I had an issue with a package A/C unit this week. In cooling pressures were normal , switch to heating and with in 10 seconds unit running in to vac . My thought was the expansion valve faulty but these were accurators and all moving and seals all ok. Next on the list was RV , what a mission to change (1 3/8 tubing)! the boss said i ll order one and you change it. I thought about it and asked about the bi flow filter could be blocked in one direction , No said boss.
So thought , ill just change filter first so i cut out the bi flow first and replacing and vac down the system , charge it up and run it , sure enough it was the filter check valve faulty , i could blow through one end but not the other.
Just something else to check out when diagnosing i quess.
Cheers Andy