Deep/Short Brake Pedal

We just put a set of rebuilt Girling 14LF calipers on the front of the Alexis MK14 and ran it at Mid Ohio last weekend. The car previously had 12sp calipers. I noticed that when the brake pedal is first pushed that the pedal goes a little deeper then when its pushed immediately after. So, subsequent presses are a bit firmer and a bit higher than the initial press.

Of course, if we wait several seconds, the first press will again be a bit deeper followed by firmer and a bit higher.

Not sure what the problem is. A little air in the system? A failing master cylinder?

Any ideas?

Thanks.

Ralph :greenflag:

A long pedal immediately followed by a shorter pedal sounds like pad/piston kick-back to me. Warped rotors? or something causing the rotors to wobble and push the pads back between application?

Ian has the right idea, could be the wheel bearings are a little loose or you still have a little air in the system.

Quote:

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Originally Posted by Ralph Z.

We just put a set of rebuilt Girling 14LF calipers on the front of the Alexis MK14 and ran it at Mid Ohio last weekend. The car previously had 12sp calipers. I noticed that when the brake pedal is first pushed that the pedal goes a little deeper then when its pushed immediately after. So, subsequent presses are a bit firmer and a bit higher than the initial press.

Of course, if we wait several seconds, the first press will again be a bit deeper followed by firmer and a bit higher.

Not sure what the problem is. A little air in the system? A failing master cylinder?

Any ideas?

Thanks.

Ralph :greenflag:

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I think it's possible that the caliper piston seals are springing back and thus pulling the pistons back more now since they're new than they will later. Once they take a set from heat and use, they should spring back normally (a small amount to keep the pads from pressing on the rotors once released).

Quote:

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Originally Posted by DaveW

I think it's possible that the caliper piston seals are springing back and thus pulling the pistons back more now since they're new than they will later. Once they take a set from heat and use, they should spring back normally (a small amount to keep the pads from pressing on the rotors once released).

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Even if we did five sessions at Mid Ohio last weekend? Do you think it still will set yet?

I have found on my car the rears tend to be a little harder to get all the air out. May be because its a longer run.
I put the rear of the car up on stands and tap the lines going back and the clipers, let it sit overnight and then bleed again. This usually does the trick.

Rob

I rebuilt all the hydraulics on my March before the first event at Roebling and that was exactly my feeling. Between bleeding again after the event, then after Road Atlanta (still not great), mine were great for MO last weekend.

Bearings and warped rotors can certainly cause similar issues but I personally have not had a warped rotor or failing wheel bearing in FF. And if those parts are the same as before the rebuild, and it didn't have the issue, it would seem more likely to be as DaveW suggests.

Quote:

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Originally Posted by Ralph Z.

Even if we did five sessions at Mid Ohio last weekend? Do you think it still will set yet?

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Depends on how hard you used the brakes. If they got pretty hot, then they should be acclimated (set) by now. But as Garey said, if that was his issue, it might take longer for you , also.

It could also be that the seals are sticking to the piston surfaces due to lack of initial (brake grease) lubrication - that should also wear in with use.