Happened to me yesterday. A previous installer set the Cooling Coil and ran the line set , electrical, etc... and all it needed was the Condensing Unit outside. I install the Unit and fire her up only to get pressures reminiscent of bad compressor valves (very high suction and very low discharge), with lots of liquid returning to the compressor. Tried pumping the system down, and it wouldnt go into a vaccuum . So, i unscrewed the liquid line Brass Coupling, and wullah....no piston inside ; how nice of the previous Installer.