Master cylinder leak?

[brother rat]

I've confirmed that the brake reservoir is full. When coming to a stop, my brakes work, but then the pedal begins to travel farther and farther and feels pretty spongy. The truck comes to a complete stop. But when stopped I can pretty easily depress the brake pedal all the way to the floor. Pumping the brake pedal doesn't really build up any pressure.
Is most likely faulty seals inside the master cylinder? Vacuum leak at the booster?
I don't see any external leak at the master cylinder. I haven't inspected the wheel cylinders yet, but the problem seems too big to be a single wheel cylinder leaking.
Thanks for your advice.

 

[adsm08]

Sounds like the MC is a good bet to me. That is more or less what my Olds did when the master crapped out. Also, at a red light if I didn't pump the pedal every 30 seconds or so the car would try to drive off on it's own.

 

[RonD]

If you are not losing fluid then I would agree, Master Cylinder