Air compressors usually don't have problems with piston rings and cylinders. The three most likely causes of failure to build pressure are the pressure switch, the regulator (if you're using one) or the reed valves.
To determine if the regulator is the problem, disconnect any air tools from the compressor, and plug the outlet,& turn it on. The motor should run until the pressure reaches the high limit setting of the pressure switch, which is usually around 100 to 135 PSI.If the motor shuts off prematurely, then the pressure switch is bad, or needs to be re-set. If the pressure won't reach the preset pressure and shut the motor off, then either the reed valves in the pump head are bad, or there's a blown head gasget. On two cylinder compressors, the gasget can leak between the cylinders, and there may not be any noticeable sound of leakage. You'll have to take the head off, check for broken reed valves, and for signs of leakage on the gasket areas.

I assume it keeps pumping, but doesn't build any more pressure. That is probably bad valves or rings. If your unit has a crankcase breather, and it is puffing alot of air, look for rings. Otherwise valves.