1993 Acura Integra Brake problem

There is no leak on master brake, bleed it well couple times, but brake slip, feel like loosing air from booster brake. How do I check if booster is leaking? There is no sign of any fluid leak anywhere, the reservoir doesn't show loosing fluid, so what cause the brake to slip? most of the time when I press on brake pedal it goes almost down to the wall. Any suggestion? Thanks