What the heck is this brass fitting for?

[davepl]

While Jim Pontius has been helping me sort out the various braces that mount the 72 3/4 ton disc setup, I found a reference photo that shows a brass fitting on the master cylinder that I have NOT installed yet. Actually, I have the fitting but didn't know where it went until all the digging.

It's marked as

K/H (D)
78497

Googled that many ways to no avail.

Male port goes into MC, line from combo valve into female port, and an unused-in-the-picture small (3/16" ?) port on top. Remember that on the 3/4 ton the front is the rear brakes and vice versa, so this would be on the rear brake side of the system.

It's not a residual pressure valve, as you can see straight through it.

So, anyone know what the heck this T is for? Heaven hope they never made a hydraulic trailer brake setup that took pressure from the master cylinder of the tow vehicle!