1970 Chysler Feels Like It Has Anti-Lock Brakes

My 1970 Chrysler 300 with power drum brakes feels as though it has "ABS"! I have owned this car for thirteen years, and completely rebuilt the brakes five years ago (even the steel lines) and it has always stopped on a dime. Then, this year, the problems started: the pedal now has less than an inch of travel from "all the way up", and there are several inches of space left beneath the pedal when it "bottoms" (with the master cylinder removed the car, the pedal WILL go all the way to the floor). The brakes slow the car, but it eventually "glides" to a stop; you couldn't lock it up if you tried! In the past, the brakes would lock up if you just LOOKED at the brake pedal!

When I first drove it this spring, it felt just as I described. I replaced the master cylinder with a new one because the rebuilt cylinder was leaking; the leak stopped, but there was no change in the way the car stops. I then replace the original booster (also rebuilt five years ago) with one of a different type. Same problem. BTW, I run DOT 3 fluid, not that DOT 5 stuff that WILL make any car with power brakes feel as though it has no brakes.

Any ideas about where else to start looking for the problem?

Sounds like your problem revolves around the booster and or vacuum.

Make sure you have direct manifold vacuum, (minimum 17 inches), next make sure your check valve is working correctly.

To check your booster, with the engine off pump your brake pedal about 10 times, then holding the brake pedal down, start the engine the pedal should go down about 1/2 inch more.

You say you replaced the booster with a different type. The adjustment on the booster rod is critical, if not adjusted properly you could either be (rod too long)riding the brakes or if to short you would not have the vacuum assist.

Vic,

Thanks for the reply. I haven't checked the vacuum reading, but the boost feels quite adequate. At low speeds in my gravel drive, the brakes are quite sensitive and will lock up -- just as they should on a big Mopar. But out on the paved road, at speeds above maybe 15 mph, the brake pedal seems to have a very definite "bottom". Still, I will indeed try the "pump-ten-times-and-crank-it" test which you suggest.

I have had two master cylinders, two boosters and two check valves on this car, and it acts exactly the same each time. This problem started after about four months of storage, so I wonder if it is a function of dormancy.

Jeff:

Let me know the results of the vacuum and 10 pump test.

Vic

Interesting comment about using dot 5 brake fluid - I have been using it ( silicon fluid - dot 5 ) for about 10 years in my 1962 Chrysler 300 and there has been no difference in stopping quality- in fact very light pedal pressure is required on a normal stop with the original single diaphragm booster. The big difference is that dot.5 fluid does not absorb water which was the problem I had when I left the car garaged and unused during winter months, and the wheel cylinders would rust up and the brakes would drag. No more problems of that type with dot 5.