Originally Posted by
genphreak
If the system had charge, the only way to know if the valves leak gradually is to test under serious pressure (hot day/run time/spit will dry real quick).
Or if the system has been working fine and you wait till everything is cool and quiet and you listen for a an ever so slight hiss when removing those cover caps. I've come across enough of those alone to not trust those valves- and even with a drop of PAG oil in those little sealer caps. There is just no way they can resist the run-time pressures if the main rubber in the valve is compromised, but they will certainly pressurise if the valve is leaking- hearing it is the hard bit, unless you can rely on a green dye to point you to the problem.