Welcome! Glad you joined.
Getting your grandfather's truck is wonderful! I say that because I'm a grandfather and would want one of my grands to get my truck(s).
You'll find the proportioning valve to be both expensive and hard to find. But I've not seen them leak, so maybe it is something else leaking?
Anyway, we are here to help and would love to see pics of the truck. But you might want to look at the page at Bullnose Forum/Forum FAQ's to see how to post pics as it isn't the easiest thing to do.
And we also have a map, which you can see at Bullnose Forum/Member's Map in the menu, and can add you with a city and state or zip.
Gary, AKA "Gary fellow":
Profile
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI