Replaced master brake cylinder. bled the system. Pedal hard when engine off but bit mushy when on.
that's typical acting of a power brake system as when there is no power the pedal is hard and when started the pedal goes down some, if you think it goes too far down re-bleed the wheels starting at the right rear, left rear, right front, and finally the drivers side front, I certainly hope you bench bled the master cylinder before you put it on.
On a power brake system it will be hard when the engine is off as there is no vacuum for the power booster. Some of the newer cars with ABS require a special tool to bleed the rear brakes properly. If your car has ABS that could be the problem. Also did you bleed the master cylinder prior to attaching the lines. If not you probably still have air in the system.
If you have power brakes, that might be the cause of it. When the engine is off, the power brakes are not power-assisted. Engine on, power assisted, so it takes less effort to push the pedal. If the mushiness of the pedal (with engine on) feels different than before you replaced the master cylinder, then you could still maybe have air in the system. Also, most master cylinders must also be bench bled, or bled by hand before they are installed in the car. If your is one of those, and you did not do it, there will likely still be air hiding in the master cylinder. If you have any doubts, take the car to a tire and brake shop and have them do a quick bleed job. It's usually pretty inexpensive, and they have the equipment to do it reliably, quickly, and easily.
