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!

[newstang79]

The old style trailer brake controllers T'd into the line like that.

[Already Gone]

Back in the day before trucks had lots of electronics when you wanted to hook up a trailer brake to had to physically cut into a brake line and run a hard line to the brake in the cab of the truck so I'm pretty sure that is what the t-fitting is for.

[FirstOwner69]

Originally Posted by 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!

It probably is a place to hook up a trailer brake controller. I don't think, however, there was a brake line to the trailer. I think the controller probably read the pressure as a way to vary the voltage to the electric trailer brakes.

[davepl]

Crazy. Guess it worked though! Given the complexity of things like cruise control and air conditioning I'm surprised they couldn't come up with an inertial controller by that point. They made it to the moon using inertial guidance, so you should be able to stop a trailer :-)

[SS Tim]

Yep, An added tee to allow routing brake fluid to a controller that in turn sent an electrical current to the trailer brakes. Easy to install and easy to use. Funny how many trucks still have them with the controller pulled off and even after a M/C change. If you look through truck handbooks from the 70s you will see warnings about not using trailer brake systems that tap fluid from the master cylinder or more than "x" amout of fluid. Just changing technology.

[FirstOwner69]

Yup, a change in technology that was, in part, prompted by the advent of anti-lock brakes. Interesting to note is that new GM and Ram trucks with the integrated brake controllers utilize an inertia system. I've heard that Ford relies on a brake system pressure reading. Frankly, I'd have more confidence in the Ford system.

[Fitz]

Originally Posted by 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!

Probably hacking by a previous owner.

[motorcritter]

Most of the trucks I run across have a trailer brake set-up like that. If there's not a little box with a lever on it under the dash between the parking brake handle and the steering column, there'll be a line from the tee fitting to it. I've got a collection of them in my storage area in the shop AKA 'house of horrors'.

[GASoline71]

My '72 GMC was the only truck of this vintage I have owned that did not have a brass T-fitting in the brake line for a trailer brake controller. The '67, '69 and my Dad's '70 all have that fitting installed for a brake controller. I have installed a few of those myself in the past.

Gary