had a service call on a 3.5 ton bryant (R-22) it was sched, as a tune up but it's been pretty hot lately and wasn't cooling that good, I found: 17 degree temp drop across coil, high head pressure, but without the high discharge air temp, liquid line was around ambient temp, 86 deg, head pressure 300, suction 35: I pumped the unit down, opened up the piston, cleared the blockage, oil was spitting out from the evap side of the piston connection, I flushed out the lineset & coil with nitrogen, replaced piston, evacuated, opened up valves and restarted. system pressures were now close to normal. superheat was 15-18 deg, subcooling was still high, around 30 deg, pressures were 72 over 225, and my drop was still 17, deg, at one point the compressor almost didn't start, amp draw was less that half, of rated amps, the unit also didn't pump completely down in itself before clearing out the piston (forgot about that) I tried to pump it down again, and would not pump down, relief valve was hissing at 35 psig. so I comdemmed the compressor, non of the distribution tubes were plugged, coil is clean, airflow is good, etc, this all happened several days ago, so it's not fresh in my memory, I'll now if I mis diagnosed it when I change the compressor next week, also, the unit is only 2 years old. I've also investigated an overcharge in unit but with the low suction , and no floodback , made be beleve it not to be.