Auto Parts: air conditioning jeep wrangler, r 134a, jeep wrangler

Question
I have a 1992 jeep wrangler. I am trying to retro from R-12 to R-134A.
I emptied all R-12 vacuumed system installed R134A.
I have about 44 oz's in system now with no low side pressure and 150 on the high side.
I can see movement on the sight glass.
The low side varies from -20 to about 15lbs but mostly stays on vacuum.
Is this maybe the drier?

Answer
    The drier should be replaced during this operation.  any time the system is opened the drier should be replaced.  But the problem you are experiencing is almost certainly the expansion valve.  Neither piece costs very much (drier about $45, valve about $18).  The valve is a very small orifice with a tapered needle in the center.  The needle moves in and out according to the temperature in the evaporator core, regulating the amount of cooling to keep the core from freezing up.  It is a catchall spot for gunk and debris in the system.  Changing the refrigerant will generate a small amount of debris just from the chemical reactions of the old oils with the new refrigerant.  So you will need to change the valve.  And the receiver/drier, which also acts as a filter.  In your case you might want to consider buying an aftermarket inline filter for the liquid line and install it between the receiver/drier and the expansion valve.  I have use these in the past with much success.  You will have to use a crimp-on version of the filter with R134A.  A local parts store or truck service shop should be able to do the crimping for you, it's a particular type of crimp that needs doing by a machine.