My vacuum pump failed and is being replaced. My mechanic tells me that part of the labor charge is checking and cleaning (or replacing) the vacuum lines back to the instruments. If the pump is sucking from those lines when it fails, why do they have to be cleaned?
I have seen the opinion expressed that, since the pump is “sucking” from the instruments, if the pump fails, there can’t be any debris back up in the hoses. I can tell you unequivocally that this is an incorrect perspective. Any gyro instrument shop can tell you stories of instruments sent in that are full of carbon powder (unless they have suction port filters). These are not from systems with pressure pumps. Those systems all have filters in the pressure line from the pump to the instruments (most commonly seen in Bonanzas). Any debris from a failed pressure pump is stopped by the filter, in these systems. The suction (vacuum) systems can’t afford to have a filter in the suction line between the pump and instruments, due to the added flow restriction. They must have a filter on the far side of the instruments (to clean the incoming air despite the restriction), and a second filter would add too much restriction; it would shorten the life of the pump, and reduce the airflow unless the pump worked harder.
When the vacuum pump is running, the vacuum exists all the way back to the central filter (on the far side of the instruments). When the pump fails, atmospheric pressure instantly “pulses” back into the system from the pump. It sends air, and anything in it, into the hoses (and instruments) from the direction of the pump, as air also continues to flow in from the direction of the central filter. The vacuum regulator instantly closes when the vacuum drops, so not much air enters there (through the filter band). The air from the central filter is harmless, as it passes through the filter (as in normal operation). The air from the direction of the failed vacuum pump is full of carbon powder and flakes. This debris can make it all the way back up the hoses and into the instruments, simply because there is the least resistance from this direction (no filter).
If you operate a pump until it fails, the hoses need to be disconnected and blown clear of debris, and the air-driven gyro instruments should have some air gently blown through them, unless they have micronic filters on both inlet and outlet nipples (they look like sintered brass plugs inside the hose nipples). This also give the opportunity to learn whether the air hoses are still supple and sound. If they are stiff, they may fragment on the inside, with pieces coming loose from the inner tube liner. If there is any debris left in the hoses, whether from the pump failure or the hose activity, the new pump will have an abnormally short life. The vacuum hoses forward of the firewall suffer the most due to the heat. Hoses more than ten years old should be replaced as a matter of good practice. Hoses should also be checked any time the vacuum instruments receive maintenance.