Can the sight glass be used with an R-134a system?

I've got R-134 put in my 300D along with new O-rings and whatnot, and I'm thinking the system may be low...but my aux fan isn't working, so that may play a part...


Thanks!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999