88 977 hemtt

Karl kostman

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Hello Folks I find myself in a position to request some ideas as to why my 977 is not keeping up with my very miserly air demands? Here is some back ground. I bought the truck and all seems to be great, really do love driving this vehicle a lot! But after driving it for a few weeks I found that I would have to be really careful about using the brakes because I not only had the low air horn sounding but I was within 10 to 15 pounds of the brakes starting to lock, (they never did)! Last week I ran to town and I had to pull over after hitting the brakes 3 times and it took at least 10 minutes for the pressure gauge to make it back up to 95 PSI, the service side has always been slow to recover and initially build but its gotten far worse over time. When I shut the truck off I hit the brakes and listen for leaks etc. but do not hear anything and yes I sure understand the rear of that truck is a long ways back there.
Later this week I will have time and a helper to hit the brakes with me under the truck to see whats going on, do you guys have any good suggestions on possible problems areas I should be concentrating on?
Any suggestions would be appreciated folks!

Thanks much
Karl
 

dk8019

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Karl,

Nothing specific to look at, but I do know that mine, when moving slowly down hill I can run it low as well, but never on the open road. I believe at some point there was an air tank upgrade on these trucks, where more tanks were added. Mines down at our farm, but if memory serves, there are two small tanks, and one very large one. Does yours have three?
 

Karl kostman

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dk I would have to crawl under and have a look to be sure, if it doesn't I wonder how hard it would be to find this upgrade kit? And mine is just like yours driving on the road all is wonderful bet get in town and it sure seems to rebuild pressure slow? DK I am sure these compressors have stopped working before, do you know of a way to test the output on the compressor to verify all is well in that dept.?
Karl
 

tobyS

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Karl, I don't have a 977 but it sounds like the problem I had with my 5 ton air pressure control valve that unloads the compressor (making it not build air). It had water and debris in the line and needed major lube with Marvel Mystery Oil at the compressor. I had to disassemble the control valve for cleaning (and new red grease lube). Run the line back from your compressor head and I'll bet you find the air control.
 

Swamp Donkey

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I'm far from a HEMTT expert but you're working with an air brake system that follows much of the standardized air brake system design. Do like we do on the M939 trucks and air up the system with shop air through the emergency gladhand. Test the brakes with the parking brake applied and released. Spring brake diaphragms don't leak unless the parking brake is released. Be sure to chock the wheels. I'm not sure of other things that use air on the truck but check those also, air dryer purge valve, driveline actuators, wipers, etc. If you find no leaks then start checking the air making side of the system. You will have a compressor, governor, PPV, and various check valves just like the M939s.

Get a couple of accurate gauges and use those for the troubleshooting. Most everything should be either compression or NPT thread so a couple of Tees will let you tap into most places for pressure checks.

The output line on the compressor should get fairly hot if it's making good pressure. Check for leaks around the compressor head as well. This might only leak with the compressor making pressure.

Kind of generalized information since I'm short of specifics on the truck. Air usage during braking will be more on a 4 axle truck due to the extra chambers on the 4th axle. Just something to keep in mind. That's about all I got without digging through the manuals.
 
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74M35A2

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As another envious 6 wheeler, the only time I had problems with air keeping up was because one of my gladhand covers was off and the valve half open.

Not sure if your vehicle has such, maybe, and another point to check if so.
 

Karl kostman

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Hey guys just and FYI but like DK said his 977 is of the 3 tank variety and that is the same as mine, I appreciate very much all your input on this subject and I will hopefully get crackin on this project tomorrow! One last question guys, I know with 4 axles and 8 dual chamber brake diaphragms is it possible that I just dont have enough air reserve volume? DK said that there was and upgrade kit the Govt sent out some time after my truck was built where they upgraded that reserve capacity with 2 more air tanks, could my issues be as simple as that? I have never run into this problem with any of my other trucks including my 920, thats got 4 axles and it shifts with air. Thoughts?
Karl
 

tobyS

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Karl, you could T an temporary air line and extra tank capacity with push lock fittings at a strategic location for increasing your volume. If it solves the problem, make it permanent.

I'm wondering if you have a bunch of leaky brake diaphragms. The 5 ton takes T-12 for service and T-16 for emergency release (I think), but I don't know about 977.

Take a look at the output of the air compressor. It it looks like it's been hot, then it's making compressed air at usable pressure. If it is shiny and painted, it's not pumping volume and pressure.
 

Karl kostman

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Hey folks I just wanted to up date you on where I am at with my 977 and my NOT enough air issue. Well this evening I fired up the truck and the emergency air pressure came up to full pressure within a couple minutes, normal right. Well it took about 20 minutes at least for the service side to come up to 100 psi. The output side of the air compressor lines are hot so its making pressure but is seems like I have a valve somewhere that is NOT letting air into the SERVICE side of the truck. I had the truck in park and listened with the engine off and there were no air leaks. I released the parking brake and no air leaks. I stepped on the brakes and held them under pressure and had my buddy crawls around under the truck from end to end and NO leaks. When I released the brakes it took about 7 minutes for the truck to rebuild the 10 pounds of air pressure it used when I applied the brakes. My question guys is this do you have any ideas what air valve might be at fault for not letting a decent amount of air into the service side of the truck. The TMs do not have a full Schematic of the air system on the truck. Thanks much and all ideas appreciated!!
Karl
 

Floridianson

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In most cases the compressor feeds the air dryer then to the wet tank / primary tank. Is your air dryer in and out pressure the same. Then go to the wet tank and look for a one way valve. Then work you way to all other tanks.
 
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155mm

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Some of the air elbows or T's have small screens in them before they go into major assy's like a spider valve or check valve. Had one that took forever to release the brakes after the button was pushed, had a screen at a spider valve clogged up.

IMG_2100[1].jpg
 

Gunzy

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Karl, my M985 has 2 tanks under the battery tray and 2 more slightly larger ones behind the fuel tank. I have to say you have a bad check valve feeding the primary tanks but I can't tell you where it is located. Sorry I can't be more helpful.
 

Karl kostman

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OK guys I moved the truck indoors this afternoon and aired up the system with shop air and started checking check valves and was going to check elbows I made it to the first check valve between the primary tanks and when the pressure got up enough to start filling the second tank I didnt get enough air out of the line to put a candle out. Removed the check valve and away I go, I have all sorts of air! I gutted the check valve and put the truck back together and all is wonderful in HEMTT land once again, next week I will buy a new valve in town and install that one!
I also wanted to say thanks to all the folks who chimed in with your ideas, it helped a tremendous amount and lead directly to me getting an easy fix out of it! THANKS VERY MUCH!!!

Karl
 

simp5782

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OK guys I moved the truck indoors this afternoon and aired up the system with shop air and started checking check valves and was going to check elbows I made it to the first check valve between the primary tanks and when the pressure got up enough to start filling the second tank I didnt get enough air out of the line to put a candle out. Removed the check valve and away I go, I have all sorts of air! I gutted the check valve and put the truck back together and all is wonderful in HEMTT land once again, next week I will buy a new valve in town and install that one!
I also wanted to say thanks to all the folks who chimed in with your ideas, it helped a tremendous amount and lead directly to me getting an easy fix out of it! THANKS VERY MUCH!!!

Karl
If the check valve has 1/2" pipe fittings on both ends it is a KN23000 valve.
 
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