This article covers checking for leaks from 1. the piston-cylinder interface, 2. the reservoir, and 3. the intensifier.
See the attached file for more detailed instructions to remove the cover and check for leaks on P3800 model high pressure deadweight testers (limited usefulness for previous generation models PM3800 and M3800 models also).
Summary is below (see attached pdf file for details and much more information.)
Measuring the piston drop rate of a deadweight tester is a better way to check for leaks than trying to do a leak check. Mention of this is made in the document attached to this article, and in this separate article P3800 Deadweight Tester - Piston drop rate evaluation