This is on my neighbor's 98 Accord. I got in it today with the engine not running. Pedal felt fine. With my foot still on the pedal, I started the engine. The pedal went almost to the floor. He told me he had been pumping the pedal to get it to stop when he drives it.
I had three thoughts:
1) Vacuum leak, causing havoc with the brake booster.
2) Bad booster.
3) Air in the system, with a possible leak since he said has been getting worse. But the MC is still full of fluid. Any help would be appreciated.

 

I'd bleed the brakes first. Sounds like air in the system but not necessarily a leak OUT.