Still working on my new acquisition and having some problems with the air brake system. Read through the TM's but not finding my problem listed. The air compressor seems ok, starts pumping up immediately but there are air leaks that sound like are coming from the brake pedal manifold and also at the service brake (yellow) knob. I have done the obvious things like checking for broken air lines and tightening all the fitting. If I push and hold the yellow knob the air leaks stop and the system continues to pump up and finally the sensor lights on the dash go out. I can then release or let go of the knob and everything basically seems ok. I can activate the brake pedal, I get the "swish" sound of air escaping like it is supposed to do etc. Haven't got the truck driveable yet so can't actually test the brakes. I have replaced the three valves above the driver's side wheel as well as both control valves in the dash (service and trailer valves). Any ideas or thoughts would be appreciated.
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LMTV air/brake problems
- Thread starter RAJ LMTV
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If air is leaking up around the yellow knobs shaft then that control may need replacing. Air should not escape from it like that. Air could be leaking thru the treadle valve also. Seals dry up and go bad from sitting especially in hot climate. Use windex to identify the leaks exact location.
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There is a thread somewhere with the rebuild kit part number. Does the pedal feel spongy or firm?
coachgeo
Well-known member
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My International was that way. Worked like this for couple years before had to repair various leaks. Don't recall all what was done so sorry not much help.
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Hard to say since this is my first experience with airbrakes. But I suppose it feels a bit spongy to me when I press on it. Haven't found the thread yet with the treadle valve repair kit info but still looking.
- 518
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- Stoneville, NC
The dash valve for the parking brake is spring loaded and will not stay down unless there is sufficient air pressure. That's why it seems OK if you hold it down long enough for the system to build pressure. The parking brakes (spring brakes) require a certain pressure to overcome the springs and disengage. Sounds like your dash valve is working fine, especially since you replaced it already.
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Brake pedal should be nice and firm even without air in the system
Thanks for the explanation on the valves. Any clue as to why there are apparent leaks in the brake valve until I press and hold the yellow valve? As soon as I press it the air noise and leaks stop......
- 518
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- Stoneville, NC
I'm not sure on that. Releasing (pulling) that valve vents the air from the parking brakes. Once the system has built up pressure and seems fine, are the parking brakes actually released? What happens if you then pull the valve to engage the parking brakes? Normally it would have a short whoosh of air as it vents those chambers and then it stops.
Well I'm not actually sure if they are released or not. I currently have the drive shafts dropped and the truck hasn't been moved since I had it delivered. So I really don't know if they release or not. Just installed a new CTIS controller and that system seems to function now. Just started it up and it pumped up to 120lb fairly quickly, but I did have to hold the yellow knob in until it reached about 60psi or it would leak from the service brake manifold. But after that the system didn't seem to have any leaks and when I **** the engine down I couldn't detect any leaks anywhere. Seems to be holding the pressure, guess I will see how long before it bleeds down
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Are the brakes caged? If it came with removed driveshafts chances are they are caged. Look for bolts sticking out of the rear brake cans. The truck will need to be chocked. You need to read the operators manual and familiarize yourself with airbrake operation. It's a big a heavy truck and things can go real bad real fast if you don't know what you are doing.
I definitely don't know what I am doing but have been faithfully reading, studying, asking etc. Truck has been chocked, I removed the front driveshaft, the rear had been removed for transport. I didn't want any chance of the truck moving when I cranked it up hence removal of front shaft. The brakes were caged, I removed 2 cage bolts previously but now looking closer I see there are two more, didn't realize there were 4 total. So next is to remove the other 2. The system held 90psi for an hour after shutting down (I released the brake lever after an hour) so I suppose there are no leaks. Just confused on the leak at the yellow service valve when charging if I don't hold the knob down.
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How were the driveshafts/u joints removed? Hammered out or removed with a correct puller tool?
The rear had been removed by the towing company and he said he had a special tool. I didn't have a removal tool but just used a flat head screwdriver to slightly pry with. The front one came out extremely easy. No hammering required
Still looking for more advice if possible. Now it seems the leaks I have when the system is pumping up are at the rear emergency glad hand as well as the vent pipe that is located behind the grill near the radiator. When the system is pumping, air is bleeding from these two locations and it will not pump up more than 30 psi. If I push and hold the yellow service knob the leaks stop and when it gets to about 60 psi I can release the knob and it will continue to pump up to approx 120 lbs. As far as I can tell the system functions normally after getting past this start up problem. It will hold air for hours after shut down so there are no apparent leaks anywhere. The red emergency button doesn't seem to do anything. What should happen when that button is pressed? Thanks again for input.
- 6,861
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- 113
- Location
- Stratford/Connecticut
Leaking all over is usually indicative of the tractor valve needing rebuild.
Tractor Protection valve maybe?
They also have a membrane inside that tears due to age. Cheap fix.