Operating the recirculation switch will lower that 40 deg reading a small amount. Sight glass observations are helpful but the system pressures tell all. I have seen many systems running at optimum with a sight glass with flash gas running through it. Make the electric condensor fan is operating at the factory set point and the condensor is clean. It looks like you are on the right track.

Where is the A/C high pressure port
 

Where is the A/C high pressure port for a 1990 Mercedes Benz 560 SEL (W126)?

Thanks.

Open the hood and look at where the grille would be. It's on the right of the condenser.