I was working on burn out compressor (semi-sealed, CARRIER-06D) DX system for a 20 ton package unit of carrier model no.50DP020, I REMOVED the burn out compressor, flush the condensor coil from liquid line and evaporator coil from suction line with ***** 22 ittself without disconnecting the metering device (Distributor type, 7 capillaries with common header from liquid line to evaporator inlet). I replaced the same type of compressor with new refurbished one, replaced the filter-drier (standard shell type) , pressure test the system, leak test, vaccumize the system, sweep the system with ***** 22 (5psig) and re vaccumize the system, Break the the vaccum and charge the refrigerant-22. And, First run the system for half an hour and did the oil test for acidity in system, found the acid is present in system (very unsatisfactory).
I removed the compressor oil & filter-drier and re flush the system condensor and evaporator coil from liquid-line without disconnecting the compressor suction and discharge service valve.
While flushing the evaporator, I flushed it with R-22 as from the npt fitting port located above sight glass of compressor crankcase without disconnecting metering device i.e. (capillary type-distributor) and release the pressure from liquid-line (drier removed). Then, I again flush the evaporator from the LIQUID LINE and release the pressure from npt fitting port located above sight glass of compressor crankcase.
I did the above procedure thrice, because when I check the oil for acidity with the help of acid test kit, I found the acidity present in system.( the colour of test kit solution turns yellow twice and third time it shows orange colour of test kit solution, which is unsatisfactory ).
Note: The drier and oil was replaced on every test of acidity in oil.

The fourth time I flush the system ONLY from a liquid-line with R-22 using liquid refrigerant and release the pressure from the charging port given on suction line, before suction service valve of compressor.

I replaced the filter-drier, charged the oil in compressor (4gs) half level of sight glass. Pressure test system, check for leak, and vaccumise the system, after vaccuming, I break the vaccume with refrigerant till (120 psig standing pressure and started the unit.

When the compressor starts, I check the gauge for charging refrigerant in system. And , I found the gauge was showing 120 psig in running condition for 5 to 7 seconds as soon as compressor starts, and suddenly pressure reduces to 0 to 2 psig (the LP switch was by-passed to charge the refrigerant) and discharge pressure is 150 psig . I check the refrigerant cylinder valve, manifold gauge valves are open properly & charging port access valve (swivel core type) is ok. Also check the filter-drier was ok, but replace to clear the doubt for any blockage, but still the problem is at it is.

Now, I would like to know, what could be the reason that the pressure is suddenly dropping to 0 or 2 psig.

1, Is there any blockage in capillary, distributor, or ?
2, Is there any blockage in suction filter of compressor or?
3, Is it something else other problem.

WHAT IS THE RIGHT & EASY PROCEDURE TO TROUBLESHOOT IT?
Pls help. !!!!!