On pitot static systems with needle valves, there is an error practiced throughout the industry. Most people are taught to maintain a 100 knot airspeed at all times. Now if they did that, all would be OK, and climbing, it is easy to do - there is no problem - one opens the vacuum on the static line to climb and bleeds the cross-bleed in order to maintain 100 knots. The trouble somes on descent. But the best rule is leave the cross bleed open except for intended airspeed tests. On some ADC sensors, a reverse air pressure of 1 psi is fatal.