I gather this is a gasser, it's important to know that. At idle, manifold vacuum is very high. When you stomp on the pedal, it approaches zero. The heating system reacts to zero vacuum by supplying max heat. You have two components which keep this from being a problem: check valve 31 and vacuum reservoir 33. I would guess that either the check valve is bad or one of the rubber connectors is leaky as has been mentioned. To help you along, here's a photo of the vacuum reservoir, it's inside the left fender:

http://www.coolcatcorp.com/190Restoration/Mercedes23.jpg