Working on a Randell Model 8411M, 111" long, 4 lower doors and full length raised rail. Original tag says 134A, but has R404 comp and tev's. Bottom works fine. With the bottom stat turned off, so only the Top Rail runs: Top rail only frosts the first half of the serpentine circuit. Fresh charge, dry, solid liquid, 10 degrees superheat, Suction Pressure 22#/-15degrees, head 240#. Pump down to 8".
When I turn on the lower air coil unit, the suctions raises to about 65#, and the bottom refer cycles normal, but the top rail never freezes past past the half way position.
I would like to see more pump down, but the capacity with the bottom refer turned off, should be enough to run the rail. Any thoughts?