Guys, im looking for some inspiration on this issue,
This is not un-common with certain Cat crawler tractors i have to admit, some you never look at and others you live with more than your wife, this is my issue detailed below and some advice would be welcomed.

The a/c system is a sanden rotary compressor , through a large tube & fin condenser with stacks of air flow, then through a small inline filter drier into a Orifice tube, through the tube direct into the evaporator ( tube & fin) out and return to the front of the machine through a accumulator and exits up and over the engine by rubber hose pipe work to the compressor.
That's the flow path, issue is this, system is correctly charged as stated by the manufacturer RED DOT, on running the system performs as expected normal high and low pressures, however once the machine has been working at full revs for some time the high begins to drop slightly from say 10 bar to 8 bar ( ambient 22C) and the low goes almost into vacuum thus tripping the LP switch at 3 psi.
Once this pattern of pressures start there is no turning back, the compressor kicks back in at around 10Psi suction pressure only to cut out again in less than a minute due to the LP switch.

I have changed orifice tubes, tried bigger and smaller bores, placed a rec drier in with a sight glass and we have liquid visible, i have blown the pipes through and removed any residue of oil build up ( cats are poorly laid out and the pipe work goes up and down with a round a 3 foot fall in places.)

Its as if there is a restriction, but some days the machine can work for hours with no problem, then for example once stopped to fill with fuel and start again the a/c blows hot and no compressor activity from the LP cut off due to suction pressure.

The only thing of note is the accumulator frosts up and the suction pressure drops, originally i was thinking it was poor routing of the discharge pipe at the back of the exhaust re-boiling the already condensed refrigerant so moved the pipe work but no change ( hence the inclusion of a filter with sight glass as opposed to the filter cat use with out.)

bigger or smaller orifice tubes makes no difference, reducing the charge by 500gr ( charge weight is 2300g) only results in poor evaporator temps, so i am at my wits end with this as i can not see what is the issue causing after a period of running the suction pressure to drop.

Its worth noting as the suction pressure drops the evap temps rise but liquid is still visible in the sight glass... what am i missing as i fear i have forgotten a basic principle and i am looking for a atom within a atom because it cant be simple???
Nothing is easy to get at, the heat generated around the engine area is immense so its not like a commercial fridge system were you can access all pipes etc, and as said not all similar machines suffer the same problem, maybe out of 20 i look after there are 4 that do this.

Any pointers will be greatly welcomed as this machine is making me look a twat TBH.

Regards

Pat