If you have no vac at the recirc servos, you have either a faulty switchover valve, a leak, or loose hose, or a bad PBCU.
The system MUST hold vac indefinitely on the GREEN plastic line from the engine compartment. If it leaks down, engine on or off, you have some leaks. Vac will drop slightly when servos are activated, but must not continue to drop when they are pulled down.
52 F cooled air is probably correct -- the evaoprator temp sensor will cut the compressor off when the evaporator gets to 35 F to prevent freezing. Don't forget that if the monovalve bellows are bad, you will also have heat in the heater core at the same time.
Pull a vac on the supply line and fix any leaks that show up before condeming the PBCU.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!