I'm pretty sure my mono valve has a leaking gasket. Since there are no kits for this model 124, 603 I'm wondering if just any mono valve will work? I've noticed that the price for differant ones are cheaper and have repair kits sold for them. So what makes them differant from one model to another. why can't I use one out of another model?

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Don
87 300D
485k km

I have both the 123 and 124 and you are right, the 124 monovalve is definitely different (the 124 actually looks cheaper). The later 124s combined the monovalve and water circulation pump but the '87 124 has the same separate setup as on the 123. I don't know why the prices should be that different. You might want to check to make sure you are pricing the monovalve alone as opposed to the later combined valve and pump.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold