Re: Brake Pressure light..
on the side of the master cylinder. you should see a connector there. that is the sensor(s) Check first to see if you can clean the contacts on the sensor/connector, that very well may fix the problem.
i'm not sure how to test the sensor with a multimeter.... I don't know if it should be either open or closed when there is no pressure on it. I suppose you could use the multimeter to see if it's either open/closed with the engine off, then switches to the opposite when the engine is started.
a good test for your brakes is to unplug your electric backup (it's the motor you hear running if the key is on and the engine is not running) If you unplug this and the bus still stops then it helps rule out a faulty ps pump. Another ps pump test it to turn the wheels from side to side while the bus is not moving. if the steering wheel turns easily and the pump doesn't whine, then it's quite likely the pump is ok.
To rule out the electric backup as faulty.....re-connect the electric backup motor, with the key off in a safe place see if the bus will stop when the engine is not running. If the bus stops, then the backup unit is likely ok.
if both of these systems appear to be functioning properly, the most likely culprit is the sensor.
Quis custodiet ipsos custodes (who will watch the watchmen?)