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Rubber gasket on lid acts as (unintentional) pressure relief at 700 psi. A thinner, harder gasket would be better.

Update

Switched back to using grease in the gun. Direct connection to a gauge showed that the gun could (barely) produce 2000 psi, although it requires two hands and there's a lot of bypassing so progressively more squeezes are required as the pressure increases. I also reorganized the plumbing to eliminate the 3/8" ball valve, since that appeared to be a weak point. Made a 3/8" thick lid. With normal clamping pressure, gasket still leaked at about 500 psi. With the bolts really cinched down hard the seal could hold 1200 psi. (ignore the parallax and the 1500 marking, the lens has rotated). Left it for a couple days at the higher pressure.