How to perform cooling pressure testing

Some cars do not have caps on the radiators. When that is the case, you will find the reservoir is under pressure at the same time the radiator and rest of the cooling system is. For the pressure test, that just means you are including the reservoir in the test. You will need an adapter that fits on your reservoir's cap, and onto the tester. Sometimes a variety of those adapters come as part of a kit, and sometimes you have to buy, rent, or borrow them individually for your application.

Here is an article that explains more about doing the test and what to look for:

https://www.2carpros.com/articles/radiator-pressure-test