it goes straight to the floor with no resistance? if so your master cylinder seals could be going. if, on the other hand it is really stiff to push it could be the booster. to test that, With the car off...pump the brake pedal a couple times & keep it pressed down, while you turn on the engine, if the brake booster is working you should still feel addl "assist" on the pedal when the engine catches & the brake booster kicks in. I have a '74 02 and when I am braking sometimes I lose the pedal usually after coasting and the pedal returns with 1 pump of the brake pedal there is no fluid loss but this started after giving the car a tune up. I'm thinking the idle might be too low. Any suggestions would be great. Thank you