O/T Hey truck/trailer mechanics

Got a problem with my trailer air brakes. They sometimes won't release, other times they are fine. These are the old style single chambers with no spring brake. There's only two things between the pedal and the brakes- the tractor protection valve and the quick release (relay) valve. I pretty sure the relay valve is bad, but wanted some other opinions. Can the TPV cause this?

(quoted from post at 05:18:35 06/19/11) Got a problem with my trailer air brakes. They sometimes won't release, other times they are fine. These are the old style single chambers with no spring brake. There's only two things between the pedal and the brakes- the tractor protection valve and the quick release (relay) valve. I pretty sure the relay valve is bad, but wanted some other opinions. Can the TPV cause this?

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here could be a few things wrong. slack adjusters, S cams could be siezing up, truttle valve,your quick release valve. I have had all those thing not make the brakes release??????

Problem is definately air related- all 4 brakes do it at the same time, and all hard parts are good.

(quoted from post at 05:53:16 06/19/11) Problem is definately air related- all 4 brakes do it at the same time, and all hard parts are good.

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f all four brakes do it at the same time more than kikely it is a valve. Just follow the air lines back to front and find the first valve and check it out, then carry on to the next. Did you say you had maxi pots or service pots?? Another thing the truttle valve is actually your brake petal valve I have had them do that before

yes: hauled a old 1970 timpte trailer for 20 yrs. it has one line per pod from relay valve. had to replace one in the 20 yrs. brakes wouldn't set/release when pushed/pulled dash valve. if you release/drain air from tank the brakes would be released. you could try to run some air dryer thru lines. depending on age, just replace valve, plus check all lines and chambers.

The TPV shouldn't stop them from releasing. It's only designed to shut off flow from the air system on the tractor to the system on the trailer in the event of a air leak, break away, etc. This would cause a sudden pressure differential between the lines on the truck and the lines on the trailer which would shift a spool in the TPV and shut off the air to the trailer. In other words it should have nothing at all to do with the brakes releasing. It might keep them from getting air to apply but not the other way around.

On the other hand the combo relay/QR valve typically is designed to use a small volume pilot signal from the treadle valve to shift a spool and in turn send a larger volume of air to the brake chambers from a seperate tank located on the trailer. The pilot and delievery sections of the valve are seperate. As such, if the spool in the combo valve is sticking, it will continue to keep pressure on the brake chambers instead of shifting to dump pressure and allowing them to release as they should, even when pilot pressure from the treadle valve is released. This sounds to me like what is happening in your case. Good luck.

That's the way I understood it. And from what I can tell, since I have the old style brake chambers, I will have to use a 7 port relay valve so the trailer tank air pressure will set the brakes when the supply from the tractor is removed. My valve is so old that I'm having trouble finding one. It's very similar to a Spearco A1000, and it seems it will interchange. There's tons of valves that will fit, but they lack the secondary supply port that I assume releases the park feature when tractor supply is applied. I'm gonna take the valve off today and look it over for some part numbers. It has the separate brass valve in the center like the Spearco, so that might be what it is.

Sealco, not Spearco. Sorry

Did a search and found this site for you. Looks like they have pretty much any style air valve you might need.
Look here

Sounds like a bad relay valve... just for fun tho... unhook the service line from the tractor when the brakes fail to release... see if it exhausts back and releases then? Just wondering if there isn't an obstructed line or problem with the treadle... but if the relay stays engaged with the service line unhooked I'd say you have a bad relay.

Rod

I have some trailers at work that will sometimes release almost immediatley one day, the next day I have to wait a few seconds up tp a minute or more. Nothing unuaual, just a little sticky every now and then. Try backing up just a tad when they wont release.

How'd you do that? This website won't let you post a reply without typing something in.

Probably should have tried that before I took the relay valve off- it is a Sealco A1000. Ordered one today online- about $58. Old one was pretty crusty, so not a bad idea even if it's not the problem. This thing stopped me twice on the way back from a pull on Saturday. Rolled up to a stop sign and could hardly pull it through the intersection before pulling over. I could get them to release if I cycled the dash valves once or twice. I wouldn't think the treadle is the problem- just the trailer brakes drag. I added trailer brakes to this truck- It was a box truck before I shortened it and added a 5th wheel. Not a hard job- followed the Bendix manual and plumbed in a TPV. If the relay doesn't fix it, I won't be pulling this weekend.

he over used his quota. lol