M1088 tractor air at spring brakes released with parking brake engaged

[Suprman]

Its an odd one. And honestly the brake system is not my specialty. 1998 M1088 tractor. Air blows out the front emergency gladhand, builds proper pressure with the offending gladhand blocked off. Was going to work on that issue eventually. Brake pedal/treadle works properly truck stops when pedal applied and it feels right. Parking brake engaged via dash control and spring brakes are still released. Followed the TM troubleshooting this morning. Replaced the inversion valve and anti-compound valve even though they appeared to check out ok per TM steps. Replaced park and trailer dash valves and air lines to the cab floor, plus verified they are hooked up properly. Treadle in-cab air lines are correct. Still no parking brake. No service brake air leaks. Kind of thinking the air coming out the front emergency gladhand is related to the issue and some lines are swapped somewhere. On to under the cab tomorrow.
Will

 

[DiverDarrell]

Dang that could be tough to sort out. At least you have other trucks to look at

 

[Suprman]

Im lucky I noticed it. Was a non runner came caged so I had it chocked. Got it running nice and then noticed the caging bolts hanging loose. Took a second of head scratching and looking in the cab at the dash valve to figure out what was going on.

 

[NDT]

Maybe the check valve behind the front bumper has rust and is frozen open. I did a thread on replacing it. Kinda hard to work through the bumper openings.

My thread: http://www.steelsoldiers.com/showth...mergency-gladhand/page2&highlight=check+valve

Also this thread: http://www.steelsoldiers.com/showth...at-rear-gladhands/page3&highlight=check+valve

 

[DiverDarrell]

I had to replace all of mine up there. Will's brakes still work, could be a small crack in the diaphragm leaking air, but not enough that the compressor can't keep up with.

 

[Suprman]

I temporarily blocked off the front emergency gladhand. Used a piece of plastic and put the cover on. Still leaks slightly but not much. Was going to work on that issue but then noticed the whole no parking brake thing and that too precedence. I don't think air could feed back thru the emergency line though and keep the brakes released.

 

[NDT]

Ok, just so I'm clear on the symptoms: with zero air pressure in the tanks, the spring brakes are engaged. Then, if you start the engine, as air builds, the spring brakes release REGARDLESS of the position of the dash parking brake control?

 

[Suprman]

Yes the spring brakes are released regardless of dash valve position. Treadle valve functions as intended and there is no tm troubleshooting left for this.

 

[NDT]

Symptoms point to the anti-compounding relay valve stuck such that air can freely flow to the spring brake chambers regardless of what the dash valve is commanding it to do. I know you said you replaced it. That is what is confusing. Could the replacement be stuck as well? Not likely.

 

[Suprman]

I replaced it off a parts truck but I know the parking brakes had worked on that truck. I opened up the original valve it did seem a bit stuck but freed up easily. I can swap that one back in tommorrow.

 

[Andyrv6av8r]

This may be a long shot, but here goes. I found out in a most horrifying way, at least on my 1078, that if you apply shop air to the front emergency glad hand with the parking brakes applied, they release. Luckily I had my cell phone to call the wife to pop the air hose lose! I assume in a tow situation that this automatically releases the parking brakes. Now maybe you have a check valve leaking by somewhere letting air make it's way to the emergency glad hand line or an air line hooked up wrong letting the spring brakes being released as air builds to pressure. Just a thought.

 

[scottmandu]

Yes the spring brakes are released regardless of dash valve position. Treadle valve functions as intended and there is no tm troubleshooting left for this.

I've got the same issue with a M1078. I've replaced just about everything between the parking brake valve and the brake chambers.. Parking brake still doesn't work.

 

[NDT]

This may be a long shot, but here goes. I found out in a most horrifying way, at least on my 1078, that if you apply shop air to the front emergency glad hand with the parking brakes applied, they release. Luckily I had my cell phone to call the wife to pop the air hose lose! I assume in a tow situation that this automatically releases the parking brakes. Now maybe you have a check valve leaking by somewhere letting air make it's way to the emergency glad hand line or an air line hooked up wrong letting the spring brakes being released as air builds to pressure. Just a thought.

Sounds like this is your answer. Your truck is self-charging the emergency front connection due to the check valve not closing. I don't recall the spring brakes auto-releasing whenever I have flat towed FMTVs but that's not to say they don't.

 

[f8617]

...with zero air pressure in the tanks, the spring brakes are engaged...

Will, Did you answer this question (above)? Rephrase -- Does the truck have parking/spring brakes when the tank is empty?

 

[Suprman]

With the tank empty there's no air to release the springs so it's parked. So maybe by temporarily blocking off the fron gladhand I created my own problem. But the system shouldn't release the brakes with emergency air input. That didn't make sense. The check valve is there to protect air in the tank from draining out but allow air in to charge the tanks from an external source. But if the front gladhand was only designed for towing purposes then that would make sense to release the springs and the tow vehicle would connect air to operate the service brakes of the truck in tow. I will check the check valve. Learn something new every day.

 

[NDT]

On most trucks with a front emergency glad hand, there is a single tube that feeds the air tanks through a check valve. On the FMTVs, there is a second tube connected to the emergency glad hand that leads to an unknown (until now) location. Looks like it's connected to the spring brake release circuit.

 

[Suprman]

See I knew there was a reason I posted this odd stuff that always seems to happen to me. I was looking into it I think one of the 3 check valves under the drivers seat has to do with the emergency air side. And some ps magazine reference to idiots like me plugging gladhands to stop leaks causing the parking brake not to set. Quite possibly the tank feed check valve is stuck adding to the mix.

 

[Castle Bravo]

On most trucks with a front emergency glad hand, there is a single tube that feeds the air tanks through a check valve. On the FMTVs, there is a second tube connected to the emergency glad hand that leads to an unknown (until now) location. Looks like it's connected to the spring brake release circuit.

Man, that sure seems like a poor trade of convenience for safety...

 

[coachgeo]

Man, that sure seems like a poor trade of convenience for safety...

sounds like one could put a shut off valve on the second line to the spring Brake release? Or cut it somewhere along the route and just cap it off on both ends?

Would guess it was originally designed and done that way as an attempt to make the truck more idiot proof and or for....... FASTTTTTTT plug and go for situations where there is a little more flying bullets that makes one tend to forget things like shutting off air brakes when your on a grab and go.

 

[tennmogger]

If you follow the tube from the emergency front gladhand it goes to the one-way valve then to a shuttle valve. That shuttle valve is most likely bad. So is the one-way valve. It is at that shuttle valve that another tube heads off to places unknown, actually to the parking brake system.

That shuttle valve chooses either truck-sourced air for parking brakes, or, air from the emergency glad hand. Connecting air to the glad hand triggers it. Or, if bad, it could put truck air back to the parking brakes if you plug the glad hand.

Putting air on the emergency glad hand does release the parking brakes. It has to! If you think about the SERVICE glad hand up front, it cannot release the parking brakes for towing because input air varies from zero to max line pressure, whatever the towing truck treadle valve dictates.

So Superman you have a bad/dirty one-way valve and a bad shuttle valve. It's the one up behind the front bumper, next in line from the one-way valve.

IMHO, of course.

Bob