brakes to the floor

hello everyone i am having some brake issues right now . on several occasions at low rpms when slowing normally like for stop signs or rr tracks the brakes will not grab and pedal is very soft goes all the way to the floor. kinda scary but i manage to stop without troubles so far. i have power disc on front 7 inch booster and drums on back. i have a 67 coronet with 440 auto and it does not have a radical cam and i think it might be stock or just the first or next one up. i didn't put car together. i do not know what vacuum my motor is pulling but i checked the lines and fittings dont hear any leak . thinking of getting reserve v. canister to increase volume of air. any help will be nice thank you all.

I have a few delivery vans (vw) and every time we have the problem you have is the brake master cilynder no leaks no vacuum problems , the o rings inside the master cylinder just don't hold the pedal pressure and goes all the way down.

I agree , sounds like a bad master. if you quickly pump the brake if it improves right away its a bad master

Yep, master cylinder.....buy one, bleed it, & bleed the brakes all the way around with the little proportioning valve "pin" held out...done. It will take you longer to bleed the brakes than replace the master cylinder, maybe 2 hrs. total.

If power disc brakes isn't OEM (?), the watch out to get the correct master cylinder, even if they are stock. Engine side brake lines, fender side brake lines + wide/fat vs. tall skinny.

And it wouldn't hurt to make sure those rear drums are adjusted properly.