. I'll check them over to see that they're straight and the right thickness tonight.
I can't really ignore the problem sadly - locking up the rears first isn't something I can live with, and the rubbish fuel economy
doesn't help either. I'd rather figure out the root cause than put a bias valve in to bodge it.
Well the first thing to do is adjust the bias-bar to reduce the rear braking and check that the rear master cylinder push rod has free play at the
same time.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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britishtrident
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| posted on 24/8/11 at 11:36 AM |
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quote:
Originally posted by DixieTheKid
quote:
Originally posted by rodgling
Good point about the diff - I've already checked for this. The diff does get very hot after a longish drive, but on a short drive the brakes
seem to get hot before the diff has time to get hot, with the discs seeming to be the hottest point, so I don't think it can be the diff.
This may sound silly but, cheap discs can expand something stupid! I had a set on my ERST with 2wd cossy calipers, i put them in the fecking bin after
they warpped. Lesson learnt.........
Actually other way round the thermal expansion coefficient of all the basic iron based alloys -- ie iron, carbon steels and cast irons is
pretty similar with the coefficient of linear thermal expansion of between 10 and 13. More expensive ferrous alloys containing chromium and
nickel generally have alphas in the range 15 to 17.
In comparison light alloys tend to have alphas in the range 20 to 25.
[Edited on 24/8/11 by britishtrident]
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rodgling
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| posted on 24/8/11 at 11:38 AM |
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The "bias bar" bit is non-adjustable, as per IVA requirements. It's a kit build so this design has worked for other cars, so I
wouldn't expect a design issue here. There is plenty of space for the pedal to come forward, so the push rods are not being impeded as far as I
can tell.
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britishtrident
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| posted on 24/8/11 at 06:42 PM |
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The problem with master cylinder push rods relates to the mushroom end not being able to come far enoughh through the big washer at the end of the
master cylinder far enough ---- not getting the last 0.5mm can be enough to cause a restriction. This is a very common problem with bias
kits. The cure is to carefully grind a little off the mushroom end and slightly enlarge and countersink the hole in the washer.
On the original Girling cylinders the washer was dished, modern cylinders copying the Girling pattern don't usually come with properly dished
washers.
the other thing is you should of course have a return spring on the pedal.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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