Air Fails To Build Intermittently; Parking Brake?

[lindsey97]

On my 1990 AM General M923a1, it was on the front of the air tank that is mounted inside the frame, on the passenger side, nearest to the transfer case.

 

[gottaluvit]

Maybe they did away with them or relocated them with the A2.

 

[Ford Mechanic]

Mine is a A2, and it's right where he said. You can actually see it from outside the truck, passenger side. Look over the back of the rear tool box at the front of the rear tank. pic inbound

 

[Ford Mechanic]

 

[gottaluvit]

With full pressure shut the engine down and then disengage the p-brake. Then check for any leaks under the truck. Like the cans or relay valves, expecially the circuit that don't have air pressure until the p-brake is disengaged

So I did this and did find a line leaking. Started to rain so postponed the removal of said line but will do so shortly. I still want to clean up and/or replace that ppv valve if I can find it. I will take some pics shortly of my wet tank's rear (first tank after dryer) and post them. Thank you very much for the help thus far.

 

[gottaluvit]

Ok. I have pics of both front and rear of the rearward tank that is immediately after the air dryer. First pic is rear of tank and second is front of tank. Can't see that valve anywhere.

 

[gottaluvit]

Well, it posted them vice-versa. Lower pic is rear of tank (closest to air dryer).

 

[juanprado]

you have a midland style ppv valve. part number kn31000. You can find on ebay with that number old stock. I can't link.....

for reference:
http://www.class8truckparts.com/Haldex-Midland-Regulating-Valve-KN31000/dp/B002HURU3Q

Midland does not make that style anymore when they merged to become Midland / Haldex. No need for duplication in their line.

Any PPV valve will work as the 1/4 npt ports are very standard. Just make sure psi is 65. Flexible air line tubing won't care what is the configuration and there should be enough slack.

 

[gottaluvit]

you have a midland style ppv valve. part number kn31000. You can find on ebay with that number old stock. I can't link.....

for reference:
http://www.class8truckparts.com/Haldex-Midland-Regulating-Valve-KN31000/dp/B002HURU3Q

Midland does not make that style anymore when they merged to become Midland / Haldex. No need for duplication in their line.

Any PPV valve will work as the 1/4 npt ports are very standard. Just make sure psi is 65. Flexible air line tubing won't care what is the configuration and there should be enough slack.

Thank you very much juanprado. That link had a real nice photo. That post is a wealth of info.

 

[Ford Mechanic]

Ok. I have pics of both front and rear of the rearward tank that is immediately after the air dryer. First pic is rear of tank and second is front of tank. Can't see that valve anywhere.View attachment 568675

Yeah you have one, like he said it just looks different. It's in the same position as mine is in those pics.

 

[Scrounger]

After reading all of the threads and posting you have on your truck I’ll give you may layman’s uneducated guess on what’s going on.
First off a little and partial refresher on the air system. Compressor to dryer to air tanks then off to the rest of the truck. Governor receives signal thru line from air tank. Air low, governor sends signal to unloader start compressing. Air reaches peak pressure governor sends signal to unloaded lift, stop compressing and sends signal to dryer purge. Now if the dryer was not working, which I believe was happening for some time, you can get a lot of water and crud in the system. And that makes all kinds of problems everywhere else.

What I would do is get a new PPV valve and set it aside.
To clean the system I would use first methyl alcohol, the kind that is used to keep air systems from freezing in the winter and then something like CRC Brakleen. Disconnect the air line from the compressor to the dryer and clean it with Brakleen then reconnect it. Then I would disconnect the airline from the dryer to the air tank. Install an air fitting to that line like the one that you would use with your airlines and air tools. The idea is to run the trucks air system off of a shop air compressor. I would start by filling a fifty foot air line with alcohol, then connect that airline to the fitting that you installed previously. Charge everything up then start draining the air tanks. Start with tank one, you may have to use two or three quarts and have to refill the airline to get is clean. You’ll know when it’s clean. Then do it all over again on tank two, then three, you get the idea. After the tanks are clean, fill the air line again with alcohol and get in the truck and start pumping the brakes. Figure on at least two or three quarts to clean the brake system. Once everything looks clean, do a final clean with Brakleen. Then I’d install the new PPV valve.

 

[gottaluvit]

Well, that sure sounds like it will get it all clean. I will follow those instructions to the tee. Seems as if you are right about that air dryer. Some water came out when I unhooked it's lines when replacing it and my first thought was that the heater in the dryer must not have been working. I ordered a new PPV valve and will get the system cleaned out before it arrives. That will make me very happy just knowing I will not have a bunch more parts going bad from water damage when I plan to be hauling quite a few loads of gravel once I quit having issues.

Thank you very much,
Jim

 

[Ford Mechanic]

There are several folks here that sell the MRAP dryer that retrofits nicely in the M939 trucks.

 

[Scrounger]

The heaters only purpose is during cold weather, it keeps the water in the dryer from freezing. I should add my air compressor has two stages and is set for 175 psi and when I’m working on vehicle air systems I have a portable pressure regulator to reduce it to 125 psi. If you only have a single stage compressor you’ll be OK. If yours it set like mine you’ll have to reduce the pressure.

 

[gottaluvit]

Oh, I figured it dryed the air with the heater as well. I see, with it being so low, and having a purge, how it could expel water out of the system though. I will be doing this process using a friends garage and will make sure it is same as truck's air pressure. Thanks, as I doubt I would have thought of that.

 

[gottaluvit]

Okay, I did the aforementioned alcohol flush/cleaning of the air system. I then installed the new pp valve (should that be ppv valve? just seems like to many v's). I am still having the same issue. It airs up great on empty tanks and then when I repeatedly pump the brake pedal it just drops low enough for the low air alarm to sound. Sometimes (rarely) it will behave and air up again on it's own, but 75% of the time the air just goes lower and lower to no pressure in the primary yet will not build at all. The secondary is usually trailing about 75% behind the primary, but still falling with each succession of the brake pedal. I just came in from cracking the drains while it was running and then it built pressure without shutting down the engine. All other times, I just shut the engine down, drained the tanks completely, and restarted the engine and it would build again. Coming back from my friend's place (close by) where I used the shop air to do the system cleaning, it wouldn't build with three shut downs and drain attempts so I caged the brakes and started it up and the stupid thing built air. (Yes, I gladly uncaged the brakes to drive home with brakes.)

I hear no leaks other than a wee little coming from the dirt blocking flapper on the rearmost relay. Definitely not enough to drain the tanks or I wouldn't think not enough to keep the thing from airing up.

I might add that this current issue came on since my owning the truck on the way back from the first time I drove it 20 some miles. Was working perfect after the unloader valve issue on two to three mile runs.

Also, the air drops only when pressing the brake pedal down, not when holding it down. I assume that means it isn't the brake chamber/diaphrams. I utilized this getting it home and limited my presses of the pedal and held it a bit longer to slow more per push.

To date: New MRAP air dryer, cleaned and lubed the unloader valve, new ppvalve, checked all glad hand valves, fixed a leaky line that has air pressure only with parking brake in released position, flushed the entire system with air brake anitfreeze, took governor apart to look for blockage/crud (looked pretty clean), banged head on bumper several times (didn't want to dent the sheet metal!).

Any suggestions appreciated.

Could there actually be some other valve of some type messing with me?

Jim

 

[gottaluvit]

Well, I messed up and started another thread on air that I really didn't think was related to this one. Mods, you can delete the air tank question thread if you think it may not be best to have that in a seperate thread. My bad. I just thought I was draining tanks wrong and didn't think it could be part of my problem but now thanks to Floridianson, I may be figuring something out. Here's the thread http://www.steelsoldiers.com/showthread.php?142184-Draining-Air-Tanks-One-At-A-Time. It was asking if I was to drain tanks waiting on the previous to drain because my last two to drain was darn near empty by the time I got to them.

So, does anyone know anything about issues with check valves in the system messing up? Locations of correct ones leaving my drain sequence #s 3 and 4 near empty by the time I get to draining them?

Thanks in advance. And yes, I was born blonde!

 

[The HUlk]

Okay, I did the aforementioned alcohol flush/cleaning of the air system.

Glad it didn't blow up!

Also, the air drops only when pressing the brake pedal down, not when holding it down.

Something is dumping alot of air when the brake pedal is depressed. I would start looking at the brake pedal and everywhere else for air leaking using the folowing method.

Fill a spray bottle with soapy water, grab flashlight and find a helper. Air up the truck to full pressure until it blows off. Turn off everything. It needs to be quiet enough to hear small air leaks well. Have soldier B press the brake pdeal as you move about the truck zeroing in on where the air is coming from. The spray bottle/flashlight tells you exactly where it's coming from. Check dash plumbing, in and out of the cab, frame rail plumbing, Brake area. Engine bay.

 

[Ford Mechanic]

There are check valves at each tank, I searched here and found a Bendix number that my local truck shop ordered for me. Their normal issue is either rust(wet tank) or getting broken on the trail.

Have you put a governer on it yet?

 

[Ford Mechanic]

It's correct to lose some pressure when you mash the pedal down, then when holding pressure it should stabilize and not lose any more unless pressed harder.