Brian, I tried using compressed air once to blow out water when I had one end of the block-to-firewall hose disconnected in a W123. It hissed and blew but I don't think much liquid came out, besides I don't think that is wise to hit the mono valve with any pressure like that or the rubber diaphragm may be damaged.
Some shops have a closed loop flushing machine that might do a thorough job, I just like to DIY my way, even if it takes half an afternoon.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!