You will have to give a clue. Engine, year etc.
system isolated and the vacuum pump turned off.
If the vacuum pump is on, the water will continue to boil. If the vacuum pump is off as the vessel is sealed, the boiling will stop at a point of equilibrium.
Vacuum hose off?
Turn off the pump first. Then remove the hose and vacuum head.
easiest way to test vacuum pump is , with engine off, pump brake pedal until pedal goes hard keep pressure on brake pedal and start engine if vacuum pump is good, you will feel the brake pedal go softer so you don't have to use as much effort for pedal to be depressed
The oil pump is located inside the oil pan; To get to the oil pump you have to take off the oil pan.
You have to find the leak in the system. Once your entire vacuum system holds constant suction, the pump will cut off and only come on when needed.
As looking at the front of the truck, on the right side of the motor above the power steering pump. Driven off the belt.
GM vehicles do not have a fuel pump shut off switch.
There is not a fuel pump shut off on any GM cars.
I found this on another site & hope it helps. Arthur in Texas "If the vacuum system functions normally at idle but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by turning off the engine and using a hand vacuum pump/gauge to check for vacuum delay while selectively blocking off vacuum hoses. I would start with the vacuum tank and check valve under the hood. This is the most likely location of a leak".
If the engine is the 4 cylinder the pump is driven off the timing belt. It is located behind the timing belt covers.