With the monovalve out of the holder, you can look down inside and see three drain holes around the outer edge.
If the outer seal is not seated and sealing properly, you will get a slow (or not so slow!) leak that will not be up at the lid with the screws.

If you only drain the coolant down until it is in the bottom of the housing, the surface tension will keep it filling the holes, until you either blow it out or mop it out with a shop rag.

 

This sounds logical. I will get back in there and see if the seal is intact. Thank you!

 

This happened to me last month!

As said, I suspect the seal, because I replaced the valve and tightened up the seal and it's fine.

I was amazed at how much coolant was lost, and suspect that since the seal wasn't perfect, the pressure was lost from the system which ultimately led to overflow...

 

I removed the monovalve again yesterday. I can see nothing out of place at all.

I begin to get a steady drip as I turn the climate control toward heat. When I click it to max, I get a steady stream.

I'm happy to buy a new monovalve but I don't see what difference it would make. Maybe I can cut a thin rubber washer to complement/thicken up the interface between the monovalve and where it seats inside the holder?

 

makes ya think about a bypass hose fabrication.....

 

Not just yet! I'm more determined to make it right. Always an interesting journey...

 

While you are in the vicinity, it may be a good time to have a look at the little aux pump under the mono valve, they tend to rot internally.
Cheers

 

I agree there. The one I pulled from my parts car was a rarely used socal car impoted to the Repbulic of Texas. The hoses, valve and aux pump looked like brand new, I hope I never need to do the rubber work to or from the heater core as that space is quite small for me.

 

WRAP UP:

Well, I installed a new Bosch monovalve. Leak fixed.

What bothers me is that I see absolutely no difference between the original and new valve. No torn diaphragm, nothing. I'm now wondering if I just didn't tighten everything down enough originally.

Well, no matter now. It's fixed, I have heat and no leak.

By the way, if you need a monovalve, Bosch have climbed from $28 in 2011 to about $100 now. And that's if you can find one. What a joke.