[Doghouse]

My hand brake was disabled many years ago, and I am trying to reinstall it. I found where the unit gets its air supply from (an aluminum bar with brass fittings behind the dash).
The problem is that I can't find where to pipe the air back into the system so it operates the trailer brakes.
Does anyone know where to do this?
I went to the dealer today, and they didn't know.

[bob h]

delivery goes to the service port of the tractor protection valve. supply comes from the front axle air tank

[Kranky]

Don't forget the double check valve, or when you step on the foot brake, air will exhaust thru the exhaust port of the hand valve, and when you apply the hand valve, air will exhaust thru the exhaust port of the foot brake valve.

[Doghouse]

Thanks guys,.....but where is the tractor protection valve? and which tank do I use if they are both placed next to each other?
I'm getting closer on this, but I sure don't want to mess it up ops: .

[bob h]

Originally Posted by Doghouse

Thanks guys,.....but where is the tractor protection valve? and which tank do I use if they are both placed next to each other?
I'm getting closer on this, but I sure don't want to mess it up ops: .

we don't want you to mess it up either... we share the same roads ; )

you know... brakes are very important, and if you don't understand how the system works (which you have demonstrated in your reply) ... i would recommend that you at least have an experienced tech working with you... imo

[Doghouse]

Thats why I ask questions and do research. Thanks for your opinion.

[RockyMtnProDriver]

Originally Posted by bob h

delivery goes to the service port of the tractor protection valve. supply comes from the front axle air tank

In a dual air system ( trucks built since the mid 1970's), supply comes from blended air. It would probably be feed into a manifold from a two way check valve between the primary and secondary air.

If you hook it up to the front axle air tank (secondary air in most systems) in a dual air system, and if you loose that tank, you will loose brake control to the trailer via the hand valve.

Get it hooked up right, by someone who knows where to plumb it for the truck you are driving.

[bob h]

Originally Posted by RockyMtnProDriver

In a dual air system ( trucks built since the mid 1970's), supply comes from blended air. ....

That is incorrect ; blend air via a double-check valve is used to eliminate spring brake dynamiting if one of the 2 supply tanks falls off the truck (ie - loss of primary OR secondary air).

The hand valve circuit is for trailer "service brake" signal air.

In a typical tractor air brake system, the rear axle service circuit is fed by primary air for the service signal as well as supply air. The secondary system provides front axle supply air (as well as signal air if a relay is used).

The secondary reservoir also feeds the hand valve ; in the bendix diagram I'm currently looking at, the secondary line feeds the hand valve as well as the supply for the antilock traction relay (... don't think they were using those in '74 ;0).

[RockyMtnProDriver]

Originally Posted by bob h

That is incorrect ; blend air via a double-check valve is used to eliminate spring brake dynamiting if one of the 2 supply tanks falls off the truck (ie - loss of primary OR secondary air).

The hand valve circuit is for trailer "service brake" signal air.

In a typical tractor air brake system, the rear axle service circuit is fed by primary air for the service signal as well as supply air. The secondary system provides front axle supply air (as well as signal air if a relay is used).

The secondary reservoir also feeds the hand valve ; in the bendix diagram I'm currently looking at, the secondary line feeds the hand valve as well as the supply for the antilock traction relay (... don't think they were using those in '74 ;0).

Bob, your wrong.

Yes, you are right, the hand valve is for trailer brake service brake application, but the air is delivered to the back of the hand valve via air from a two way check valve that connects between the secondary air tank and the primary tank.

Once again, the reason for this is that if you lost the secondary air reservoir then you would loose the ability to apply the brakes on the trailer if it only came from that tank. That is why they use blended air, so that in the event of loss of either of the tanks, you would still have the ability to apply the trailer brakes with the hand valve.

We don't use schematics to teach air brakes, we use an operational air board. And I have been teaching it this way for over five years, and was taught to teach by one of the leading experts in Air Brakes in North America.

And every manual I have ever seen shows it to be that way.

[RockyMtnProDriver]

By the way, ABS was available in the mid 1970's.