Yes, I've replaced the cap and it came with an 0-ring. The caps are made of plastic like most cars. My leak detector shows a leak on the low side port. The high side shows no leak.

I have a leak at the low side service port. I have tried valve cores from 3 different sources. I tried tightening the valve core to spec, under tightening, over tightening, still leaks. Core appears centered, not cross threaded or anything. The port is part of the hose-line. Is there any tricks or t...