Brake issue

[fattboy123456]

Jumped into my truck yesterday and noticed that when I let my foot off the brake the truck wouldn't move
(the ebrake was off)
As I tryed to figure it out scratching my head i believe that when I press the brake and the system pressurizes and when I take my foot off the Air is not being released
(im not hearing it)
I would get out and open the valve on the back of the truck for the service brakes a gush of air would come out and the truck would relax and would be able to move until I press on the brakes again
My question is what controls the releasing of Air when you take your foot off the brake is it just the brake peddle valve or is there another item that releases that

Thank you

 

[gimpyrobb]

We had this issue on a truck we were working on. It turned out the plastic air line had got hot and melted shut. Once we replaced the air lines we were good to go.

The vent lines are no fun to trace, good luck.

 

[Floridianson]

Do believe there should be a quick release valve somewhere in the frame work if your is a 939 series.

 

[fattboy123456]

I will check those 2 things out thanks for the suggestions

 

[98G]

The front service glad hand has a vent that is likely stuck.

 

[98G]

The front service glad hand has a vent that is likely stuck.

.

 

[gunsgreg]

The front service glad hand has a vent that is likely stuck.

That is exactly what was wrong with mine. X2...

 

[fattboy123456]

The front service glad hand has a vent that is likely stuck.

OK so with that info I checked my front gald had Which I moved to the back of the truck because there was none in the back the day before i had this problem and caped the front off because I picked up a M989a1 Trailer and I needed the glad hand in the back to hook up You where right since in removed the glad hand and caped it off it couldn't vent I had no clue there was even a vent on the front glad hand!

Thank you for you help guys!

 

[Floridianson]

Yea have to agree but the glad hand does not do the quick release of a air brake system. It has to exhaust fast so as to release the front and rear brakes. Thought the front had a quick release and so did the back. That how it was on my civi dump truck

 

[Floridianson]

Good thing you found it. It has been awhile since I went over the air system on a 939 series. The front glad hand has to have an open vent but it also has a check valve. Air can go into the front glad hand when it is being towed but not come out when the brake peddle in the truck is being use and not towed. Believe with all the valves that are on the truck when the brake peddle is applied the front one way valve closes, rear service works and brakes get applied. When the brake peddle is released the charge that is in the brake peddle system is then routed out the front glad hand and the brake chambers are vented through the quick release. If the charge in the brake peddle is not released ( front glad hand vent) then the quick release for the chambers can not dump.

 

[JohnnyBM931A2]

It certainly is an interesting setup on these trucks.

 

[Floridianson]

It certainly is an interesting setup on these trucks.

Yep that's why I love 809 series. The 939 series is a fine truck but!! As 98G posted the first thing to check is the glad hand. That info needs to be a sticky if it is not already. No thread just the facts mam as Joe Friday would say.

 

[M35A2-AZ]

I found out the hard way about the service glad hand on the front, my truck has a ball valve also on the service glad hand and I turned it off after washing the truck. The brakes started locking up on me. After about three weeks of trouble shooting it I found that the valve being closed was the problem.

 

[fattboy123456]

Thanks again guys for all the info that was my issue just drove the truck 400 miles to day to get a M989a1 trailer (again) and it worked perfectly!