Just jump the A + C terminals on the switch to test it, pump should run when the black and Wht. wire are shorted together, or the white wire goes to a good ground.
Black wire should be a good ground read with an ohm meter to battery ground, low ohms.
Make sure the 5A relays fuse is good and working, Brown wire Pin B of the powermaster wire connector should be +12 when IGN is in RUN position.
Pin A the black wire of that connector should be a good ground and is the ground to run the motor.
Pin C the Red wire should be +12 at all times and is the hot for the motor.
Internal relay will activate when the IGN is in RUN and the white wire is shorted to ground simulating the brake pressure switch calling for more psi.
Several tests to do before buying and swapping the switch, unless you have a spare handy then a quick swap is easy.
Make sure the +12 signals are correct too.