Towards last fall it seemed like my house A/C wasn't running as well as it usually does, but the hot days ended and I haven't messed with it until now. Today I started it up and the delta T between the return air and closest vent is around 13.5 deg F. After running for 10 minutes or so, I measured the outside suction line temperature and it is not very cold - 77 deg F. It is a 19 year old system, but are there any tests that a non-HVAC person can do to see if it is just low on refrigerant or if the compressor has weak valves or something?