1996 Mitsubishi 3000GT have to step on brakes unusually har

Stop the engine, then pump the brake pedal a half dozen times to exhaust all power assist. Then, while holding steady, normal pressure on the pedal, start the engine and watch how the pedal reacts. If it drops a few inches under your foot pressure, the power booster is working. If the pedal moves very little, remove the vacuum hose from the power booster and check for strong vacuum while the engine is running. It should suck your finger to the hose and hold on tightly.

If there is little vacuum there, look for a kinked or leaking hose, loose connections, or a plugged check valve. If the check valve is suspected, it can be removed temporarily and bypassed. If the power assist comes back, replace the check valve. The purpose of the valve is to hold enough vacuum in the booster for two or three power assisted pedal applications if / after the engine stalls.

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