rodgling
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| posted on 20/2/13 at 11:39 PM |
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long brake pedal
Trying to get to the bottom of my long brake pedal travel. Firstly I've looked at the bias bar and noticed that in the first few inches of pedal
travel, the front moves a lot but the rear doesn't really. So the front system obviously has some give in it before the system allows pressure
to build up and the pedal gets firmer.
I tracked the front issue down to one caliper which I thought was flexing. I've replaced the guide bushes and the pedal is maybe a little
better, but not right still.
I have noticed that the piston on that caliper (it's a single-pot floating caliper from an E36 BMW) retracts a lot, so there is at least 1mm
clear between pad and disc when the pedal is released. I think that the soft/long pedal is because it takes a few inches of initial pedal travel to
push the piston out to where the pad contacts the disc.
Is that plausible, and what can I do about it? What makes it retract so far?
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britishtrident
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| posted on 21/2/13 at 07:27 AM |
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A few questions:
Are the pads new and not worn tapered ?
Is the spring clip on the outside of the calliper in place ?
As the suspension at that side suffered impact damage ?
[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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Grimsdale
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| posted on 21/2/13 at 08:20 AM |
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it's probably the piston sicking on the o-ring in the caliper. I've had this myself on a new set of calipers.
Pop the piston out and put some red rubber grease on the seal and piston and reassemble.
[Edited on 21/2/13 by Grimsdale]
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MarcV
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| posted on 21/2/13 at 08:34 AM |
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The pistons indeed retract only because of the O-ring / seal. A sticking piston sounds like a good thing to check out, especially if you see a
significant difference between the two sides.
Drain fluid from caliper (press the piston back in) then pop it out with compressed air (without your fingers in between!  ). I just changed the
caliper on my E46 because it wouldn't be pressed back in, severe corrosion of the piston.
I wouldn't put rubber grease on, normal instructions are to use brake fluid.
However, do remember that you are running similar size MC for front and rear while there is a significant difference in piston area. Therefore the
same pad movement surely needs more travel at the front.
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rodgling
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| posted on 21/2/13 at 08:43 AM |
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quote:
Originally posted by britishtrident
A few questions:
Are the pads new and not worn tapered ?
Is the spring clip on the outside of the calliper in place ?
As the suspension at that side suffered impact damage ?
New pads, although I had to grind them a bit to get them to fit. Previous pads appeared to wear pretty evenly though. Spring clip present. No impact
damage.
Don't understand the comment about the sticking piston - the point is that it retracts too far when the pedal is released, so it can't be
sticking?
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MarcV
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| posted on 21/2/13 at 08:53 AM |
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Piston is held by the seal and deforms the seal during application. This springs back when the brakes are released, making the piston retract. After a
certain amount of deforming, the piston just slides past, which is why wear is accounted for.
If the piston has corrosion behind the seal (ie in the caliper) it won't slide to the correct position. Instead it will deform more on
application of the brakes and spring back more when released.
Was this effect also present on the other pads?
It sounds a bit odd to me that you had to grind the pads to make them fit. Isn't that an indication that you were unable to press the piston
back far enough? Or does the manufacturer say it could be needed?
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Grimsdale
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| posted on 21/2/13 at 09:42 AM |
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I tried brake fluid, which worked for a matter of weeks before the piston stuck again.
Agreed it could be a corroded piston, either way it needs to come out for inspection/lubrication.
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GeoffT
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| posted on 21/2/13 at 10:56 AM |
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It's an obvious point and I guess you've checked this, but does the pedal adjustment allow the master cylinder(s) to fully retract to
their rest position when the pedal is released?
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