Hi, all.
We've had a problem with inconsistent flush power on our Am Std w/Flushmate 501A tank for a quite a while. The upper and lower supply group had been changed recently due to a leaking pressure regulator in lower goup, so I assumed that they were still ok. The actuator cartridge showed no air bubbles during testing.
Tech support at Flushmate suggested cleaning the cartridge. I took the cartridge out and discovered that one is able to disassemble it most (if not all) of the way. The innermost control rod (has a ball shaped bottom w/cross shaped plastic arms) unscrews from the top, allowing cleaning of most of the surfaces.
After cleaning and inspecting the o-rings (they all looked fine), I reassembled it. The wording in the online manual is a little more clear than my printed version regarding how to properly install the cartridge. It suggested screwing the cartridge in until 1-2 threads of the tank appear above the body of the cartridge. Then turn on the water and let the unit fill for 30-40 seconds. If the water doesn't shut off, turn the cartridge in until the sound just starts tapering off (like a normal refill). Keep turning it in small increments until the water finally stops filling. Flush and listen for a complete refill. If it refills normally, you're done!
We have not had a single sluggish flush since doing this procedure, though I am unsure as to whether it was the cleaning or the installation method that has caused the improvement. My guess is that it was the latter, since there was only a little iron staining on the cartridge.
So, if you've got similar problems w/your Flushmate, give this a try before buying a new toilet or Flushmate replacement unit.