OK, I thought air conditioning systems have both a low side and high side, with appropriate taps for monitoring the system's pressure. Now, I had a new to me engine put in the 300D, but cannot find the high side tap. The low side is obvious, but I can't for the life of me find the high side tap. Is there one? The system has the GM R4 pump...

thanks to all...

__________________
71 220D 169K wrecked
83 240D 118K sweet 4 speed
91 350SDL (one of the 60% good engines) 156K
84 300d (loaner to my sister) 189K
79 300SD (partswagen)
86 420SEL partswagen
70 220d (partswagen)
68 280s GASSER!!! under construction now
85 300sd 310K miles winter beater car retired
93 300d 2.5 turbo 168K wife's car
83 280SL euro 5 speed 155K
69 250S newest project 54K

The high side valve is under the air filter just behind the alternator.