I own a 1972 Datsun 510, that I just changed the front disc-brake pads on and then changed the rear master cylinders in both drums. The front brakes worked well before I changed the rear cyliders then they stopped working. When I push on the brake pedal the car slows down lightly, if I pump them twice the rear will lock up and the front still dont work. If anybody could help me I appreciate it a lot. Thanks.

It sounds like the brake system is either not bled properly or you might have a defective master cylinder.

Try bleeding your brake system first. If this doesn't cure the problem, then you may have to replace the master cylinder and/or brake booster.

Good luck,

I had this same problem with a 1968 Chevy Impala that I used to have, what it turned out to be was that I had to adjust the rear brake shoes into I should hear the pads scraping against the brake drums.

But considering it's been over a month since you last posting you have probably already figured it out.