PITOT MASTS with incorporated Static Ports. Again, the aircraft manual is vital. Special fittings are often necessary. There's always a few crazies like the Islander mast. It's war surplus from the Spitfires. It has a tricky internal baffle to drain melted ice water, and does not have the leak that the aircraft manual test will find it has. FLUSHES. Blowing out the bugs and water. if ever the lines are contaminated and a flushing is called for, get very very well organized and pay the strictest attention to the manual. Sometimes there are sensors in the system you weren't aware of that will blow up with the flushing pressures - sometimes sensors added by STC and not in the main maintenance data. ALTERNATE AIR SOURCES. A general aggravation. They wear and get dirty. They are often in such a place as to get kicked. They are a source of leak. LEAKS. You may want to pressurize you system down to sea level and then use bubble stuff to help find leaks. Boeing have a neat idea. They put refrigerant into the lines of a new aircraft and then use a halon detector to sniff for leaks. The best test strategy is to try and clear one half of the system at a time. Continued dividing in two finds troubles much faster than starting at the end. HYSTERESIS. Altimeters built with aneroid capsules are subject to the stretch memory of the metal having a "memory" of its recent history. So coming down from altitude, it remembers that stretch of materials and lags the truth. Depending on the quality and age and the range of pressure under consideration this can be up to 100 feet. FAA require that accuracy tests be therefore done only ascending. Descent is NOT appropriate for decisions about accuracy. Silicon chips and other modern sensors do not have this problem.