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?

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 ?

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]

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.

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 ?

---

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?

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.

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?