Prob'ly doesn't need repair, just lubrication
A piston/cylinder toilet is no different from a piston engine...it needs lubrication, or it binds. The main difference is, in a toilet it's rubber o-rings around the piston rod that create the seal that rubs against the cylinder wall...operating the toilet without lubrication wears 'em down, which is why many toilets need rebuilding far more frequently than they would if they were kept properly lubricated. Most people wait to do it until the toilet becomes hard to pump, then pour something down the head that washes out in a few flushes...just again when the toilet starts to squeak again. That approach to toilet maintenance is like adding a cupful of oil to an engine only after it starts to smoke each time. Buy a tube of thick teflon grease--the same thing that's in every toilet when it leaves the factory. Remove the top from the pump...depending on the age of the toilet, that requires either removing 6 screws or just a nut. Put a healthy squirt of the grease in it...pump a few times to spread it all over the inside of the pump cylinder...put the top back on. You're done. Do this annually as part of your preventive maintenance schedule and your toilet will be trouble free for years longer, without any need to rebuild, for at least twice as long as it does if you don't. If lubricating it doesn't fix it, the toilet may have more serious problems that rebuilding won't cure. But try that first.