I take that to mean the pistons aren't getting pushed out to make the pads hit the rotors. If that's wrong, correct me.
The first thing that comes to mind is there's still air in the lines. You'd have a soft, mushy brake pedal. I'm not so sure that's the case unless you let the master cylinder run empty while the calipers were off. Brake fluid will run into the calipers due to gravity, but usually very little air goes back up the lines. That means it's pretty easy to get the air bled out.
The next thing is a little less common but it does happen enough. Crud and corrosion build up in the bottom halves of the bores in the master cylinder where the pistons don't normally travel. Anything that allows you to push the brake pedal all the way to the floor runs the lip seals over that crud and can rip them. That includes pedal-bleeding with a helper, (which I never do), the sudden surprise of a ruptured hose, or just trying to run those pistons out of the calipers. Most commonly the symptom is a slowly sinking brake pedal, sometimes not until a few days later, but if both seals got ripped at the same time, you'll be pushing the pedal but not moving any brake fluid. Usually you'll feel some resistance in the pedal, but not always.
I have to bring up a serious question for a moment. Is there any chance the brake fluid got the slightest hint of petroleum product in it? That includes engine oil, transmission fluid, power steering fluid, or axle grease? It just takes a little residue on your fingertips to create a major disaster, but that will take at least a few days to show up. If you think that's a possibility, I'll go into more detail.
The last thing has to do with the old seals? The inner square-cut seal would have to leak for brake fluid to leak out. That is extremely uncommon. The outer dust boot that usually has a metal ring on the outside on GM vehicles, doesn't hold brake fluid in. It just keeps dirt and water out. The inner seal would have to be leaking, but the outer boot is all you can see. How did that get bent? My suspicion is something else happened to cause that bent dust boot and now it's preventing the caliper from sliding freely on its mount.
First, the most common thing that prevents the caliper from sliding smoothly are those two mounting bolts. Be sure they're not bent and have no dirt or rust on them. Rust pits can be sanded off, but that's a permanent fix. Once the chrome plating has lifted, the problem is going to come right back. The bolts should have a coating of special high-temperature brake grease to prevent that rust and to insure the caliper slides freely. If it sticks, the piston will come out when you press the brake pedal, but instead of the caliper sliding to self-adjust, it will spring back on the rubber boots and push the piston back in again.
The brake rotor can do something similar. You have a rotor that just slides onto the hub and is clamped in place with the wheel. Sometimes a heavy rotor will flop over a little. Applying the caliper will straighten up the rotor, but before you can stroke the pedal again, the weight of the rotor pushes the piston back in as it tips. That can be verified by installing two lug nuts lightly to hold the rotor in place.
Look at the mounting bracket too. If that wheel got hit in a crash or a ball joint broke, that bracket could have gotten bent. If it is making the caliper sit anything other than perfectly parallel to the rotor, applying it will turn the rotor, then as it straightens out, it will push the piston back in. The clue here is only one caliper will be affected, not both of them.
You're working with something rather uncommon so it's hard to give you a better answer. I'm drawing mostly on theory now and less on experience.
One more thing I'd like you to look at. Your vehicle was available in a front-wheel-drive version as opposed to all-wheel-drive. Almost all front-wheel-drive vehicles have gone to a "split-diagonal" brake hydraulic system. That means the left front and right rear brakes are on the same hydraulic system. Or, ... To say it a different way, the two front brakes do not share the same hydraulic system. For a long time GM has used a master cylinder design that has a valve inside that will move to block two ports when one of them fails to build the same pressure as the other two. In other words, there's a leak. The most common complaint is "I can't get any brake fluid to flow from one front caliper". Also, due to changes in the design of the suspension geometry, you will barely notice any brake pull even though one caliper isn't applying at all. The less common complaint is the new brake pads wear out very quickly on one side but on the other side they look like brand new. This will happen multiple times before some people realize they have a problem.
That valve will be tripped too by pedal-bleeding if the pedal is pressed more than halfway to the floor. One piston will always be more willing to move and its pads will hit the rotor first and the caliper will start to build pressure, ... THEN the other one will start to move, unless that valve has already tripped.
The service manual spells out the bleeding sequence, and a lot of people think that is to prevent that valve from tripping, but that would defeat its entire purpose. All you have to do is never push the brake pedal more than halfway to the floor. That is standard procedure for any vehicle with a master cylinder more than about a year old, again, because of that crud that builds up in the bores. If you aren't sure if this is what's happening, when a helper applies the brakes, you won't be able to turn one front rotor by hand but the other one you will. The only way I ever found to reset that valve is to go to the opened bleeder screw of one of the two brakes where fluid won't flow, and give it a very quick, short burst of compressed air, then let it gravity-bleed. You want to avoid pushing air all the way up to the master cylinder because you'll just have to bleed it out again. You just want to move the brake fluid back a couple of inches to push that valve open.
Once fluid flows freely and the air bubbles have stopped appearing, close the bleeder screws, then "irritate" the brake pedal a little by hand. It's enough to push the pedal just an inch or two a few times. Any little air bubbles close to the caliper will get pushed into them where they will pop out when you open the bleeder screw once more for a couple of seconds. Any air bubbles that might be close to the master cylinder will wash back into the reservoir.
Friday, March 19th, 2021 AT 10:32 AM
