But not for long.

Those "121" systems as they were known back then were so problematic that nearly everyone removed the anti lock controllers and replaced them with standard non ABS relay valves.

By about 77 or 78 the truck manufacturers were again using standard non ABS brake systems, and ABS was gone until it was again mandated in 1998.

The system in use from 98 to the present is much improved from the earlier attempt, and causes few problems.

 

When the old type ABS failed sometimes it actually caused the entire braking system to become nonoperational or limited it so much they might as well have not been there at all.

It was a complete disaster.

Now when the ABS does not work, you still have brakes.

 

FMVSS121 is the current air brake regulation

 

Bob, try this experiment.

Hook up a tractor and trailer, make sure all the wheels are blocked and the truck is in gear, and do it on a flat ground.

Drain the secondary tank, this is where most trucks get their front service brake air from.

Push in both yellow and red dash valves.

Now make a brake application using only the hand valve.

Tell me if the service brakes on the trailer make an application or not. If they do, then the air must be coming from blended air. If they don't, then it will be coming from the secondary tank.

then do the experiment in reverse. Drain just the primary air, and see if you can apply the service brakes with the hand valve.

If they apply in both situations, then it must be blended air that is being fed into the back of the hand valve.

Or anyone else can try it also, and let me know the results.

I did it on my air board. And my air board is no different than any truck, except I do not have ABS on it.

 

And it was FMVSS121 back then too, but a vastly inferior system.

 

Agreed

 

I tried a '97 fld and a '95 9200 today; both units used primary air for the trailer hand valve supply.

As soon as I opened the cab door it was evident; 2 green lines and a black.

I drained the secondary first; trailer service was still applying. Then, I filled the sec and dumped the pri and no more hand valve application.

 

My apologies Bob, upon further investigation, I found out that depending on which system you use, it can come from Blended, Secondary, or Primary air.

Nice to learn something new each and every day. Even for someone who has been teaching Air Brakes for 5 years.

It just befuddles me why it would not always be blended air in a dual air system. All you would have to do is instead of taking directly from one or the other tank, you would take from a manifold being fed by blended air. It would require no more plumbing or parts.

 

In the case of the tractors that I tested, the primary from the foot valve and primary from the hand valve both go to a double-check (supply ports). Delivery from that dc goes to a supply port for a "blend air double-check", and the other supply connection comes from the treadle sec delivery. Delivery from that second dc supplies the service (control) signal at the tp valve.

... if that makes sense ?!?

IE - the trailer service signal does use blend air... but, NOT from the hand valve... *"supply to the hand valve"* was the original question

 

There should be a two way check valve between the delivery side of the Primary valve and the delivery side of the foot valve, and this then goes to the Delivery side of the Tractor Protection valve. So you would have a blended delivery signal to the trailer service brakes from the foot valve.

There would be another two way check valve that would connect that signal and the signal from the hand valve which would then give you a signal from both the foot valve and the hand valve, depending on which one you use, or if you where using both, in which ever one which had the highest PSI would then be the signal going to the Tractor Protection valve. In other words, if you where putting 40 PSI with the foot valve and 60 PSI with the hand valve, then you would have 40 PSI going to the service brakes on the tractor, and 60 PSI going to the service brakes on the trailer. Or if you where putting 40 psi with the hand valve, and 60 PSI with the foot valve, then you would have 60 PSI going to the service brakes on the trailer and the tractor.

Now of course, you need supply air going to the back of the hand valve. This is where we started the conversation. On my air board, I use blended air, but as we have seen, it can be blended air, secondary air, or primary air.

As I said, I don't understand why Blended air is not used always. Because if it is, and you lose either tank, you would always have supply air to the back of the hand valve and you would need to lose both tanks to lose application air to the trailer brakes.

Of course, if that where to happen, you have larger issues. Like your spring brakes locking up, or more than likely, the unit is upside down in the ditch.

I am sure there is more than one way to skin this cat, and it would depend on the manuafacturer and the valves they spec for the truck.

A long time ago, I stopped trying to figure out why they build them the way they do, and just usually test them to make sure that they are working the way they should.

I always remember the story about NASA and the $3,000,000 that was spent to make a pen that would work in Zero gravity.

The Russians took pencils.