you want to use a ptfe/ss check valve underneath your stainless steel prv. this will prevent leaks under vac, as the prv valves are made for positive pressure. you should have a prv on every part of the system you are able to sequester between valves, this prevents hydraulic rupture of the hoses/spools. Select a prv that is half of what the lowest part of the system is rated for. We use a 90 psi relief valve, due to the fact that the largest part of the system is 12".
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