Braking works but gets Super strong

There are a few things to look at. The most likely suspect is a rear parking brake cable that is rusted in the partially-applied position. Remove the drum, then look at the two shoes where they hit the large anchor pin at the top. If the parking brake cable is not fully released, one of the shoes may not be hitting that pin. Even if both of them are resting on the anchor, look at the parking brake strut bar between the middle of the two shoes. You should be able to push that forward 1/8" against the anti-rattle spring pressure, with your thumb. If there is no free play, the parking brake is partially applied. That will hold the front shoe out and make it grab the drum very aggressively under light brake pedal pressure.

Compare the length of the linings on the front and rear shoes on each rear wheel. The front one, (toward the front of the car), must be shorter than the the rear lining. If they are reversed, that can also cause that brake to apply harder than it is supposed to. The shorter front lining is only supposed to try to grab the drum and rotate with it. Doing so pushes on the lower link, the adjuster in this case, which pushes on the bottom of the rear shoe to apply it. The rear piston in the wheel cylinder pushes on the top of the rear shoe. This is called a "duo-servo" brake because two forces are acting on the rear shoe. The rear one is longer because it is the one that does most of the stopping.

Rear gear lube leaking onto the rear shoes will make the shoes grab real hard. Once they have been hot from normal braking, there is no way to get that grease out. The shoes and drum have to be replaced. Both are porous, and that grease will soak into the cast iron drum, then leach out later to recontaminate the new shoes.

Same goes for axle / bearing grease. There cannot be any contamination on the friction surfaces. If there is, it can be washed off with brake parts cleaner as long as the parts have not gone through their first heat cycle yet. Professionals even wash their hands first with soap and water to prevent getting fingerprint grease on brake parts.

A drum brake can apply too hard if the drum was not machined or replaced. No two linings ever ride in exactly the same place, so the new one could hit a rust ridge on either side of the drum's friction surface.

There are six "lands", or raised spots, on the backing plate that the shoes slide on. Those must be lubricated with a special high-temperature brake grease. GM has a design issue with their shoes where just a thin edge of the frames slide on the backing plate. That promotes the formation of grooves in those lands. Those can cause a shoe to stick and not apply under light pedal pressure, and it can cause a shoe to not release right away. Other manufacturers have bent-over tabs on their shoes to distribute the wear. The grease is still used to prevent an annoying squeak when the shoes retract. On GM vehicles it is also important to reduce the formation of those grooves.

As long as we are discussing brake jobs, if an old drum is machined, look at the hub it mounts on and you will see at least one access hole, and sometimes three. Water can get through those holes and form a circle of rust on the inside of the drum's mounting face. Those must be scraped off before a drum is machined so it sits squarely on the brake lathe, to prevent machining a warp into it. When a used drum isn't machined, if it is reinstalled in a different orientation, those rust spots will be trapped between the hub and drum and prevent the drum from sitting squarely. You may feel that as a vibration when braking.

Be aware too that a real lot of research goes into making the brake system balanced front-to-rear. Any time a friction part is replaced or machined, the linings will not make one hundred percent contact until they have as much as one hundred miles of use. Until then, you have to apply harder pressure to the brake pedal to stop the car, and that will cause the linings to get hotter than normal, leading to one type of brake fade. Once they cool down they will be fine, but you can have one brake working more effectively than the rest making it appear to be locking up. That problem would have started after you replaced parts, not before.

What have you found out JBUC?

I don't see anything obvious. There's only two other things I can think of. The first is since you replaced the wheel cylinders, did you buy the correct diameter? Did you look at one of the old ones or go by what the person looked up at the auto parts store? On the old one the diameter is molded onto the inside of each rubber lip seal.

The other thing is if there's a height-sensing proportioning valve. Those are used at the rear axle on most pickup trucks and minivans because there can be such a wide variation in weight distribution based on loading and number of passengers. As the rear is loaded more heavily, more braking power goes to the rear brakes. Reducing braking power to the rear when lightly loaded reduces the tendency for the rear brakes to lock up during hard braking.

Cars don't normally use a height-sensing proportioning valve, but you should check to be sure. In the late '80s, Chrysler minivans had an irritating characteristic where the rear brakes would lock up very easily when the road was wet. That was due to slightly sagged rear springs making it falsely appear that the rear was more heavily-loaded. The adjustment for that valve took less than ten seconds, and I carried a 5/16" wrench with me on all my alignment test-drives in case I ran into one doing that.

The springs on all vehicles sag with age, and if your car has this valve, it is almost surely the cause of this problem. The valve is almost always above the left rear axle, bolted to the body, with a linkage connected to the axle housing.

This isn't a symptom I run into very often, other than the sticking parking brake cables, so I don't know if the proportioning valve will cause this. If you have it in the combination valve under the master cylinder, you likely don't have a height-sensing valve in the rear. That combination valve also has a metering valve, aka "hold-off" valve that delays the application of the front disc brakes to give the rear shoes time to expand and contact the drums. That makes all four brakes apply at the same time. On some older Fords, that valve had to be manually held open to gravity-bleed the front brakes. There was a small tool made for that, and if you'd forget it and leave it on the car, the front brakes would apply too soon and they'd apply too aggressively under very light pedal pressure.

I can share an unusual story related to wheel cylinder diameters. I worked at a Sears Auto Center in the '80s, specializing in Brakes and Suspension and Alignment. Most of our brake jobs included rebuilding wheel cylinders and calipers. I liked to team up with a friend because we had similar work habits. Also, rebuilding one front brake was okay, but the second one got boring, so we would each do one front and one rear. On one Chevy in particular, our brake kits included the correct shoes to cover a number of vehicles, and three or four sizes of wheel cylinder seal and boot kits. You were supposed to use the kit that fit your car and throw the rest away.

Once I had the wheel cylinder honed out and washed, I grabbed the 7/8" kit to replace the seals and boots, but I noticed my friend had already opened the 15/16" bag. Upon closer inspection, we found out there were two different size wheel cylinders on the rear, ... From the factory. The owner said the car had a brake pull to the right ever since it was new. This was its second rear brake job. No one caught the difference up to that point. I don't remember, but I suspect the wheel cylinders weren't rebuilt during the first brake job.

Goes to show that 1/16" difference on a big, heavy car can cause a brake pull, so you can see how much effect wheel cylinder diameter can make.

Let me share another story. Front disc brakes are expected to need to be replaced twice as often as rear shoes. Dodge had a problem with their trucks in the mid '90s where the fronts were wearing out four-to-one, ... Much too quickly. This was only on those that regularly carried a heavy load. There was a factory service bulletin to address this issue. It called for replacing the wheel cylinders with a pair that were 1/16" larger in diameter. In most cases that solved the problem by making the rear brakes work harder. If you had a truck that was always fully-loaded, (we had two that were seriously overloaded all the time), you could go a full 1/8" larger in diameter, but the clinker was that size wasn't available from Chrysler. We had to buy them from the Chevy dealer. This problem was mainly on the 3500 diesels because of the higher weight on the front end.

Dandy. Lets hope this solves the problem.

Dandy. Come back again when you need my wondrous wisdom!