buy one of these...
Brake Bleeder & Brake Bleeder Vacuum Pump Kit
you can use it for other stuff also like brake bleeds etc...
Find a vacuum line that dead ends or cut one and use the T fitting (you will replace this with a straight fitting after test)... run hose from gun to fitting... squeeze till tight and see if the pressure is dropping... make sure all hose ends have holds, even zip ties work... you should not have any drop.