I just bought an 85 lx hatchback, the ac blows moderately cool. Where is the low pressure check valve located? I want to add coolant and check pressure levels.

There is a check valve located right at the ac compressor were the main hose moves back towards the fire wall.  Is that the check and fill valve?  This cars got miles on it but it has had an extensive service then sat for a while before I bought it.  I think it is just low on 134a but want to be sure.