Air backing up through dash valves when setting brakes

I've hunted the internet for people having similar problems, but only found a few clues with no real closure with their problem, so I guess I'll start my own thread. Maybe this will help someone in the future too.

I'm working on getting a 1981 Peterbilt 359 ready for inspection, and on the most recent test drives and to get new tires, I have a quirky problem with setting the brakes. This wasn't an issue the month prior when I drove it to get new windshields in similar cold weather.

When setting the brakes, there is an excess amount of time, and flow of air back through the dash valve for the brakes to set. At least a steady 10 seconds of air flow and delay before I feel the brakes grab; not the instantaneous and crisp "Psht!" and immediate brake grab you should have when you pop it.

Most signs seem to point to quick release valves, and I have replaced two of the standard quick release valves (3 port), with the same symptom. I was starting with simple first, but I will be swapping both anti-compounding quick release valves (4 port) as well. I have them on hand, but it's 14 out right now and would prefer to not chance cracking a line or causing more damage in haste. Cold weather does not make a cool head when making repairs.

So in the meantime I figure I would pose the question that if the next course of action does not solve my problem, where should I look more closely next? The dash valve does not leak when supplying air, it's not loose, and otherwise works fine as far as I can tell.

I certainly appreciate the input or experiences, stories and lore of similar.

 

Ran out of patience and decided to deal with the windy 20 degrees today and swapped the anti-compounding quick release valves. Aired up the system, and there was an immediate difference.

Even at only about 90 psi, as soon as I released the brakes and got out of the truck and walked to the tandems, I could rock the truck with little effort (I'm about 130lbs for reference). Before this, even when the system was near 120 psi before a drive and brakes released, it would take a minute or so with the brakes cold and a little light rocking before I felt they were comfortably released. I try to give ample time on brakes and old trailers to air up, especially in the cold and/or after sitting.

Now when pulling the valve, there is a more distinct release of air externally, and only a minuscule bleed off from the dash valve. Not the extended hissing from excess pressure, and I cannot rock the truck within seconds of setting the brakes. I didn't wanna fire it up in the cold just to do a roll test in the driveway, so I'll be checking it closer when it warms up and I take it for a drive. A little engine power/momentum would make sure the brakes are setting quick enough.

 

I can only say the tractor as I've yet to hook a trailer, but it seems you could have a similar problem to mine with quick relay valves, only on the trailer.

If it only does it when the trailer brakes are set, but ceases when you pull both of them, it may be escaping through the tractor quick release valves instead, if it cannot exhaust through the trailer valves. When the tractor brakes are left released, it could reach the dash valve since the air circuit is theoretically open.

I'm still trying to school myself and ingrain what activates what, where and why. Found a pretty helpful chart, but it's too big to attach here.

 

Definitely nice to have some confirmation on this in the future.

With the fairer weather and further air system diagnosis, I did find a leaking brake chamber with brake pedal applied. But - it did not present itself until after all quick release valves were changed. Seems this was a combo failure with varying symptoms.

Brakes did not leak, but dash valve did. Dash valve stopped leaking, now a brake can does. Perhaps the proper allocation of air via valves was able to fully illustrate the weakness of a certain brake chamber.

Not sure which one it is yet as I did not want to clamp hoses in the cold, but I have a new spare and I will just have it changed out by whoever I get to do my inspection. Not real inclined to deal with brake chambers just yet.