I guess I'm not sure. There's a 20-year-old embossed label on the bulkhead next to he cylinder that reads "30psi." Previous owner said he kept it at 40 because that was the only way to get the upper helm to stop weeping. When we found the excessive leakage today I went down to depressurize the system and it was about 20, which to me reads as "as pressure falls, leakage rises."