I need some help. I have a 71 Chevelle with Power 4 wheel drum brakes. My problem is that all of a sudden my brake pedal goes to the floor. If I press hard it will grab, but if I hold it, it slowly goes down. If I release it completly it will grab again and do the same thing. If I only release it a little and press again it will go to the floor quickly until it is bottomed out. I have bleed the brakes and am getting straight fluid at a all 4 wheels. I have not replaced any components in the brake system recently. What could it be? Help.

master cylinder has gone bad

I think that is it. I took a closer look and sure enough I am leaking fluid where the MC mates up to the booster.

Now another question. When I took off the old MC The hole the push rod from the power booster goes into is about 1.5 to 2 inches deep. There is virtually no hole for the rod to go into on the new MC. Can I get a shorter rod? I tried to get the same MC, but nobody has it.

They're are two size rods. You need the short one...

It is fixed!! Did some searching on Partsamerica.com and found the correct master cylinder. Thanks for all the help. Did I mention I hate bleeding brakes?