Quote:
Revving the engine makes the coolant level drop... The degas jug is typically tapped into the system on the suction side of the water pump (in the lower radiator hose or near it's connection to the front cover). This is for safety, because this is the lowest pressure area of the system. Rev the engine, the water pump sucks harder, and drops the pressure of the coolant in that area. Downstream of the water pump the pressure increases and expands the upper radiator hose, heater hose, and whatnot. This increases the volume of these areas, coolant fills the extra space, and coolant is drawn from the degas jug, dropping it's level.
I have read that this is why it's installed on the low side of the pressurized system. It only makes sense to protect the cooling system.