What is your year, make, engine, etc? Are you getting a brake warning light on your instrument panel? Does it take alot of pedal pressure to get it to bottom out?
The check/combination valves could be faulty or clogged with debris, causing them to not close & seal properly.
Steady pressure on the pedal that causes the it to slowly sink to the floor indicates a system leak.
Also, if the system hasn't been flushed in a while, do so. Brake fluid is hygroscopic, absorbing water over time. You can bleed it many times, but if it is still weak or spongy, then the right procedure isn't being followed or there is air in the ABS system.
Autozone has a check procedure for the brake booster:
Vacuum Leak Test
- Operate the engine at idle without touching the brake pedal for at least one minute.
- Turn off the engine and wait one minute.
- Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.
System Operation Test
- With the engine OFF , pump the brake pedal until the supply vacuum is entirely gone.
- Put light, steady pressure on the brake pedal.
- Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if the constant pressure is maintained.
Good luck!