I need a 1" 15psi soft seal pressure relief valve.

We are building a high voltage test chamber that will be filled with sulfur hexafluoride (SF6) to prevent arching. The plan for the chamber is to pump it out to around 10^-3 torr or lower (aka higher vacuum) then to fill it with SF6 up to 1 atm (gauge pressure not absolute). My understanding is that if the pressure is kept below 15 psi above atmospheric pressure we do not have to certify it as a pressure vessel so that's the reason we need the relief valve. We bought a kunkle valve from mcmaster carr (http://www.mcmaster.com/#6017t13/=7qe10e) and when we tested it we realized it did not hold pressure and started weeping well below 15psi. So I need help in finding a soft sealing (because it has to hold vacuum) 15psi pop safety valve. It can't start weeping much before 14.7 psi.

The main reason I believe the Kunkle valve failed is that they are adjustable and 15psi is the lowest value offered so the tolerances were horrible when they would be acceptable at 150 psi. If someone can find a non adjustable valve I think that would be best.
I forgot to add I am located at Triumf in Vancouver BC, Canada so local or Canadian companies would be preferred.

Put a NON-FRAGMENTING rupture disc under it, or eliminate it entirely with a rupture disc.  Even a soft seat relief valve will leak inward under vacuum.

If you do put a disc under the valve, make SURE you put a proper means of leak detection in there between the disc and relief valve inlet.  There are numerous options for what to do there, and which one you choose depends on how frequently it will be attended and how probable leakage through the rupture disc is.

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