Do you have a picture of the proportioning valve?

Sounds like you have a plan now.
But I'm curious about this plunger thing. I have a GM normal looking valve on my truck, and have never had a problem like you have, nor have I noticed a place to push and re center the valve. Sounds like in the future I'll use the kind I already have, and avoid the kind you have, but what does yours look like?

Thanks,
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-