Measuring the piston drop rate of a deadweight tester is a better way to evaluate performance rather than trying to do a leak check and measuring a pressure decay rate. Leak checks can be difficult to do because the piston is typically floating and dropping slowly, and some oil leaks from the piston-cylinder interface.
Use a dial gauge (or similar) to measure the drop rate of the piston at full pressure of the deadweight tester. From DWT Uncertainty Analysis (Technical Note 2170TN13), the drop rate specification for P3800 deadweight testers at full pressure is -1.5 mm/min (-0.059 in/min).
If the drop rate is excessive, or there are other leaks, see this article P3800 Deadweight Tester Leak Check