Pedal Soft at top and then gets hard at bottom? [Archive]

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PhillipM

10-04-2009, 10:47 AM

So here are the contents of the system:

Baer 13's and 12's
MP 8.5" dual booster
Baer supplied master

We have bled the system many times and the problem has yet to give up. . At the top of the swing the pedal is soft but I can feel the brakes starting to work, even through the soft pedal. Then as the pedal gets about 3/4 of the way down the pedal gets really stiff and I have to use ALOT of input pressure to get things to happen, this is followed with the fronts locking up and rears never do. Any ideas??

NOGO

10-04-2009, 04:26 PM

Check your brake pedal to master cylinder rod adjustment- should be about 1/4" of play. You also want the front brakes to lock up before the rear brakes. You can dial in your bias a bit with an adjustable prop valve.

sik68

10-07-2009, 01:02 PM

Hmm, I have this same problem on my daily driver, soft pedal through most of travel then hard pedal at the end, or if I pump the pedal quickly it gets hard. Also, if I am holding the brake at a stop light, while giving the engine some revs, the brake pedal falls further. Haven't had it repaired yet, but my guess on my car is the check valve on the booster is faulty. Hopefully that helps...let me know if you diagnose it!

NOGO

10-07-2009, 08:15 PM

sik68

10-08-2009, 06:07 AM

A pedal pumping up is a sign of air in the system.

Typically yes. But if he and i are confident that the systems are properly bled, another way you can obtain a hard pedal is with a bad check valve not letting the booster properly store boost.

CarlC

10-08-2009, 06:48 AM

Does the MC have bleeders?

David Pozzi

10-08-2009, 08:29 AM

Have someone observe the calipers while you apply the brakes, - sitting still of course. Look for caliper movement, especially twisting of the caliper. You might have a miss-alignment of a caliper bracket. Also check rear axle end play, it should be very small like .010" to .020". If you have a non-posi diff, the end play is huge.
David

PhillipM

10-09-2009, 09:36 PM

Thanks guys these are great ideas that I will look into and report back!

PhillipM

12-07-2009, 03:33 PM

I just wanted to give an update so far on this issue. We found that we have quite a few things going on as we dig further into this issue.

#1 - Torn grommet at the booster. Most likely letting vacuum escape.
* Fixed with a new gromet at check valve just for good measure.

#2 - Proportioning valve had been modified by our builder to "increare rear pressure". Well any one with physics 101 could tell you that drilling out the orfice size reduced rear pressure not increased it. We were only getting about 700 max psi out of the rears.
* Installed new combination valve and are now seeing 950 - 1150 out of the rears.

#3 - Pads glazed over in rear. Guessing this was due to redueced pressure to seat the pads.
* Cleaned up rotors and installed new pads.

So, here we are and the car has never stopped this well. Now I am not saying that it is the best stopping vehicle that we own but it is WAY better then it was. Only problem is that the pedal is still spungy. In fact it doesn't feel hardly any different. Only thing is that it actually stops. I am not sure how to check for push rod length so if someone could chime in with a procedure that would be awesome. Also, I did see that our builder claimed that the left rear caliper mounting bracket was welded on the rear end in the wrong location. We have about a 1/4" thick spacer inbetween the bracket and the caliper. This has been there since the day we finished the car for Power Tour. I am wondering if there is a way to check the bracket on the rear end. Like a measurement from the hub surface to the bracket? Thanks for all the help, I think we are on our way to trouble shooting these brakes!