i started to have this problem so i just thought, simple fix, replace master cylinder. wrong. didn't do a thing. my brakes do work better (other than pedal travel) because i unstuck a caliper piston and unstuck both wheel cylinders. i do have the drums adjusted so that it takes plenty of effort to turn the tire when its off the ground.the brakes work allot better than when i had it adjusted with little resistance. i bled the master cylinder and bled the system twice went through 70 ounces of brake fluid. i bled back to front to lspv. i also replaced the brake booster. after all this the brake pedal still goes near to the floor if i press it slowly. could this be the symptom of a bad lspv ? im out of ideas.

 

It sounds like your system is not holding pressure. There are a few causes: air trapped in the system, an external leak, an internal leak (master cylinder seals letting fluid pass through the pistons), misadjustment of brakes (usually rear drum brakes).

Did you bench bleed the master cylinder? Sometimes air can get trapped inside one of the bores, and a manual bench bleeding will allow the air to pass through and allow brake fluid to take the air's place.

How confident do you feel the new master cylinder is good?

 

All so closely inspect all the rubber brake lines. They could be ballooning when you step on the brakes. What brand master did you install?