Brake Pedal going to the floor on 1974 Dodge Charger SE

[rob1684]

I have a 1974 Dodge Charger SE, power brakes, front disc and rear drum brakes. The problem I'm having is my pedal will occasionally go almost to the floor and then I let up and press it again and it's feels normal. This usually happens when I am not pushing on the gas pedal and coasting, whether it be letting up early while approaching a stop light or backing out of garage when you are backing out or pulling into garage. I have removed rear wheels and drums and manually adjusted the  brakes to where they just barely touch when reinstalling the drums. This has made no difference. I have 20 inches of vacuum at idle so it doesn't appear to be a vacuum leak.  What am I missing? I have checked with several car buddies that are very knowledgeable and no one has been able to come up with a answer. HELP please.    PS I have owned this car for 47 of its 50 years its not a new acquisition.

thanks Rob Black

[LaOtto70Charger]

I've got a 70 that did the same thing. Mine the pedal would stay firm for a couple miles. Than it would go back to 3 pumps to get firm again. Originally all drum brakes. I changed the front to ssbc disc's and it still did it. My drum brake booster went out and converted to manual and doesn't seem to be as bad. I have a new dual diaphragm booster to go in but haven't done it yet. I don't think it was the booster though I just don't think it is as noticeable. Plus I had replaced the drum booster in the past with no difference. Had it 31 years to date.

[b5blue]

Check the pushrod that enters the master to be certain it just touches. (The correct length from booster to master.) A small over shoot can prevent proper reset.

[Back N Black]

Sounds like air in the system, did you have the lines open recently, changing a caliper or line?

[John_Kunkel]

I have a 1974 Dodge Charger SE, power brakes, front disc and rear drum brakes. The problem I'm having is my pedal will occasionally go almost to the floor and then I let up and press it again and it's feels normal.

Internal failure of one system in the master cylinder...should also illuminate the BRAKE warning light. Sometimes they fail and never recover, other times the failure is intermittent. Replace the MC.

[rob1684]

Thanks for the replies, all are appreciated. Just took my Charger to the body shop today for some paint touchup and will investigate all your suggestions when I get it back. I was asked if I had done any recent brake work. The most recent was 2013 when I replaced rotors, pads and brake lines and all was fine afterwards.

If anyone else wants to contribute a suggestion for me....post away!

[Kern Dog]

It is a bad master cylinder, it is as simple as that.

[375instroke]

I've had the same problem.  Randomly goes to the floor, and pumps right up.  First with power disc brakes.  Converted to manual discs and new master.  Same.  Replaced master.  Same.  Three bad masters in a row?  Same with alternators.  Right out of the box, I can see the diodes are cracked.  Three in a row right out of the box.  Can anything be made today that's at least half way decent?