Re: Air pressure on the brakes of an M1081!

[FloridaAKM]

Re: Air pressure on the brakes of an M1081!

I can't build air pressure past 60 psi in the rear brakes & the front brakes gage is not registering any air pressure. The front brakes work well, but the rear brakes won't release anymore due to low air pressure. Since I can't wrap my mind around this problem. I came here for the experts to tell me what I can't see. Last night after shutting down the engine, I could hear air escaping & located the leak coming out of the service gladhand in the front of the truck. The TM's haven't helped either so far! What am I missing?

 

[Overdrive]

Hi there FloridaAKM,
Not building pressure beyond 60psi due to escaping air. Sounds like a shuttle valve blew its little diaphragm and is leaking out the gladhand. Others have more experience on which one than me and have discussed the same issue. I believe its one of the three shuttle valves behind driver side step. Others please correct or add.

Double check the air is coming out the gladhand and not the exhaust port behind the grill in front of the radiator (tucked up behind the lip of the bottom of the cab in the center). That port vents the park/trailer and brake pedal valves.

OD

 

[FloridaAKM]

It's the gladhand for sure, so I'll go look at the shuttle valve to see the condition of the diaphragm after disassembly!

 

[Overdrive]

It's the gladhand for sure, so I'll go look at the shuttle valve to see the condition of the diaphragm after disassembly!

I don't want to steer you wrong... There are 3 or 4 shuttle valves under there all located in the same spot. I am trying to remember who knows EXACTLY which one to check, Suprman, LMTV772, ?

 

[Suprman]

There is a check valve behind the bumper that will cause air loss. Clogged air dryer will contribute. Usually the 3 shuttle valves wont prevent air being built since they are service brake but a tractor valve in need of rebuild will make air bleed out all over.

 

[Ukraine Train]

Is there a comprehensive description of the air brake system in the TM's somewhere? I'd really like a better idea of how the whole system works.

 

[Suprman]

Tm has diagrams and there are troubleshooting flow charts. I have come across several trucks with air lines and such switched and that makes the troubleshooting flowcharts useless. Look for recently replaced parts.

 

[FloridaAKM]

Pray tell where the tractor valve is located as after many hours looking thru the TM's equals brain lockup! Not to mention that the mechanics removed all the caging bolts from the rear axle & the rubber sealing plugs!

 

[Suprman]

Tractor valve is in the back on the frame rail on 5 ton or against the rear crossmember on 2.5

 

[FloridaAKM]

I replaced all three shuttle valves & now have air to the tractor protection valve which leaks profusely, more fun things to repair!

 

[tennmogger]

Hey floridaAKM, you can't get a break can you! That's a lot of bad parts. Hang in there!

 

[FloridaAKM]

The rubber diaphrams crack & blow out, I could have replaced just the diaphrams, but they were out of stock for several weeks! Its a learning experience. I just wish I was 40 years younger so it would be easier!

 

[FloridaAKM]

Under the truck again, results in the tractor protection valve being removed, all parts repaired & replaced. Upon starting the truck, both air pressure guages went to 110 -120 psi, the stop light indicator went out, the air drier started purging & the front/ rear brakes released like they are supposed to. Since there were no more indicated problems on the dash & the truck rolled freely, time for a test drive. Two miles down the road, everything is good to go; a few more miles & a stop @ the mailbox equals all four wheels getting hot & the rear brake air pressure peaks @ 90 psi. The air drier quits purging and there is a leak behind the cab, so more fun stuff to go figure out. TM's do not indicate what is up! Maybe next weekend will figure out the next air leak!

This coming week will deliver a center console to replace the one that came with the truck that is bent, dented, missing parts & does not work! More fun for an old guy!

 

[FloridaAKM]

I found the leak that started last weekend. The drivers side air leveling bag is leaking, but you can't hear it till you shut off the truck with full air in the system. Rear brakes guage shows half way between 90 & 120 or about 105psi. Before the airbag started leaking, the pressure was 120 & the air dryer was cycling; now, now the air dryer is silent! There was a post about the air leveling bag replacement, but my search was futile. The system has an exhaust lead on the right side of the truck, but it is silent & not dumping air. Where are replacements for the air bags?

 

[Suprman]

You can shut the air bags off. Bottom right control on the hyd manifold over the fuel tank. I have the air bags.

 

[FloridaAKM]

In another thread, guys were saying with the airbags actuated & you shut off the truck, the airbags deflate for 10-15 seconds, then stop. This is what mine are doing, so I'll guess that my bags are not leaking, which is great news. All the brakes are not releasing still, so there is more investigating what is up. Front air pressure in 120 & rear is 90-110 psi; the truck is slow to build pressure, but it does. When the air dryer starts dumping again, I'll know I am on the right track!

 

[Suprman]

The compressor unloader is rebuildable they go bad. Kits are cheap online. Dump that old style air dryer it could be clogged. At 90 psi your brakes should release. Make sure none of the gladhands are clogged or blocked in any way. Bugs can build a nest in little holes blocking air flow.

 

[FloridaAKM]

The rear glad hands are free & clear as I tested them Saturday afternoon. The air dryer was working great a few weeks ago, purging every 4 or 5 minutes, dumping water out till everything cleared out. I guess I will check on the compressor unloader kit @ the same time I look up replacements of the service & trailer brake controls. Correct me if I am wrong, but the service brake control is Haldex KN 20035 & the trailer control is Haldex KN 20025!

 

[Suprman]

Front gladhands are the usual brake locking culprits the passenger side one in particular. The service brake valves only get air when you are stepping on the brake pedal. The air dryer purges moisture out the bottom that it has collected in the last period of time after it purged last. When you reach governed air pressure and the governor sends an air signal to the compressor unloader to stop making pressure, it also send that signal to the air dryer to purge collected moisture. If it was me I would replace the air dryer with a newer model and then adjust the air governor up slightly. Did the brakes that wont release ever release? Did the truck come caged?

 

[FloridaAKM]

When I got the truck, the rear service & emergency brakes had the rubber covers removed, but were not caged. The caging tools were not in the holders, So I am lead to believe that it might have been caged when it went to DRMO & they robbed the caging tools. The truck came off the transport with 0 psi on the front brakes & 60 psi on the back. The truck rolled freely, so I drove it 12 miles home on the urging of a coworker. Half way home, I started loosing pressure & then the rear brakes started getting hot a mile from the house. The front brakes stayed cool.

After replacing the shuttle valves & repairing the tractor protection valve, the air pressure would build to about 80 psi on the rear brakes, then the front brake needle would jump to 120 psi instantly, then the rear brakes would catch up to about 110! Everything then had released & the truck would roll freely down hill by itself. My funds have been depleted till Friday, so I will get back with you on the dryer after that.