90 740ti, black 75kPa cap.

My cap is venting pressure (and LOTS of coolant) after the car warms up. After losing a good portion of my coolant many times, and replacing it with distilled water, I'm sure I'm running close to pure water.

Is there any way to test that the cap HOLDS pressure up to 75 kPa before I buy a new one? I bought it just recently, and can press the vent open with my fingernail. The top rad hose does seem to hold quite a bit of pressure, but I can squeeze it fairly easily.

Yes, I'm having cooling system issues. Right now I've been dumping distilled (doesn't leave deposits) water into it. I think it may either be venting because
1) Air bubble, which I keep clearing out. (water pump may be leaking from weep hole)
2) Pure water boils at 212.... I need more antifreeze in the thing.
3) Black pressure cap is either too low a pressure, or is defective.


Any help would be appreciated. I am broke BrOkE, BROKE, so any free solutions would be good.... my financial aid is 18 days late, and I'm going to school full time...

Anyway,
Happy Bricking,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond