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I have a new (to the car, about 3 yrs ago) green? cap rated to 150KPa or around 20psi, off the scale of my ancient but serviceable STANT pressure tester, and the new cap holds pressure and vents vacuum. OTOH, my system won't hold above 14psi pressure (new nissens metal radiator and new hoses, haven't tested it since I put in new upper hose) but haven't been able to find any leaking antifreeze and level stays at Max. Caps shouldn't hiss unless they're venting and they're either venting because of weakened/defective cap or excess pressure in the system (note...filling the system too far/to the top WILL vent due to expansion of the fluid which is quite a bit more powerful than simple vapor pressure of the coolant).
Point is that there are probably several pressure rated caps around...check your owner's manual for the right one for your car. I'm beginning to think that 20psi is a little high for my ancient B230FT although it's held so far...
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