The POH for a PA28 (engine driven vacuum pump) says a decrease in pressure on the gyro suction gauge may indicate a possible sticky vacuum regulator. A zero pressure indication means a sheared pump drive. What exactly does that mean?
There is a small vacuum pump attached to the engine by a small rotating shaft which generates vacuum whenever the engine is running. That vacuum is used to spin up the gyro instruments in the plane.
Because the pump produces different amounts of vacuum at different engine speeds, its vacuum output must be sent through a device called a regulator to furnish a constant vacuum to the instruments at all times.
If the vacuum pump drive shaft is broken ("sheared"), the pump produces no vacuum and the gyros go dead- and the suction gauge reads no suction. If the regulator mechanism is stuck or jammed, it stops working as a regulator and allows full suction pressure to be applied to the instruments- and the suction gauge reads too much suction.