Air conditioning trouble

I'm having a bit of trouble with the a/c on my 400i and was wondering if a few of you might know the following answers:

1) Anyone know how many pounds of freon it is supposed to hold?

2) I suspect the high & low pressure switches may not be operating properly. Over 2,000 or so rpm the a/c blows cool but at idle the compressor won't come on.
Does anyone know a crossref for these switches?

I sure appreciate the help

Dave
'80 400i

[Pcar928fan]

What was the final outcome and cost on your A/C repair?