Some cut and pastes from similar threads I've responded too, hope this sheds some light.
Compressor Cycling: My "bottom line" is 5 minutes. If they cycle sooner than that you have an appreciable leak. Why 5 minutes? Well I honestly pulled that out of my head... based on my experience with the hundreds of ARB setups I've sold and installed, some might feel comfortable with a bit more or less time, but I wouldn't send anything out the door or using something on my vehicle that had a lower bottom line. I usually only have my Air Lockers on when needed, sometimes 30 seconds or less... however there are times I leave them on for hours on end while tooling around in the snow or sand, last thing you want is a compressor running all the time, potentially damaging itself. If you are nearing the bottom line for compressor cycling, first check & tighten all of your airline fittings. As the system does not hold much volume, even a very small leak can be enough to cause the pressure switch to instigate a cycling of the compressor, make sense?
Near or below the bottom line and have checked and re-checked your fitting using soapy water or similar medium? Perform a leak-down test on your actual differential, from the bulkhead fitting to the seal housing inside the axle. Doing so will require you to either find a shop with the specialty tools, or to build a test apparatus. Ours allows us to charge the differential to a variable pressure (using the pressure regulator on the compressor). We then block off the input air source (ball valve) and disconnect. Our apparatus has a pressure gauge between the bulkhead fitting and the ball valve, thus allowing us to see how much pressure the locker system is retaining. We will actually charge the system to 100psi (slightly higher than the standard operating pressure) and allow it to sit for at least 10 minutes, periodically spinning the differential to make sure the pressure doesn't spike (indicating a bad seal to cylinder cap interface). Assuming less than 5 psi is lost over the 10 minutes, we call it good to go and either ship or install into the vehicle. In most cases, the locker will lose 0psi, even when sitting for 20 or more minutes. However, I have had customers bring vehicles to us for locker failure analysis, we've seem them losing air rapidly, going from 100psi to 0 in under a minute, likely the result of bad o-rings, a damaged cylinder cap or an improperly installed seal housing. One other possibility is a leak at the bulkhead fittings themselves, check and re-check before going after o-rings.
O-Ring Seals: Keep in mind the technology and design of ARB o-ring seals and seal housings have changed in the recent years. Where they used to utilize a round cross-section o-ring, they are converting to an almost square cross-section o-ring, thus increasing the surface contact at the cylinder cap. Also, older ARB units utilized a clamped seal housing, which required great care while installing to maintain a concentric seal interface. New units use a "clip" style that actually "floats", thus ensuring a better interface between th o-ring seals and the cylinder cap. In some applications it is possible to upgrade your seal housing to the newer units (requires several other components for the complete install).
All this will eventually end up on my website in the ARB tech page