I believe the circuit that activates the booster pump on IH vehicles is turned on when the switch is on, but pressure is low. I'd start with the simple stuff - fluid level in the steering pump (which also supplies the brake booster), lines from the pump and lastly the pump itself. Also check the pressure sensor and it's associate circuit.
Remember these systems are designed to be "Fail-Safe", which does *NOT* mean fail-proof! Consider this - if something fails, what's the safest possible thing? Well, I'd want that pump to run so I can steer and stop that bus, no matter what else. (Of course it's still possible to lose power assist, but it's at least engineered to try to provide that if everything is otherwise in order).