Sounds like a cylinder issue.
You should be able to pump the brake pedal up hard when the engine is off.
Keep your foot on the pedal then start it and the pedal should sink a bit.
Also just googled this :
"When a brake master cylinder begins to go bad or fails, you will notice a soft or squishy feeling when you press down on the brake pedal. When constant pressure is maintained on the brake pedal (like when you keep your foot on it at a stop light), the brake pedal will begin to sink to the floor as the brake fluid leaks internally in the master cylinder. This is called "extended travel" when the brake pedal goes farther down than normal or than it was intended to. When this occurs you will have to "pump" the brake pedal to regain normal pressure and to keep the car from moving forward because the brakes are slowly releasing. The red brake warning dash light should come on to indicate low brake fluid, or excessive movement "travel" in the brake pedal.
You will not normally see the brake fluid leaking out externally of the brake master cylinder or from the wheel areas when this pedal softness occurs, so fluid level alone isn't an indication of a good or bad brake master cylinder. The fluid will usually not be low or in need of topping off…remember the leak is internal and the brake fluid is leaking past internal O-rings, so an obvious external sign that the master cylinder is "bad" is usually not existent. "
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