Rear brake piston pops out

The travel on the pistons are designed to hit the end of their travel when the shoes are at their minimum. So when you run all the brake material off the shoe and then the metal on the shoe wears away the drum the travel increases because the drum diameter has gotten bigger. Make sense? Because the diameter got bigger the piston falls out.

You should be fine when you replace the drums and shoes.

Rich

The first thing to suspect is the drum is worn beyond the legal limit, especially if it was grinding. There is always a published maximum diameter the drum can be machined to, and another slightly larger diameter it can be allowed to wear to. Those are called the "machine to" spec. And the "discard" spec. You'll need a special drum micrometer to measure them accurately. Don't waste your money on buying one. Every auto parts store that offers drum and rotor machining will have one and they will check your drums for free. No reputable mechanic will machine a drum or rotor beyond the machine-to spec to try to save it. That can leave them open to a lawsuit or liability issues. For that reason, all independent repair shops that offer brake service will have this gauge too.

The second chart below is just a blown-up copy to make it easier to read. While it isn't listed this way, drums and rotors are measured to the thousandth of an inch. Yours started out at 9.050" when they were new, and they can be machined to 9.080", which is only 0.015" per side. That is a fairly standard amount although many larger drums can be machined quite a bit more. Your drums can be allowed to wear another 0.010" to a total of 9.090". Once they get bigger in diameter than that, the shoes' diameter will prevent them from making full contact until the center of the lining wears down a lot. That leaves you with a front-to-rear brake balance considerably different than what was carefully designed in.

It takes a lot more wear on the drum to allow a piston to pop out of the wheel cylinder. A better suspect is the shoe frame is moving out far enough to let it drop down into the "labyrinth seal". That's the curved groove around the outer edge of the backing plate. The drum has a matching groove. When those are assembled, they form somewhat of a seal that prevents water from splashing in.

Look at where the piston is sitting in relation to the shoe. If the piston is away from the backing plate, meaning you can see it in front of the end of the shoe it's supposed to push on, the shoe has dropped into the groove and is allowing the piston to bypass it. If the piston is behind the shoe, the better suspects are worn shoe hold-down hardware or weak shoe return springs. All of those springs and hardware are available as a complete kit for the entire axle.

Your comment about the old shoes burning up concerns me. There's two things I would be looking for. The first is if a parking brake cable is stuck in the partially-applied position. Given the age of the vehicle, if you live in a northern climate like I do where they throw a pound of salt on an ounce of snow, rusted cables are pretty much a certainty unless they've been replaced previously. A stuck cable will also prevent the self-adjuster from operating. Over time, instead of adjusting closer to the drum, the shoes will have to move out with a longer stroke to reach the drum. With sufficient wheel cylinder wear, that can allow the piston to twist enough for the shoe to slip off the end. Once that happens, there's nothing to hold the piston from being pumped out when you press the brake pedal.

My worse fear is when the brake fluid is contaminated with a petroleum product such as engine oil, transmission fluid, power steering fluid, penetrating oil, or axle grease. Those will cause all rubber parts that contact the brake fluid to swell and become soft and mushy. The first thing that usually happens is the rubber lip seals in the master cylinder grow past the fluid return ports, then the brake fluid is trapped and can't release back into the reservoir. As the disc brake pads heat up from staying applied, the heat migrates into the brake fluid. The fluid expands, and since it can't get back to the reservoir, it applies the brakes harder and harder, and they get hotter and hotter. We customarily see this with overheating front brakes, and not so much with rear drum brakes, but it can affect both axles. I think you would have noticed other problems or symptoms before now.

Your self-adjuster works by spreading the tops of the shoes further apart as the linings wear. With this design, the pistons work their way out of the wheel cylinders, and that leaves them prone to problems caused by overly-worn drums and linings. If the new drums and shoes don't solve this, try to post some photos, then we'll see if we can figure out what is happening.

Be sure to use a click-type torque wrench on the lug nuts. Torque them all to 46 foot-pounds first, then to 92 foot-pounds. Please keep us updated on your progress.

First it is not the boot quality that is the issue. Brake fluid rapidly swells and destroys rubber. This rubber is a dust shield and was not designed to be wet with brake fluid so this one wasn't an AcDelco issue.

Next, Both adjusters work if you are under the car looking at the dust plate you want to rotate it top to bottom or in a down motion to tighten them. I personally do not recommend putting the drums on and adjusting them as it is too easy to over adjust them. If you adjust them and slide the drum on after it is much safer. To do it this way you want to keep adjusting them a bit at a time and keep sliding the drum on to check them. Keep doing it until the shoes drag on the drum as you put it on and once it all the way on you should feel it dragging just a bit if you rotate the drum. If the drum goes on stuff and then spins freely once on then you only need 2 to 3 clicks of adjustment from the back. You will have to do this when a groove is worn in the drum on the next set of shoes. With a new drum no back adjustment is required as there is no lip on the drum. You generally want to do it this way all the time anyway cause its much faster to spin the adjuster with your fingers than to do it from the back.

If the lever does not reach the adjuster then the adjuster is in backwards.

Rich

There is also a gauge for adjusting the shoes, but the method just described by 94 TRANSAM is much better. With the gauge, you adjust it out to match the drum diameter, then you use the other side of the pair of fingers to set the shoes. I've never had good results with that tool.

Be aware that GM is one of the manufacturers that makes most of their pwn parts, as high as 80 percent, and they buy the rest. I would be suspicious that they buy their wheel cylinders since there are companies that specialize in that and do a perfectly fine job. With those dandy resources, it doesn't pay for GM to invest in making their own wheel cylinders. When you buy an aftermarket wheel cylinder or one from the dealer, there's a real good chance they both came from the same factory, just with different names on the boxes.

This is also true of Chrysler fuel pumps. When you buy an aftermarket replacement from Napa, it is a reboxed new part that came from the same supplier that sold them to Chrysler.

Given the unusual nature of the problem, it might be time to look for uncommon failures. For example, your car uses a fixed anchor on the bottom for the brake shoes. Could that be broken loose? Also, GM had a big problem in the '70s and '80s with wheel cylinders that just set in an hour-glass-shaped hole in the backing plate, then it was held on with a snap ring on the backside. Those holes commonly rusted out, then the wheel cylinder could rotate, the pistons popped out, and the brake pedal went to the floor. There were redesigned backing plates available, and you had to install the regular wheel cylinder with the two mounting bolts. That required removing the axle shaft.

Also related to GM, back in the '80s, I worked at a Sears Auto Center and I had one coworker who had the same work habits as I did, so we often teamed up on jobs. Both of us enjoyed rebuilding the first brake caliper, but we both got bored when doing the second one. Instead, we each worked on one side of the car, each rebuilding one caliper and one wheel cylinder.

Our brake kits came with shoes, all the hold-down hardware, return springs, and all the seals and dust boots to fit every possible size of caliper and wheel cylinder those models could have come with. One of the cars we worked on was a mid '70s Pontiac station wagon. The owner said the car had a left-hand brake pull since it was new, and the brakes had been replaced two times already, and now, due to high mileage, it was ready for the third service.

I rebuilt the left front caliper and assembled that brake, then moved to the left rear. To rebuild the wheel cylinder, the diameter of the lip seal is molded on it, so it was easy to select the right ones from the four sizes included in the kit. All of a sudden my coworker hollered to me that I used the wrong size from the kit. Turns out his rear wheel cylinder was 1/16" diameter different than mine, and that caused the brake pull for all those years. No one had figured that out since the car was new.

That's a perfect case of where someone grabbed the wrong part on the assembly line. Finding the shoe adjuster on one wheel is for the other side is not unheard of.

Take them back. Don't accept damaged parts. As one manufacturer's trainer used to tell us, "we not only sell you new parts, we sell them to you pre-broken". Supposedly that was to save time, so you didn't have to wait for them to wear out again.

It doesn't cost any less to make rubber parts out of the wrong compounds or additives, so there's no reason for a manufacturer to intentionally use the wrong material. Part of the problem, as 94 TRANSAM mentioned, is any rubber part that contacts brake fluid has to be compatible with that fluid, and those parts will absolutely not be compatible with any type of petroleum product. One drop of engine oil, for example, in the master cylinder, or penetrating oil sneaking in when trying to loosen a rusted brake line nut, can contaminate the entire hydraulic system to include every rubber seal, o-rings, and flex hose.

Wheel cylinder dust boots, on the other hand, live where they are going to come in contact with axle grease or gear lube, and as such, they have to be compatible with those and other petroleum products. It's normal to see a little of what looks like brake fluid seepage, even with brand new wheel cylinders, but that is Brake Assembly Lube and is considered normal. That doesn't swell the dust boots, but it doesn't have all the characteristics of brake fluid either.

Regardless, dust boots are designed to do what their name suggests, ... Keep out dust. Most of us have seen Ford Escorts with the pistons rusted tight in the wheel cylinders, yet the dust boots were in perfect condition. It's also common to see mushy dust boots that can't be doing much good, yet the wheel cylinders are still working just fine. I consider the new pistons and seals, and the rust-free wheel cylinder to be the important parts I'm replacing, and for my own vehicles, I'd reuse the old dust boots if I had to rather than make a special trip back to the store. That's a different issue when the customer is paying for those new parts and they aren't exactly getting what they paid for. That's when we get on the phone to the store that supplied the parts to us.

I don't think so. We hit them with Brake Parts Cleaner all the time, and we use special high-temperature brake grease on the six raised surfaces the shoe frames ride on. Gear lube doesn't hurt them. In fact, I don't know of any common chemical that will deteriorate them.

I used to do demonstrations for my students to show what happens when wheel cylinder lip seals get high with one drop of engine oil, but I never tried that with the dust boots.

Brake fluid will rapidly eat the type of rubber those are made of. I you are having issues with the pistons falling out it is very possible the pistons were on the edge and leaking for a while and given those dust seals are a tight fit at both ends they may have been full of fluid on the inside and you couldn, t even see it.

Rich

You are very welcome.

Rich

Be aware too that when you install new shoes with the old / resurfaced drum, that drum is going to be slightly larger in diameter, so the linings will only make contact in their center area. For the first few hundred miles until they wear down to match the drum, you won't have the normal amount of braking power in the rear that was designed in. The engineers designed them to have a carefully-balanced brake system, front-to-rear.

It is customary on the test-drive to hit the brakes pretty hard a few times to help speed up that matching process so the customer doesn't have to do that. It is also okay to have that slight drag on the drum when the shoes are adjusted up.

When you have these new drums and shoes, that drag isn't necessary because the shoes will match the drums in diameter. Even if you leave them with a 1/16" gap, you won't notice that in how far the brake pedal travels.

Sounds like you solved this. Please come back to see us again.

You should have no problem leaving that adjuster out. All it will do is cause a delay for that side when the brake is applied because it will have to travel further not being adjusted out like the other.

Rich

Dandy. Please keep us updated on your progress.