I bought one, like this, for a really, really, good price. (gotta love an on-line Auction)
http://www.ebay.com.au/itm/Leak-Down-Engine-Cylinder-Head-Leakage-Detector-Tester-Tool-Kit-Car-Truck-Auto-/131647121913?hash=item1ea6c7a9f9:g:OOsAAOSwzgBY0cAKIdea is to block the outlet (create 'zero leak') then set the testing pressure with the regulator, (therefore a constant test pressure).
Connected to a cylinder, (2 or 4T) you read the leak rate on the second gauge as a percentage pressure loss.
2T's Loss from rings or head or plug seal with piston rings above exhaust or wherever with inlet & exhaust blocked.
4T's you can listen at the exhaust and inlet ports, for leakage from valves.
Best as a comparison test, against a known good (new parts & bedded in) assembly, than an absolute reading.
Use to monitor wearing in & then wearing out.
One TRAP for Young(ish) players with the Chinese kits. The hose must be the same one they use for the Compression Tester Kit.
It has a Schroeder valve at the bottom.......No leaks are detectable with this valve in place.
I also replaced the air fittings with Nittos (personal preference).
Cheers, Daryl.