W123 Monovalve - Building a replacement with new parts

[Demothen]

So I've still been fighting monovalve issues in my 85 300d. Primarily the issue is that at high coolant temp/pressure, the monovalve is not able to stop the flow of coolant to the heater core, meaning the heater overpowers the AC system.

Last night I swapped in a new MTC core, and so far it's working well but I don't really trust it long term, and i just want this problem to go away.

My plan is to pull the entire monovalve assembly and replace it with modern parts. I've found a 3/4 12V normally open solenoid valve made from brass that's seems like it's overbuilt for this situation: 3/4'' 12V DC Normally Open Electric Solenoid Valve 12-Volts - this is rated to 90PSI, 265F, and runs on 1.5a. I still need to check my existing monovalve to see how much current it draws, but I imagine worse-case I can just add a relay to the circuit to protect the logic boards. I'll also need to build an adapter to add a 1.5a fuse and convert from the Mercedes style plug to the standard spade terminals that the valve seems to use/


Next up is the check valve. The monovalve housing contains a ball type check valve that seems to be intended to prevent back-flow from the auxiliary water pump side of the block to the heater core. My plan is to find a brass check valve to replace that. Something like this:


Next is the adapters. I'm thinking I'll order a few 90 degree 3/4" NPT to 3/4" barbed elbows, as well as some 3/4" NPT to 3/4" barbed straight adapters, so I can work out the plumbing. I'll need to use teflon tape for the NPT connections, right?

Finally, the mounting bracket. I'll have to figure this out as I go, but I want to mount this using the same mounting location as the original monovalve used. I've got a welder and reasonable fabrication skills, so this shouldn't be too much trouble. I might need to get some bulk heater hose to deal with differences in port locations.

Does anyone see any flaws in my plan? I would think brass would hold up well to the coolant - I believe the stock monovalve has a brass filter element and other brass bits. I can't imagine the cooling system ever reaches anywhere near 90PSI, and it certainly shouldn't exceed 265F.

Of course the other alternative is to add a manual shutoff valve so I can swap between heat/cold as needed, but I'd really like to maintain the automatic climate control functionality, as I've spent a lot of effort rebuilding the rest of the system.