international 4300 with hydraulic brakes, what activates booster pump?

The pump should come on when the key is on, engine off, or any time the key and engine is off and the brake pedal is pressed. Power for the pump only comes on when it is supposed to be on. Power comes from the pump relay which should be mounted right on the master cylinder right there close. The relay gets power constantly at 30 from the 100 amp mega fuse at the firewall. The 86 terminal should also have power all the time which comes from the body controller. The ground side of the relay, terminal 85 is what triggers the relay through either the flow switch, pressure circuit differential switch or brake control module to supply a ground path to turn the pump on. The 5 amp fuse you found is a power feedback sensing circuit to the brake control module so it can sense when power is supplied to the pump to aid in warning and diagnostics. Motors do fail. If you jump 12 volts to the power post of the motor and nothing happens, I would say you need a new pump motor.

 

Hello, just found this post, my hydro boost won't shut off, only way is to disconnect