I have a one year old $7500 snap on A/C recycler machine. When i first bought it i was told to buy a stone filter that goes between the R134 canister and the input line .The welds on the inside of the 134 canister’s have very small particles of slag . Well over time some one did not install the filter when they changed the freon and our recycler machine would not take in new freon.
Our local tool repair shop told me a trick . Remove the cover on the recycler machine and find the inlet solenoid from the new freon to the valve body unplug the voltage.
Rapidly energize the solenoid with 12 volts a bunch of times bam it worked.
Moral of story buy a stone filter so you never have this problem. Ollie