I have a Carrier 12 ton split system. Original issue was bubbles in the sight glass, SC and SH also indicated low charge.

Charged system to clear sight glass. Ambient temp at troubleshooting time was 65-75 and minimal load in the zone.

Before initial charge of system, head pressure was at approx 160 psi and suction was 75-78. Motor Master type c/fan control (Hoffman) was running c/fan at full speed even at lower ambient when initially started troubleshooting based on the LL temp (as per the manual, it goes to high speed at LL temp over 80 degrees).

After charge, Head pressure increased slightly (but still had c/fan running at full based on LL temp), but suction pressures stayed about the same. This is odd, since everything indicated low charge except the suction pressure at initial beginning of troubleshooting

What is also odd is that as my LL temp increases, my head pressure decreases and as the LL temp decreases, head pressure increases.

There are 2 non adjustable TXV's for the stacked evap coils and they seem to be metering properly and do have temp/pressure drops through both TXV's (indicated by temp drop across TXV). I will do diagnostics on those today to confirm though.

I doubt that there are any non condensables, as my head pressure is running more on the low side, even when ambient was closer to 75. I did have about 29 degrees air temp diff across condenser coil, but only about a 13 degree drop across the DX with both coils active.

We do have re-heat coils which is more of what we use to maintain temps. Cooling is more to help maintain humidity levels since we have no other source for de-humidification, so there is an artificial load when temp is low enough in suite to activate RH coils.

I did pump down the compressor using the service valves, and was able to get it into a slight vacuum, but pressure did slowly begin to climb every time (pumped down 5 times), however, I have had service valves leak by in the past around here, so I am not ready to condemn the valves just yet, unless it corrosponds to the questionable LL temp/pressure issues.

I cleaned the snot out of the condenser coils, which are 2 coils sandwiched together. I have not yet pulled the top of the C/U to seperate the coils and do a better cleaning. This unit runs one of our clean rooms, and can't shut it down completely to do that for another month.

I will update with more specific information relating to pressures, temps, SH & SC, but was more curious about the LL temp/pressure issues to start with as I can't really do much more until I figure out what is causing that as I can't get the c/fan control to control in low ambient with out switching to a pressure switch...