You should locate the vacuum controlled water valve. It is in the cowl area more toward the passenger side.
Verify that vacuum control responds to inside controls. If vacuum is OK, verify the condition by using an appropriate device to squeeze the hose shut. This should shut down the water flow. If it solves all your A/C conditions you will have to replace the valve which is cheap (less than $20).
If vacuum is your problem you could cheat your way through the A/C season by running a straight vacuum source through a check valve to the heater valve. The point of the check valve is to keep the valve from opening when the throttle goes to the floor (and vacuum goes to zero).
Bosch Master, ASE Master, L1
33 years MB technician