Help with Brakes???

[jevose]

I just added the monoblock kit (front boxster calipers and cross dirlled rotors on all four) to my 82SC. Also added new SS lines. Problem is my front calipers are not engaging while the rear work perfect. I have bled the brakes and do see brake fluid pump out of all calipers when bleeding. Also, pedal pressure is very, very hard. Only travels 1-2 inches. I was told that it might be the proportioning valve?? Any ideas?? If it is the proportioning valve, where is it??


Thanks,

Jim

[Jim Smolka]

Just a quick check, are the bleeders of the front calipers on the top???

[jevose]

Yes...two bleeders on the top.

[Superman]

A hard brake pedal is a good thing. We have had trouble at times with caliper pistons that would not rest out against the rotor. Instead, they tended to retract after each use such that there was an air gap between the pad and rotor. Check for that. At rest, there should be no visible gap between pad and rotor. I don't believe there is a proportioning valve unless you put one it, but I could be mistaken.

[jevose]

With the car on jackstands, when I press the pedal, the back brakes engage and the front hubs can still be turned with my hands. The pedal is extemley tight. Fluid is getting to the front calipers when I bleed them?? Calipers are just not engaging. Any ideas??

[Bill Verburg]

You have too much front bias. Your hydrailic bias ratio is ~2, in practical terms it means that the front pistons move half as far as the rears but will exert twice the pressure once fully engaged. The problem is compounded when the front rotors are too thin. A relatively small mc will make the situation even worse.

I have never run into a problem as severe as you are reporting with the Boxster caliper on the 24 mm carrera rotor. But have seen similar problems with mismatched Brembos or with Brembo fronts used with M rears.

[jevose]

Thanks for the feedback Bill...These calipers came off of car setup exactly like mine..Same rotors. Why would they work on his but not on mine? What can I do to correct the problem?

Thanks,

Jim

[Bill Verburg]

Did you check for free movement of the pistons before installation? What thickness rotors on the original car vs yours?

[jevose]

22mm rotors on all 4 wheels. Stock calipers on rear. Rear pads have 70% life remaining, same with front.

Any ideas??

I bled the entire system again and cannot get the front calipers to engage. Rears work great. Pedal is not as tight, but moves down about 2 inches ans stops dead solid. I checked the space between the rotors and pads, also looks good. They are just touching the rotor.

Any ideas are welcome!!!

Thanks again,

[Bill Verburg]

Those calipers are designed for 24 mm rotors yours are either worn below wear limit or mismeasured, I suspect mismeasured because stock rears are 20mm thick when new. Other possible causes too small mc should not be a problem with 82SC, pads worn too far in combination with rotors worn too far or too thin, malfunction of front circuit, pistons frozen in bore(rusted in place, you'd be surprised at the amount of corrosion that will develop if the fluid isn't changed regularly especially if the pistons aren't exercised regularly to their full travel).

Those calipers are used by many people successfully if not optimally. They should be used with the 84-89 Carrera 24mm thick rotors and should have either the 84-89 Carrera rear caliper with its own 24mm thick rotor or with matching Boxster or Brembo calipers paired with the 24mm rear rotor.

[jevose]

You are right about the mismeasure..They are 24mm. Calipers and rotors are almost new and came off working car. I am fresh out of ideas. How can I check front circuit? If I see brake fluid come out during bleeding, does than mean anything in regards to the MC?

Thanks,

Jim

[Bill Verburg]

The seals in the mc on the front circuit can be leaky and still push fluid, you would not get a hard pedal for long though.

What you described is a cut seal in your master cylinder.
Try this to test it.
Pump the pedal several times and then do a hard application, if the pedal sinks and then gets hard as you descibe. You have as Bill said a cut seal for the primary circuit. The hard part is the secondary (rear brakes) working.
You have allready done this according to one your posts.

This is a common ocurance durring brake jobs becouse most people dont change the brake fluid on a schedualed basis (2 years) And most cars have the original master cylinder, which happen to form rust in the areas of none useage ie full stroke (Becouse the fluid is not changed/tested)
So what you get is a cut seal when you bleed your brakes, remember that seal has been operating in a limited stroke untill you changed it by depresing the pedal and running the seals through the rust and such.
Also remember brake fluid is hydroscopic ie it attracts water. This can be tested by using a brake fluid boiling point tester. So if you have a can that has been sitting around for a couple of years it may test below DOT specs if it has been opened.
Good luck

[Early_S_Man]

I suggest removing, disassembling, and cleaning your master cylinder ... don't remove the cups and seals on the master cylinder piston, just rinse of with spray brake cleaner and rub off any residue with paper towels, then take pictures if a digital camera is available.

[rattlsnak]

A little more to the story,,,,,,, As Jim mentioned the brakes came off of a working car. -mine!- They have been working fine with no bias problems for the last 3,000 miles, and had been sitting only a week before the exchange, so rule out any rust or bad fluid, etc. It is the complete Boxster kit with all 4 carrera rotors. We did the swap at my house, and I noticed that his front brake circuit (I guessed calipers) had apparently been sticking and the front rotors were worn past the limits and very tight to turn even with the wheel off. After we put the kit on, the pedal kept going almost all the way to the floor with little resistance, but the brakes were "catching". We decided at that point, since he still had the original rubber brake lines, that maybe it was the lines that were clogging and not letting the fluid return on the old system, or not enough to supply the bigger calipers. He drove it later that night, and after it got hot, the brake pedal became very tight, and the brakes became applied without pressing the pedal. (this was before he put the new lines on) After it cooled down he was able to drive it home, where he put the new lines on, and now we have this problem. I'm also suspecting the master cylinder, but as you can imagine I feel bad that this has happened because I sold him the brakes, but I feel that there was a problem in that circuit beforehand anyway, and of course now it is exaggerated. Any other thoughts on what to do?? I'm going over to help him on Tuesday. Thanks in advance....
Marc

The rust would be in "his" master cylinder and "his" former calipers, lines etc.
So what you describe and what he said you cut a seal in the master cylinder. Or one of the hard lines from the master cylinder to the front circuit is loose/cut and leaking.
So first check the fluid resevior has the level gone down?
If so check the master cylinder and the hard lines to the front brakes.
Also pull back the carpeting from the pedal board and inpsect where the push rod enters the MC is it wet?
If you have not lost any fluid it is either an internal leak or air in the system.
A master cylinder is not that expensive so you might pick one up.
Good luck