I don't like to hear that, but rather than "brittle", the bladder / seal should have been mushy. Just to be clear, a contaminated seal will be hard to force back into the reservoir. It will keep blubbering out as you chase it around. If you meant you just had to reshape the seal until it popped back into the cap, that is normal. As fluid is bled out, or as it gradually takes up the space behind the caliper pistons as the pads wear, the lower fluid level produces a slight vacuum in the reservoir. The bladder seal in the cap pulls down to keep the fluid sealed while eliminating the vacuum. I aplogize if I'm over-simplifying something obvious that you already know, but that's better than mis-diagnosing contaminated fluid.
If contaminated fluid is the problem, the calipers should release when opening the bleeder screws. The exception is if the square-cut seal in the caliper has also grown. It will not stick to the moving piston and bend slightly. The seal straightening out when the brake pedal is released is what pulls the piston away from the rotor to prevent brake drag. Still, with fluid pressure gone, the caliper will release. Normally GM has very little trouble with sticking calipers. For that reason, you should be able to easily pry the pistons back into their housings with a flat blade screwdrver, and the fluid will go back into the reservoir. If the piston will not pry back unless you open a bleeder screw, the seals in the master cylinder have grown past the return ports. That's what causes the heated and expanded trapped fluid to apply the brakes while driving.
Very sorry to say the only proper fix is to replace all rubber parts. The combination valve has rubber o-ring seals. I would be confident finding one in a salvage yard because they too give very little trouble. Watch though that there are no signs of fluid contamination on other rubber parts from that donor truck, and look for a valve assembly either with the same part number, (if you can find one), or from a truck simlar to yours. The proportioning valve part of the assembly is calibrated according to the front-to-rear weight distribution of the truck. This may be less of a concern if you have a height-sensing proportioning valve attached to the left side of the rear axle. They are often used on trucks and minivans because there can be such a wide difference in weight distribution between empty and fully loaded. That height-sensing valve has o-ring seals in it too.
Before you go through all the expense of replacing all the rubber parts, if the calipers will release with the bleeder screws open, but not when they're closed, try the same thing by loosening the steel lines at the master cylinder. That SHOULD give the same result. If you can not release the pistons that way, ... We gotta talk. Then there would appear to be something simpler and less expensive wrong. It should be fairly easy to narrow down.
Tuesday, March 2nd, 2010 AT 8:16 AM