Frosty
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| posted on 6/5/09 at 08:24 AM |
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Poor brake pedal feel from rear brakes
My car stops really well, but I have never been that happy with the brake pedal feel. The pedal has a bit of extra travel in it, and it's due to
the rear callipers.
I have the following setup:
AP master cylinder.
AP 4 pots.
Sierra rear callipers.
I know there is no air in the system, and that the front brakes are spot on. The problem is that the rear pads always retract a little too much -
probably due to their simple design.
When the handbrake is applied, the pedal feel is rock solid since the slack is already taken out of the read brakes, but as soon as you take it off,
the pads are uncompressed by the handbrake system, and so the initial travel in the brake pedal is used to take this slack back up again.
I was wondering if the pads (OE Ford Sierra) are very soft and actually compressing a little thus contributing to the play int he pedal, and if it
would be better to switch to another pad like an 1144?
I was also wondering if a residual pressure valve would help keep the brakes a little closer to the disc. Is this type of thing recommended?
Things I have tried already:
* Bleeding brakes - no air at all as pedal is rock solid with hand brake on and does not pump up.
* Wedging pedal down overnight to allow piston seals to stretch a little (or whatever it does).
* Abusing them - they have done several track days, and the feel improves when the brakes get hot, but goes back to normal once cool.
Any suggestions on the above?
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nick205
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| posted on 6/5/09 at 09:34 AM |
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Sure you'll have considered/checked, but what condition are the discs/pads in?
If the discs/pads are worn there will always be more travel in the system. Replacing the discs might take up some slack.
I have rear drums so unfamiliar with the Sierra disc set-up, but how are the pads retained in the caliper? if they're simply held in place with
pins/clips rather than actually attached to the pistons could you pack them out with a shim of some kind?
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40inches
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| posted on 6/5/09 at 10:23 AM |
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quote:
Originally posted by Frosty
I was also wondering if a residual pressure valve would help keep the brakes a little closer to the disc. Is this type of thing recommended?
I had exactly the same problem, annoying init? In my case it was rust on the part the pad slides on, holding the bottom of the pad, it was obvious the
way the pads had wedged from the top in the way they had worn, I cleaned all the contact areas with a smooth file,new pads, sorted 
If that didn't work I was going to fit a 3lb residual valve, only thing to lose was the cost of the valve.
Should add that I had a similar problem on a SAAB front caliper, but the problem there was that the old pad had worn a groove in the carrier and I had
to replace the carrier.
[Edited on 6-5-09 by 40inches]
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Frosty
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| posted on 6/5/09 at 10:45 AM |
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Thanks for your replies guys, but I don't think this is the problem in either case because the items were brand new and have done 1500 miles.
Pads have hardly worn at all.
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40inches
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| posted on 6/5/09 at 01:45 PM |
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Residual pressure valve is what I would try next, but that is what I would do. Especially if the master cylinder is floor mounted.
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nitram38
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| posted on 6/5/09 at 02:25 PM |
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Try the run out on the disc and the wheel bearing or the caliper mount.
If the disc is not true, then as you drive the pads will be pushed away from the it.
When you press the pedal you will be taking up this gap first before you get the brakes to bite.
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Frosty
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| posted on 6/5/09 at 02:28 PM |
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quote:
Originally posted by nitram38
Try the run out on the disc and the wheel bearing or the caliper mount.
If the disc is not true, then as you drive the pads will be pushed away from the it.
When you press the pedal you will be taking up this gap first before you get the brakes to bite.
This is when stationary - the pads always fall back a small distance away from the pad. The feeling on the pedal is a lot like pad knock-back though,
where the pedal sinks slightly lower than usual to take up the slack of the pad and disc contact.
My master cylinder is floor mounted, but the problem doesn't seem to affect the front brakes at all, just the rears.
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MikeRJ
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| posted on 6/5/09 at 06:05 PM |
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Are the rear calipers mounted absolutely square to the disk surface? If not they will be distorting the disk and making the pedal springy. Check
with the DTI on the disk.
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Frosty
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| posted on 6/5/09 at 08:40 PM |
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quote:
Originally posted by MikeRJ
Are the rear calipers mounted absolutely square to the disk surface? If not they will be distorting the disk and making the pedal springy. Check
with the DTI on the disk.
Yes they look absolutely square. I'm going to order a Wilwood residual pressure valve which should prevent the pads falling back as far. Once
fitted I'll post my findings.
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britishtrident
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| posted on 8/5/09 at 01:25 PM |
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Two things come to mind
Firstly this might be the same problem seen on the MK Indy ---- bleed nipple on rear calipers is in the wrong position to get all the air out, usual
way round this is to unbolt the caliper and turn it so the bleed nipple is at the very top when bleeding.
Another good trick for getting all the air out is for each caliper in turn to take the pads out, then pump the pistons out 8 mm or so then push/wind
them back so the fluis is foreced back to the master cylinder.
Second thing is are you sure you have given the handbrake cable enough slack for the self adjusters to work --- the handbrake tension should be
adjusted by
(1) Slacken back the cable
(2) Pump the brakes hard several times
(3) Tension the cable so it just lifts the levers at the calipers off the stops.
(4) Pull the hanbrake on and off several times and then repeat steps (2) and (3)
If both these points are OK the problem is much more likely to be from the front brakes.
However if you want to test the master cylinder just unscrew the pipes from the master cylinder outlets and replace them with bl;eed nipples and give
it a quick bleed
[Edited on 8/5/09 by britishtrident]
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britishtrident
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| posted on 8/5/09 at 01:26 PM |
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quote:
Originally posted by Frosty
quote:
Originally posted by MikeRJ
Are the rear calipers mounted absolutely square to the disk surface? If not they will be distorting the disk and making the pedal springy. Check
with the DTI on the disk.
Yes they look absolutely square. I'm going to order a Wilwood residual pressure valve which should prevent the pads falling back as far. Once
fitted I'll post my findings.
A residual presure valve is probably not an ideal solution.
[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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bob
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| posted on 8/5/09 at 02:03 PM |
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Agree with BT, my sierra rear calipers are a pig to bleed because of the position on the MK. What position are the rear calipers on your car ? ideally
they should be at 12 o-clock on the disc to remove all the air locking in the caliper,as said by BT i had to unbolt the calipers and roll them to the
top of the disc to bleed properly.
I had the same problem when first built, front are ok as they are on a different circuit.
[Edited on 8/5/09 by bob]
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Frosty
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| posted on 11/5/09 at 10:37 AM |
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quote:
Originally posted by britishtrident
Two things come to mind
Firstly this might be the same problem seen on the MK Indy ---- bleed nipple on rear calipers is in the wrong position to get all the air out, usual
way round this is to unbolt the caliper and turn it so the bleed nipple is at the very top when bleeding.
Another good trick for getting all the air out is for each caliper in turn to take the pads out, then pump the pistons out 8 mm or so then push/wind
them back so the fluis is foreced back to the master cylinder.
Second thing is are you sure you have given the handbrake cable enough slack for the self adjusters to work --- the handbrake tension should be
adjusted by
(1) Slacken back the cable
(2) Pump the brakes hard several times
(3) Tension the cable so it just lifts the levers at the calipers off the stops.
(4) Pull the hanbrake on and off several times and then repeat steps (2) and (3)
If both these points are OK the problem is much more likely to be from the front brakes.
However if you want to test the master cylinder just unscrew the pipes from the master cylinder outlets and replace them with bl;eed nipples and give
it a quick bleed
Thanks for your reply.
I am certain there is no air in the system since the pedal becomes firm with the handbrake pulled on - it's just caused by the pads falling back
too far. You're right about the bleed nipples and their location, but I have managed to get round the problem by swapping the bleed nipple and
brake hose locations so the bleed nipple sits at the top. This makes bleeding much simpler and avoids the need to remove the calliper and rotate to
bleed.
I think the problem may be with the handbrake cable. I have mine completely slack and under no tension at all. I wonder if by introducing a little
tension, the pads will sit closer to the disc at all times? I think I'll give this a try as per your process above.
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Frosty
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| posted on 12/5/09 at 08:48 PM |
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Ok I adjusted the hand brake and removed all of the slack, but the pedal still travels too far for my liking.
Once again though, if you apply the handbrake firmly, the pedal feels superb.
I don't know what to do with it!
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