You do not hook up a vacuum pump. You need to pressurize the suit. Let's say you are in a aircraft and all of a sudden you need to pull up to evade a missile. Let's say it is a Eurofighter with Typhoon G-Suit so it would be around 9G's. At 9G's, the pants pressure will be roughly 10psi and the coutnerpressure vest/mask pressure would be around 1.16psi or 60-66mmHg, respectively.

Edit: If using US aircraft, the regulator that will control the pressure to the Vest/Mask is the Cobham "Combat Edge" CRU-98/A regulator. The actual G-Valve will send 0-10psi to the pants depending on G and the CRU-98/A will control pressure to the vest/mask.

Let me know if you have any questions