The CTR series of reservoirs are a unique item that is advised to be used with a Pressure Check Valve for maximum safety. While not mandatory, it's still a good idea. The modularity of the CTR does involve a trade-off in that it’s not meant to hold as much pressure as a typical traditional reservoir can. A properly installed pressure check valve resolves this issue, however, by simply not allowing pressure to build up in the system to an unsafe level. While this precludes full-system leak testing with a pressure tester, you can still test individual components with it, which will also help more immediately recognize where any potential leaks are coming from if you have one.