Refrigeration Technologies Nylog Gasket and Thread Sealant RT201B Blue or RT200R Red 30 ml RT201BP RT200RP (2 Pack, Nylog Blue + Nylog Red)

Worked great! Held vacuum for 2 hours.

Nylog is a lifesaver. I used this when I replaced all the O-rings on my 1987 Honda Civic's air conditioning system, and on the threads of the receiver-dryer and elsewhere. The 100 psi dry-nitrogen pressure test demonstrated that the O-rings were working...as was the Nylog Red I used. The manifold gauges seemed like they were glued in place at 100 psi 36 hours later. As in love, politics, and other aspects of Life, you pay for what you get. In this case, I paid only a little compared to what I got: the benefit of knowing that I've done all I could to create a leak-free system.

This is the stuff you should use. I bought the red label on On HVAC suggestion