When I press down and hold the brake pedal it will slowly go to the floor even if I pump the pedal first. The brakes will not grab enough to lock up the wheels when I'm driving. There are no leaks in the system. The master cylinder is near new and was bench bled. One possible issue is that I have a fourth gen disk rear but the MC and prop valve are for a rear drum car, though I don't forsee that causing the problem I'm having. Any suggestions? Maybe the MC wasn't bench bled completely?

 

The three most common problems for this are
1. leaking piston seal
2. leaking master cylinder
3. leaking brake line or connection.

Here are others that could also cause your problem.

1. problem with metering valve.
2. restricted brake fluid passage way.
3. problem with proportioning valve.
4. problem with power brake booster.