Disconnect the wires so they don't twist up while loosening the master cylinder switch assembly. It doesn't matter what you do with the terminal screws, just don't lose them
the master cylinder switch assembly will take something like a 13/16"-7/8" or metric equivalent to move it. With pressure applied and holding, loosen the switch threads that enter the master cylinder valve body (you won't have to completely remove it and only loosen enough to get air or fluid to come out past threads) until you get a little fluid out or the pedal goes all the way down. Still holding the pedal down, tighten the switch assembly just snug tight. Pump up the pressure again and while holding the pedal down repeat the process until nothing but fluid leaks out past the threads. If this doesn't fix it, the new switch on the new master cylinder may be bad. Check continuity across the switch terminals with the brake pedal pushed down; the switch should be closed with the pedal down and open with the pedal up. If you have a good switch, make sure that one wire goes directly to the brake light and the other goes to battery + or is connected to a wire going direct to battery +. Brake lights are not supposed to be dependent on any other switch including the ignition switch. As long as you have some juice in the battery, pushing the pedal down should turn on the brake light no matter what condition the buggy is in (engine running, not running, ignition sw on or off, light sw on or off).