Is it safe to flush a high pressure toilet that hasn't been flushed in > 3 years?
All the ones I've work on have a canister with a diaphragm that compresses the canister when it's filled with water. Just flush it. If nothing happens, which is probably the case since the diaphragm has probably lost pressure, try it again and see if it recycles. When these unit would go bad, there weren't part to replace, just buying a new canister. What should you do first?? Have a towel handy. good luck and stay safe out there.