If the pedal sinks slowly after initial resistance, the master cylinder is failing. It is leaking internally and pumping or sudden braking works but holding the pedal would result it it sinking gradually.
When the pedal sinks to floor and holds after pumping it, there could be air in system. Get the system bled and retest.
Is rear brakes disc or drum?
Do you have brake fluid losses?
Thursday, November 4th, 2010 AT 9:19 AM