Mercedes Benz repair: can i bypass servo with a manual valve, mechanical valve, manual valve

Question
I own a 1980 300SD Mercedes and have no heat (no A/C either but don't care). Tracked problem to the servo and would like to know if I can connect incoming  hose from water pump directly to outgoing hose from servo (hose to fire wall to heater core) with manual on/off valve (so as to turn on valve the odd time I want heat).  Am concerned that in doing this there will be no flow of water to other side of engine (return hose from heater core)when valve turned off but does this matter in an MB (in an american car, water must flow continuously to cool other side of moter..dumb design)
Do not want to spend the money at this point on a servo but may later.  Help and thankyou from Canada (now you know why I want heat and don't care about A/C)


Answer
If you replace the mono valve with a mechanical valve, you will have heat above idle speed.
How you will regulate it is unknown.
Does your blower still work?

You could join http://www.mercedesshop.com/shopforum/forumdisplay.php3?f=15

Several members go junk yard shopping every month at pick and pull = very cheap high volume junk yards, and will get the part you need for a very small fee.