Found this proportioning valve at Carolina Truck...
https://www.carolinaclassictrucks.com/73-78-GMC-MotorHome-prop-valve.html
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'78 Palm Beach. car nut new to RV'ing 26 ft. 403 rebuilt in 2018, rebuilt tranny in 2014, Dave Lenzi knuckles & hubs, yada yada yada OEM chromed
wheels,Green Brady Bunch stock
https://www.carolinaclassictrucks.com/73-78-GMC-MotorHome-prop-valve.html
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'78 Palm Beach. car nut new to RV'ing 26 ft. 403 rebuilt in 2018, rebuilt tranny in 2014, Dave Lenzi knuckles & hubs, yada yada yada OEM chromed
wheels,Green Brady Bunch stock