A/C 101 revisited

[Kneale Brownson]

As Alan Cordeiro suggested when I posted about my A/C test gauge reading
Zero pressure on the high side, I learned today I didn't have the hose
connected tightly enough to the high pressure test valve.  

So, getting back to my original status, I have about 45 pounds pressure on
both high and low sides of the system when it's cold and static.  When I
run the engine with the CC head set to "auto" and the temp at "lo", the
compressor clutch engages and disengages every few seconds and the pressure
on the low side swings between about 10 and 30 pounds.  The high side
swings from about 50 to about 75 pounds.

If I read the Bentley right, my low pressure switch is opening and closing
at the proper pressure range, but the high pressure side is way low.    Can
I add refrigerant through the high pressure side?  The Bentley says
refrigerant can be added to either, but I always understood it should be
added through the low pressure valve.