It is rare that an aircraft is not found to have a leak. Yet most aircraft do not actually often have leaks. But sometimes there is a real leak and the false leak phenomena can drive you crazy. The False Leak phenomena comes from two sources: the elastic nature of the test hoses, and thermal differences across the test environment. It is easily dealt with. Simply climb 5 to 10 % beyond your target altitude (depends on your hose quality and length). Then descend to your test altitude. The overstretch of this will quickly allow the hose wall to reach equilibrium and the false leak will not be a bother. A real leak will still show - in fact you will be more confident that it is a real leak that remains. Lastly, the temperature variations. Many pitotstatic test sets have pumps and get rather hot, thus radiating the instruments and hoses and warming the air. This is opposite to leak. The hose to the aircraft lay on the hangar floor and are very subject to breezes from opening doors cooling them off. Cooling the air inside the hose makes it more dense and so again is like a lower altitude and simulates a leak. Enough time to allow equilibrium is the only way to be sure and that may take an hour or more. Good reason to buy a modern ADTS.