[xj-s] A/C Overcharged?

I had my A/C serviced a few months ago, new rear seals in compressor, new
drier and new R12.

This cost NZ$200, about US$130. Since then, the cooling performance has
been less than I expect, and the expansion valve hisses quite loudly all
the time.

The evaporator outlet temperature never gets to freezing, and the
compressor thermostat never shuts off. There is no frothing at the sight

Are these symptoms consistent with a system with too much refrigerent? My
theory is that the low pressure side has too much pressure, and this is
limiting the low temperature of the evaporator.

Is there any way to check this without gauges? What should the low
pressure side read?

In reading the factory manual, I discovered that section 82.00.00 which is
the general description, and fault finding of the refridgeration system has
been lifted almost unchanged from the V12 E-type manual, which explains a
few peculiarities, like testing the system at full cooling with the heater
turned on!