Over a period of a couple of years my central vacuums suction and flow
seemed to be decreasing. Early-on we had ensured the hose was clear;
inlets were clear; motor fins were clean; and more recently were
suspecting the pipes & outlet doors. These were also inspected except
where the pipes were hidden. Still the performance was very low. The
final check was to remove the input and output pipes from the unit
where we discovered that covering the inlet did not greatly decrease
the output, ergo the problem had to be a leak within the vacuum pump
or the filter canister (two separate units on mine). Only now did it
occur to me that the filter can had become increasingly more difficult
to latch into place, which was also due to the same cause; a 1/2" high
x 2" long pile of crud between the filter's upper container and the
lower dust can. We were getting bypass air directly into the dust can
preventing full vacuum at the inlets. Much thanks to those of you at
this site that provided me with diagnostic direction.