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Rawze

Acually, checking the coolant system on a big truck should be done at 20-psi, not 15 psi. This is done by applying the test kit in place of the pressure regulator cap and not the fill cap. This ensures that you will find any leaks that may only occur when pulling a hill and working the engine really hard. -= You know... that area of engine operation where it would count the most,., like pulling a steep mountain at 80,000lbs, and its 120-F outside, and at 6,000 feet altitude.. when you need it not to fail because your in the middle of nowhere.

next: checking for coolant leaks does not involve watching the air pressure gauge except to ensure it is kept at full pressure. It is actually is quite normal for some of the pressure it to fall off a bit as the truck and engine cools down, or sits over and entire night while testing it. -= and if it falls off, just top it off again, no big deal. 15+ gallons of coolant will contract b y a lot as it cools down, making it seem as if it lost pressure somewhere.

It is not "AIR" your checking for leaking.. it is COOLANT. -= SO WHAT YOU LOOK FOR AND WATCH FOR IS actual COOLANT leaks, both externally, and internally to the engine if necessary.

Lastly, coolant leaks tend to almost always get worse and easier to find as "cold leaks" too. This means that any leaks will show up more when the engine has cooled off and has sat all night with a pressure test kit on it. Hot seals are soft, but as they cool, they stiffen. So ... because metal components will shrink slightly and seals will get stiff, that is why the leaks show up more after, or as the engine has cools off during the night.

This means that you mentioning a simple "5 minute test", and at only 15 psi .. is only a simple quick check. This if fine if you have an immediate issue, but if you want to test the system properly.. it has to be done over night.. over several hours, and at 20-PSI. This is important to do every so often for someone who has to make their living with the truck every day.

Hauling freight for hours on end with heavy loads and working that engine and truck hard is nothing to skimp about. Running up and down the roads day after day.. across god knows where and at what altitudes.. at god knows how much weights... an over-night test is the proper way to test the coolant system if you want to stay on top of any future coolant leak issues or potential failures.

AND ... if your loosing a gallon every here and there. .. and you do a coolant leak test over night and find zero leaks.. then it is time to drop the oil pan, open up the EGR cooler exhaust piping .. and find the issue, because it is most likely an internal leak and those are the most damaging.