1/4" tubing to the back. It connects to a fitting where it changes to the rubber hose which runs to the tee on the rear axle.

9/16" - 18 refers to the threads in the combo valve. So, when you buy your 1/4" line it should have a fitting with 9/16" - 18 male threads to fit the combo valve.

All the CPP instructions (for all their products) can be found here: https://www.trifive.com/forums/showthread.php?t=34212

Wish they didn't call a combo valve a proportioning valve; it's much more than that. They also call an anti-roll bar a sway bar...

Gord 🇨🇦
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1954 1/2 ton 235 4 speed