DialCaliper posted a brake bias/knee point chart a few brake threads back, and if I recollect correctly, the rears don't see pressure untill the fronts are already doing some work. This means just a little restriction somewhere in the system could cause only the fronts to drag if the residual pressure is under the threshold of where the rears see pressure
With the pedal up, cracking the bleeders open on the calipers when the brakes are hot and binding will tell the tale.
If you don't get fluid out the bleeder, it's very likely an issue with the calipers. (Sticking sliders, or junked up/corroded pistons, and the like)
If you get fluid out the bleeder with the pedal up, you can go to the master and crack the lines there, (using a 10mm flare wrench.) If you get pressure relief there, the problem very likely involves the booster/linkage adjustment or the master cylinder itself.
When the master cylinder piston doesn't retun all the way to it's internal stop due to a misadjustment/internal booster problem/or linkage issue, the thermal compensating port in the master won't be uncovered. This port, (located just past the tip of the seal with the master cylinder piston at rest) allows the system to vent all the fluid back into the reservoir when the pedal is released. It also allows for thermal expansion of the fluid. If the port is blocked, the brakes *may* "appear" to apply and release normally when checking it in the shop cold, but as the fluid heats up and expands, the trapped and expanding fluid will progressively apply the brakes harder and harder. As the pads fade, it might be difficult to tell they are being apllied ever harder by the trapped pressure.
Discharge/pressure at the caliper and not at the master suggests something gone awry between the two units, as something is not allowing for the fluid to return to the master cylinder reservoir.
A little time with the line wrench should find the problem. You'll need to put the system back in "bind" mode, so a short drive may be required between tests... (carefull, it shouldn't take very far or fast to build some heat up, so easy does it!
The prop valve would be another easy place to check. Crack the inlet line (master side) to the proportioning valve, and see what comes out there. No pressure relief/fluid flow there would suggest you test the prop valve a little further. Cracking the line at the outlet (caliper side) of the prop valve would finish telling that tale. Pressure on the outty,(caliper side) but not the inny (master side) of the prop valve suggest a blockage or an internal issue inside the prop valve.
Just continue breaking the system down into short lengths, checking for fluid/pressure relief in beween the segments, and you will find the problem.
Remember that brake fluid will soften/remove paint, so wipe any and all spills promptly!
Quoting 656of1000:
Another probable cause for your problem are the front brake hoses. If, after 200k plus miles, your brakes and suspension have been apart, those hoses probably endured a lot of stresses from being moved around and stretched. Additionally, old brake fluid, after it has begun to break down, will start to deteriorate the insides of the hoses, causing further weakness. So with all of those possible factors, the chance you have collapsed hoses are good.
^^Nice post!
Pressure at the caliper, and not at the outty of the prop valve points to a problem with some kind of blockage in the lines/hoses. To have both front hose sets deteriorate to the level where it could cause this problem is very unlikely but, at this point, who knows. I've seen flat rate line tech's f*** up both hoses and even hardlines before, trying to do a 3o second pad swap, so I know it can be done. ( not supporting the calipers during a pad swap and then damaging the hoses at the fittings by bungy jumping them off the knuckle /ubbthreads/images/graemlins/banghead.gif ) One thing that makes this unlikely is that the usual symptom of hard line/hose issues is there would be a delay in building pressure, and then it will slowly release. The usual complaint when there's a hose restriction is a pull away from the bad corner when the brake is applied, and then a pull towards it when the brake is released. This occurs because the pressure is delayed getting there compared to the other corners, and then slower to leave when the pedal is released. To have both sides affected at the same time is unlikely, but then again so is having both front brakes binding at the same time...
It's also possible to get a one flapper effect inside the hoses at the fittings, but that's extremely rare, and to have both hoses manifest this problem at the same time is very unlikley
Quoting Bimmubishi:
I think it's a brake booster issue.
Your point of the push rod or pedal being out of adjustment has some serious merit.
About the only thing I can add to Alex's list of things to check is that the pedal is returning to the "up" stop. Slop/wear or binding in the linkage could cause some problems with the linkage returning all the way. Pulling the pin on the pedal where it attaches to the linkage would be a quick way to check for adjustment issues.
In addition to the tests Alex listed, a good fuctional test of the booster system operation is to pump the booster down with the engine off, and then recharge the system with vacuum. This verifies that vacuum is being apllied to and released from the booster correctly.
With the engine off, applying the brakes ten times or so should bleed all the residual vacuum from the system. The pedal should get progressively harder to push, and will rise on each stroke untill all the vacuum has been released.
Then while holding the pedal down, start the engine.
The pedal should drop substantially, showing the effect of the vacuum assist.
This test will help verify the correct operation of the booster diaphragm, check valve and vacuum release systems.
If the pedal doesn't change, the check valve may be faulty, or there may be something wanky in the release of vacuum from the booster.
edit The only other thing I can think of is that the master cylinder leaked enough fluid out the back over time that it somehow filled the booster enough to liquid lock the diaphragm... /ubbthreads/images/graemlins/dunno.gif
Needless to say it is very dangerous to drive the car with the brakes malfunctioning, so it's best to get to the bottom of this sooner, rather than later.
Akuma, I pm'ed you my phone number if you want to ask any additional questions.
Good luck troubleshooting this issue, and please update the thread with what you find.
