When I sit in traffic the pedal gradually goes all the way to the floor. Once it bottoms out the car starts to creep. I can pump the pedal and have brakes again but it gradually goes to the floor every time.

I did the functional test that is described in the 91 Civic service manual, with the engine off and pumping the pedal a few times then holding, the pedal sinks. The book says it's either a line, wheel cylinder, or the master cylinder.

I'm leaning towards the master cylinder because I'm not loosing any fluid. I think I'd notice fluid loss if it were a wheely cylinder or a line.

BTW, the booster passes all the functional test

 

replace the master cylinder would be the first step i did.. I think you can get htem for under $100, maybe less with a core exchange..