Diagnosing a bad rollover/vent valve

I replaced the tank in my '90 MJ right after I bought the truck because the OE tank leaked at the seams. Since then I've noticed what seems to be a lot of pressure buildup in the tank. Even when it's cold I'll have a lot of hissing when I open the cap. The tank has 2 rollover valves in the top and I know the vacuum lines aren't kinked. I tried hooking the vacuum pump to the lines and it holds 15" vacuum but that doesn't translate into any type of hissing/sucking noise when I remove the gas cap. I suspect one/both of the vent valves are not working. What else can I do to diagnose this problem??

You probably need a new fuel filler cap or your EVAP system is not functioning correctly.

If you have a 4.0 your EVAP purge valve and EGR solenoid are a combo on the driver's side fenderwell, the charcoal canister is passenger's side of the engine at the firewall.

The fuel cap is not vented, correct? It would hold pressure unless the seal was bad. I have the aftermarket Stant locking cap and it worked fine on my old 2.5.
Yes it is the Renix 4.0. I actually pulled the vacuum line at the canister and applied vacuum from there back to the tank as well. I was thinking it would draw something and/or leak down but it held 15" Hg even after I unscrewed the gas cap.

Actually, the correct fuel cap has either two vents or a combination vent.

It has to let too much pressure out and also has to let air in when there is too much vacuum.

Check the part number for the correct Stant cap and compare it to yours, it may not be the correct application.

If it is Ok, then check the EVAP system including the charcoal canister, they will go bad.