How's the booster? Got lots of vacuum? When the master cylinder leaks out the back some brake fluid ends up in the booster. It usually wrecks the rubber bladder inside sooner or later. The end result is a firm pedal with no assist. Just an idea.

HOw do I tell if the booster has enough vacuum? It doesn't feel the way manual brakes feel...

A lot of the early 80 and 90 vehicles had proportioning
valves that were 50 x 50. This was because people complained of front pads wearing 3 to 1 to the back shoes.
If your vehical has this it is normal. The wheels will not
lock up. They don't do that any more because people complained of no brakes. Most all cars are front drive
and all the weight is on the front now.

In a large parking lot drive to 30 mph and jam on the brakes.
You should see 4 equal length black marks, the fronts may be longer..
The front pads may be really old and glazed over..

worm--that's the problems. NO black marks, no locking the wheels. I can stand on the brake pedal with both feet and still no skidding. A "panic" stop just becomes a lot more "panicier"...

If the front pads are good and not glazed over bleed the system again, there may still be a little aur un it. I had the same peoblem on my pickup a second bleeding cured it.

First you are correct about the fact that if you want to lock up the brakes you should be able to do that. The parking brake when you try it should be able to lock up the rear brakes and slide the wheels. So the question arrises, why can't you?

First check. Pump the brake pedal about five times and hold it, memorizing how much pressure your placing on the pedal, and how far from the floor the pedal is. (You can use your other foot for a gage if you want to). Now release the brake pedal and wait about ten seconds. Step on the pedal a single time, using the pressure that you think you did when you held the pedal a few moments earlier, and see how high the pedal is from the floor. Is it exactly in the same place? Is the pedal lower? Memorize the pressure your applying, and the height of the pedal, and quickly pump it three times, and see if the pedal height changes.

If the pedal ended up at the smae point each time, then your adjustment of the rear brakes is correct, and you don't have any air in the system. If the pedal height incresed when you pumped the brakes, then you could either have an adjustment situation, or still have air in the system.

Next step. Apply brake with same pressure as before. Without releasing the pedal, start the engine. Vacuum from the engine should cause the booster to begin to assist you in applying the brakes, and you will feel the pedal fall from your foot. Maintain the same pedal pressure, and see how much further the pedal travels. It takes some experience to feel when this assist is sufficient, but at least you can tell if you indeed have assist or not. A bad booster, or vacuum problem will only give you about an inch or less of pedal travel increase. You will need to investigate the cause if you don't get a good assist.

The next possibility requires an assistant. Put the car up on jacks and jackstands with all four wheels off the ground. Put the transmission in neutral. Pump the pedal three to five times to bleed off booster vacuum. Now have an assistant start to lightly press on the brake pedal while you turn the LF wheel by hand. Keep telling the assistant to increase pedal pressure slowly until you cannot turn the wheel with a moderate effort. have the assistant memorize the pedal travel at that point. Release the brake pedal, and repeat the process at the right front wheel. Now do it again for both of the rear wheels. The pair of front wheels should take the same pedal travel, and the rear wheels will each take slightly more than the front in most circumstances. All four should not need to have the pedal pushed more than half way before you cannot turn the wheel.

Having made all of these checks, and not making any errors, you are now faced with the likelyhood that you have glazed linings, or at least linings with poor friction characteristics. All of the linings, the rotors, and drums may need replaced on a vehicle this old. Remember to include the hardware for the calipers, as well as the brake shoe hold down, and return springs (Maxipack). Make sure that if you replace the pads and shoes, use top of the line components, thats the only way your going to make sure you have something that should perform correctly.

If all of this looks like a lot for doing something as "simple" as brakes, ""you aint seen nothing yet"". Lock-up charateristics, and point of lock-up are aspects that even as techs we normally do not have to concern ourselves with because normally the cars are engineered well enough that they almost always work. We only run into problems like that when we are dealing with improper materials, contaminated materials or the very rare, failed proportioning (or combination) valve. We actually have tools available that allow us to remove the bleeder screws, and directly measure the brake fluid pressure. Some shops have tools that fit in the caliper in place of the brake pads and rotors, in order to measure the systems ability to squeeze the pads and rotors. All in all it can be extremely complex especially when we have to deal with the opposite complaint of an early lock-up, or an antilock brake issue which essentially adds an entire system on top of everything else I have already mentioned here.

I hope this helps. If there is any doubt, take it to a real pro to have this checked.

The tests prescribed by John are much more exacting and scientific than jamming on the brakes at 30mph and checking out black marks. These are supposed to isolate any problem.
It seems as if we still have, for some reason, air in the system.
Go by what John has written..