The subject of leak down testers has arisen on this forum numerous times over the years. I recall a discussion once of how to make an LDT from a compression tester and some weatherhead-type fittings.

While looking on eBay for some Snap-On tools, I stumbled across an LDT - not Snap-On brand - for under $30.

At that price, it's really not worth fabricating one, and not cost-prohibitive for even most mechanical dilettantes to own one. Of course, you'll need mechanical knowledge to interpret the results. If you have to ask what an LDT is good for, you probably have no business owning one.

Merely go to the eBay site and search for leak down. The item I'm referring to comes in a red plastic case.

Keep in mind that the genuine Snap-On LDT is about $300. These inexpensive items on eBay are probably Made Overseas. 

At this time I have not purchased or used one of these items.