Had a local Mopar guy meet me this morning to show me an issue he was having with the drum brakes on his 70 GTX. It is a power drum car with 11" drums. I drove the car and it is scary! When you apply the brakes the brakes @ about 1/4 of a pedal they will grab and about throw you through the windshield. He has changed the brake lines, hoses, wheel cylinders, distribution block, master cylinder, booster, shoes and had the drums turned. Feels to me to be a friction issue with glazed shoes or overheated drums. Anybody ever experience this or have any suggestions?

Sounds like he has the shoes mixed up?

check pushrod rounded nub clearance (want ~.020") then do a visual on the linings: take the drums off & have a helper CAREFULLY SLOWLY press the pedal then jab the pedal (barely) to see if there is any hanging up on the shoes & you see if there is anything not right/hanging up (I think you are OK there) just be carefull to not let him move the wheel cyl cups too far. if all good so far then it about has to be a booster/check valve issue (did he reuse the same check valve?) EDIT my bet is a booster/check valve/pushrod clearance issue, one of them

I did ask about primary and secondary shoe. He told me he had the longer shoe on the rear which I believe is correct. Looks like the check valve has been replaced. Sounds like the previous owner had the same issue. He also told me when he bought the car the left front drum got so hot it locked up and was smoking. That was before replacing all the parts that he has. He has thrown about every replacement part at it trying to fix the issue. The only thing not replaced were the drums and they were turned. He adjusted the brakes kind of loose to make it somewhat drivable.

Brake booster out of adjustment?

Sounds like a pedal or master problem...your saying its applying full pressure on only 1/4 pedal travel to lock it up..

He said it was as soon as you hit the pedal and the only reason it is @ 1/4 pedal now is because he backed off the brake adjustment to make it somewhat drivable. What booster should this car have? The one on my 69 GTX is the larger bendix unit but I have disc on my car.

in the op it says manual drum car. not power drum.

If it is power drum, there is an internal valve that failes some times.
It is supposed to gradually open, but can sometimes stick and then fully open. If tht happens you get a sudden burst of assist.
I have had one that did that and replaced it.

Sorry it is power drum. Said he had booster rebuilt but it was the one on the car when he bought it. I believe he said he replaced the check valve too. The pedal is easy to push throughout its travel. The reason I asked about the correct boosters is I don't know if the pivot linkage that is attached to the pedal is the same for both.

it isn't the check valve. that just holds vacuum inside the booster.

there is an internal valve that allows vacuum to pass to start the assist.
that valve for lack of a better word gets sticky. It will hold shut then instantly pop open allowing more assist than anticipated.

you can narrow it down to the booster by removing the vacuum assit and you can see if it still does it.
plug the vacuum line to the booster and pump it till the pedal is hard.

then try it and see if you still have an issue.

I will pass that on. That will at least eliminate a vacuum issue with the booster.

so the car had this problem before all the parts were changed out..some cars has 2 spots to hook up the rod on the brake pedal...then some have a z linkage also..I would try to compare to another same car

I did look @ the brake pedal and there is a z linkage. It was that way when he got it but not sure if it is correct. My 69 GTX has a z linkage and a Bendix booster but has disc brakes. His is a 70 and has a z linkage and a different booster but has 4 wheel drum brakes. Does anyone have a 70 GTX, Charger or Coronet R/T with power drum brakes to compare to?

have you tried it in reverse? do the brakes react the same? that would help eliminate brake shoe issues from the goose chase.