R134a is a mixture while R12 (Freon) is a single
compound.  I have read that the sight-glass is only
useful with R12.

Also, the charge of R134a is typically ~80% of
R12 and should be calibrated by gauges and
output temps.

Good info at
http://www.griffiths.com/achelp/achelp1.htm

                                    Richard A. Jones
                                    Boulder, Colorado
                                    '81 Vanagon  Mr Bus
                                    '87 Syncro Westy    El Jefe