Hydroboost on 81

[Old 403]

Been a while since I checked in here but I'm having brake problems and looking for ideas. The original hydroboost brake system has worked great from day one on my 81 Sierra Classic pickup but recently I'm getting only front brakes applying and takes more pedal pressure to stop. Rear wheel up on blocks test shows the rear brakes will apply on a quick pedal push but gradual pedal application gives almost no braking and the pedal goes down more as if pressure is by passing somewhere yet there are no visible leaks. Brake fluid is old and dark. Maybe the master cylinder is weak? The truck is a former diesel but now gas and have run with the original diesel steering pump and brakes since the swap in 86.