1998 Chevy Suburban FRONT CALIPER

Two possibilities come to mind. Both have to do with bleeding the new lines. If you pedal bled the system, did you press the pedal all the way to the floor? If you did, there could be debris blocking the lines near the master cylinder. Normally the pistons and seals don't travel in the bottom halves of the master cylinder bores. this video will how you how.

https://youtu.be/OYmzl1LSfX0

Corrosion and debris build up there. By pressing the pedal to the floor during bleeding, (or when a hose pops), the seals can tear when they run over that crap. That will cause a sinking pedal when you hold steady pressure on it. Whether that happens or not, some of that crap could be knocked loose and sent down the lines. I think that's a stretch, but heck, . . . haven't you already looked for the simple or common stuff?

Another often overlooked problem has to do with brake fluid contamination. The slightest hint of petroleum product in the fluid will cause rubber parts to swell. Seals in the master cylinder will grow past the ports to the reservoir trapping the fluid. As the brakes heat up, the expanding fluid can't release back into the reservoir so pressure builds up that applies the brakes even harder. You might see the seal under the reservoir cap balloon up and feel soft and mushy. The only proper fix is to replace ALL rubber hoses and ALL parts containing rubber seals or o-rings, then flush and dry the steel lines. Any rubber part not replaced will leach contaminants back into the new fluid and will contaminate the new rubber parts. If this is suspected, the calipers will release when the bleeder screws are opened. This takes about a week to show up after the fluid became contaminated.

Common causes of conntaminated fluid include resetting the bladder seal in the reservoir cap with oily fingers, and using penetrating oil on hydraulic fittings. A less common, but still lethal cause is wiping out a funnel that was used for engine oil or transmission fluid, then using it for brake fluid. The slight residue is enough to cause problems.

To show my students the importance of keeping brake fluid clean, I put two wheel cylinders in two beakers half filled with brake fluid, then added one drop of engine oil to one of them and stirred it up. After a week, the contaminated one had grown by about 10 pecent and was soft and mushy. The rubber used for brake parts is definitely not compatible with petroleum products.

If this doesn't sound like it fits your problem, holler back for some more ideas.

caradiodoc

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.

Caradiodoc