Hi James. That's a lot of detail to follow, so let me start out with a few suggestions to get you started. GM uses an unusual design for the master cylinders on their front-wheel-drive cars. They incorporate a valve assembly to block two ports if a leak develops. You likely tripped the valve by performing normal bench-bleeding procedures. Once the valve trips, you will not get brake fluid to flow at one front brake and the opposite rear one. You may be told you can avoid this by bleeding the brakes in a specific order, typically starting with the right rear, then the left rear, etc. You may even read that in some service manuals, but it doesn't make sense. Suppose the metal line to the right rear brake rusts out and starts leaking. The procedure implies the valve won't trip, but that is not the case. For all vehicles, I always start bleeding at the wheel I'm standing closest to. You will not prevent the valve from tripping from anything to do with which wheel you start with.
When you do bench-bleeding, and if you use a helper to do pedal-bleeding on the car, never push the master cylinder over halfway down. That should prevent the valve from tripping, but there's one more important point that applies to all car brands and models. Once a master cylinder gets to be roughly a year old or more, crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal to the floor runs the pistons, and their rubber lip seals over that crud and can rip them. That results in a slowly-sinking brake pedal, and that often takes two or three days to show up. This corrosion doesn't apply to a master cylinder less than about a year old, but we still have to consider that valve in GM units.
It is not necessary to replace the master cylinder when that valve trips. The only way I have found to reset it is to go to one of the wheels where fluid is not flowing, open the bleeder screw, then give it a quick, short squirt of compressed air. The cover should be loosened on the brake fluid reservoir too. You don't have to get carried away. I just slap the handle on the air nozzle with the side of my hand, (like I'm doing a light karate chop). After that, let that circuit gravity-bleed. The air will be expelled in less than half a minute.
When you have a problem with brakes that are dragging and smoking, especially right after a recent service, it is due to brake fluid being trapped under pressure and it is not able to return freely to the reservoir. The goal then is to get the brakes to lock or drag, stop on a slight incline, place a block about a foot downhill of one tire, shift to neutral, then open the hydraulic system in various places to determine where the fluid is being blocked. I like to start with loosening the lines right at the master cylinder. They're easy to get to, and effectively open each circuit right in the middle. If opening those fittings doesn't let the fluid release, go underneath and open the bleeder screws on the front calipers. If that works, the first thing to consider is whether there is a metal anchor bracket crimped around the rubber flex hose. Rust can form inside that crimp and squeeze the hose closed. You'll be able to force brake fluid through that restriction with your foot on the pedal, but the fluid won't be able to release until the bleeder screw is opened. This is common on Chrysler products, but the crimped bracket is pretty easy to open up with a large flat-blade screwdriver or with a pair of large pliers. The same thing can happen on Toyotas and other imports, but they use a different design that requires the hose to be replaced. To my knowledge, this is not a common GM problem.
When opening the bleeder screws doesn't let the brakes release, it's not due to trapped fluid under pressure. Look for a mechanical problem such as debris behind the piston in the caliper, (especially common right after pressing the piston in to make room for new, thicker brake pads), or the caliper is sticking on its slides or mounting bolts. That will keep the outer pad applied. Calipers on slides should have those contact points lubricated with special high-temperature brake grease after cleaning off any rust, dirt, or high spots with a file. Mounting bolts should also be lubricated only after they've been inspected for rust or lifting chrome plating. Mounting bolts with rust must be replaced. Once the chrome plating has lifted, the rust will come back and cause the same sticking problem.
If the brakes do release when you loosen lines at the master cylinder is when I get worried. One of three things has happened. The least serious is the brake light switch is out of adjustment and is holding the brake pedal down a little. The second suspect is the push rod is misadjusted on the front of the power booster. This is more of an import thing. While they are adjustable, that is not something we are supposed to adjust. It's done by the rebuilder or on the assembly line and shouldn't be touched after that. The push rods on most domestic vehicles are not adjustable, but to see if either of these causes is the problem, when the brakes are locked, loosen the two mounting nuts holding the master cylinder to the booster. Let the master cylinder slide forward about 1/8th inch. If the brakes release, look at the brake light switch or the booster.
The most serious case is where the brakes do not release when moving the master cylinder forward on the booster, and when they do release when one or two lines are loosened. That means the fluid is being trapped from flowing back up into the reservoir even though the pistons in the master cylinder are fully released. That is caused by the brake fluid being contaminated with a petroleum product such as engine oil, transmission fluid, power steering fluid, penetrating oil, or wheel bearing grease. Petroleum products cause rubber parts to swell and become mushy. In the master cylinder, the rubber lip seals grow past the fluid return ports and block them. The only way to get the trapped fluid to release is to open the line anywhere between the line at the master cylinder, and the bleeder screw. This gets to be a very expensive repair. All parts containing rubber parts that contact the brake fluid must be replaced, otherwise the contamination from one part will leach out and recontaminate the new fluid and new parts. This means replacing the front calipers, rear wheel cylinders or calipers, four rubber flex hoses, master cylinder, rubber bladder seal under the reservoir cap, and the combination valve. Follow the lines from the master cylinder to the combination valve, usually right under the master cylinder on a frame rail. If steel braided rubber hoses are used coming off the master cylinder, those must be replaced too. If the car has anti-lock brakes, the hydraulic controller must also be replaced as it has rubber O-rings and gaskets inside. Those parts must all be removed at the same time, then the steel lines must be flushed with Brake Parts Cleaner, then dried, then the new rubber parts can be installed. Due to the wide variation in loading on pickup trucks and minivans, they usually also have a height-sensing proportioning valve near the left rear wheel. Those have rubber O-rings, so that valve must be replaced too.
The power booster shouldn't be involved in any of this, including the low brake pedal or when the brakes are locking. All it does it makes the brake pedal easier to push. If you do replace the booster, a professionally-rebuilt unit will come with a plastic cone in front, around the push rod. Don't remove that until after the booster is bolted to the car and you're ready to bolt on the master cylinder. If you get a used booster from a salvage yard, it is important to not let it lay flat, either when carrying it or when transporting it. GM uses an odd double diaphragm design that uses a valve that will fall out of position if the booster is transported flat. The typical mechanic or driver won't notice anything unusual, but the owner of the car who is familiar with how it feels will know right away the power assist is not normal.
See how far you get with these ideas, then let me know what you find.
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Monday, March 25th, 2024 AT 4:18 PM
