Still got a leaking pressure relief valve

Had the keg vendor send me some more pressure relief valves, 4 of them, and they all leak. He sweared he pressure tested them all to about 40 psi and they all held. Wonder what the problem is? I managed to wrap one of them in teflon tape and it seems to be holding around 14 psi, but it still doesn't sit well with me. Terrified I'll go to the keg and the CO2 tank will be drained. Anybody got any other ideas???

Soapy water.

Seems a couple of my lids have that problem. I use the soapy water solution and sometimes I find a relief valve will leak until you fiddle with it a bit and it gets seated properly. Even though some of my relief valves leak like this, I haven't yet lost any CO2.

Have you tried a little keg lube on the threads? I put a dab on those and the tops of the poppets after I break them down to clean them. Never had any leaks in any of my kegs. <knocking on simulated wood desk>

edit: I put some on the tip/seat of the relief valve too.