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Brakes? 15 miles from home - M923A2

Elijah95

Certified Rookie
1,239
1,183
113
Location
Georgia
We need to discuss basics of how your system operates.

The compressor is pumping 100% of the time, but when it builds pressure is determined by the D2 governor located on the passenger side firewall under the hood

There’s only a few things that can STOP you from building air,
1) most common is the Pressure Protection Valve located under the truck gets rusty or frozen inside, and will not allow air to pass through to give the “signal” to build air, it’s job in life is to protect your braking ability if you blow an airline on your wipers, horn etc. see picture 1

2) the governor mentioned above, see picture 2, it has a cut in and cut out pressure, generally it cuts in 85-100 psi, closes the compressor valves and the air builds until 120-135 depending on settings and then STOPS building pressure, purges drier, all is good.

3) the compressor unloader valve is an integral part of the head of the air compressor itself. It slides up and down with O rings in the bore, if it gets sticky it can hang up and never build air. This is what you were supposed to spin the line off, on an a2 truck looking at the compressor the line comes off the TOP of the compressor at a 45° angle pointed slightly back towards the cab, it’s like a little 1/4” line or smaller, when you spin that line off, stick your finger on top of the fitting of the compressor, is it “dead” (meaning no air moving) or is it pushing air out then pulling it in?

4) last but not least, the drier commonly sticks open on the purge valve, making people falsely thinking they “aren’t building air” yet the truck is building air, you just have a leak so massive it loses it as fast as it can build it.

There are other things, such as blown unloaded head gaskets and main air compressor head gaskets, faulty discharge lines etc but those are a little more rare.

Since you’ve gotten the truck home, I advise airing the truck up via your deuce or air compressor, and see if your horn and air wipers work. If they don’t and you have 100+ psi of air, there’s your problem. If they do, it’s time to check your governor followed by the air compressor unloader.

Remember, everything on these trucks are stupid simple, accurately go down a diagnostics tree and you’ll find your root cause. Work down that list and let me know


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simp5782

Feo, Fuerte y Formal
Supporting Vendor
12,091
9,248
113
Location
Mason, TN
You can air the truck up through the emergency connection on the front of the truck on the left side behind the bumper. It has a quarter turn valve which has to be open to let air in and closed to keep air in. You need an air line with a glad hand on each end and another truck or compressor to supply air. The rear emergency connection doesn't work, I believe, because there are check valves in the line to the rar.
m939s can be air'd up from either end of the trucks.
 

Slate

Well-known member
Steel Soldiers Supporter
682
377
63
Location
Ozona Texas
So I drug my truck home turned it off ate some lunch come back out to check out where my issue is and now I'm making air....??? What the heck.

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simp5782

Feo, Fuerte y Formal
Supporting Vendor
12,091
9,248
113
Location
Mason, TN
We need to discuss basics of how your system operates.

The compressor is pumping 100% of the time, but when it builds pressure is determined by the D2 governor located on the passenger side firewall under the hood

There’s only a few things that can STOP you from building air,
1) most common is the Pressure Protection Valve located under the truck gets rusty or frozen inside, and will not allow air to pass through to give the “signal” to build air, it’s job in life is to protect your braking ability if you blow an airline on your wipers, horn etc. see picture 1

2) the governor mentioned above, see picture 2, it has a cut in and cut out pressure, generally it cuts in 85-100 psi, closes the compressor valves and the air builds until 120-135 depending on settings and then STOPS building pressure, purges drier, all is good.

3) the compressor unloader valve is an integral part of the head of the air compressor itself. It slides up and down with O rings in the bore, if it gets sticky it can hang up and never build air. This is what you were supposed to spin the line off, on an a2 truck looking at the compressor the line comes off the TOP of the compressor at a 45° angle pointed slightly back towards the cab, it’s like a little 1/4” line or smaller, when you spin that line off, stick your finger on top of the fitting of the compressor, is it “dead” (meaning no air moving) or is it pushing air out then pulling it in?

4) last but not least, the drier commonly sticks open on the purge valve, making people falsely thinking they “aren’t building air” yet the truck is building air, you just have a leak so massive it loses it as fast as it can build it.

There are other things, such as blown unloaded head gaskets and main air compressor head gaskets, faulty discharge lines etc but those are a little more rare.

Since you’ve gotten the truck home, I advise airing the truck up via your deuce or air compressor, and see if your horn and air wipers work. If they don’t and you have 100+ psi of air, there’s your problem. If they do, it’s time to check your governor followed by the air compressor unloader.

Remember, everything on these trucks are stupid simple, accurately go down a diagnostics tree and you’ll find your root cause. Work down that list and let me know


Sent from my iPhone using Tapatalk
Negative on your # 1.

The compressor runs all the time. The pressure protection valve allows air to pass thru top the governor that sends a signal to the unloader to shut off once the governor reaches the correct setting PSI. This is why people have issues with the overpressure valve blowing when the PPV is iced up. The compressor runs the whole time. WITHOUT air the compressor will not turn off. It will run non stop.



The OP needs to remove the air line off the rear of the compressor, install a gauge, start the truck and measure the output of air pressure and go from there.

Something as simple as a clogged with soot discharge line can cause the compressor to not discharge air as fast. especially if it has to compensate for leaks.

 

Slate

Well-known member
Steel Soldiers Supporter
682
377
63
Location
Ozona Texas
I guess I will be going through everything. I have a band new air governor. I will put a new pressure protection valve on and. My dryer is making odd noises before it's purge. Or it will continue to hiss a bit after purging. And I'll check my pressure coming out of compressor. Thank you. Also I can hear air leaking from Purge valve guess I need to replace it.

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Last edited:

simp5782

Feo, Fuerte y Formal
Supporting Vendor
12,091
9,248
113
Location
Mason, TN

Slate

Well-known member
Steel Soldiers Supporter
682
377
63
Location
Ozona Texas
Known to be a pain to rebuild the purge... even after rebuild they will stick wide open.

Is this a suggestion to replace mine?

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simp5782

Feo, Fuerte y Formal
Supporting Vendor
12,091
9,248
113
Location
Mason, TN
Is this a suggestion to replace mine?

Sent from my moto z3 using Tapatalk
Yes. Those pure air plus units are old old old. Even if you get the purge kit and rebuild it. It's going to continue to stick open.

The one In the ebay listing is the alot of people upgraded to when they were readily available
 

Slate

Well-known member
Steel Soldiers Supporter
682
377
63
Location
Ozona Texas
Yes. Those pure air plus units are old old old. Even if you get the purge kit and rebuild it. It's going to continue to stick open.

The one In the ebay listing is the alot of people upgraded to when they were readily available
Thank you sir I'll have it ordered in just a sec.

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fasttruck

Well-known member
1,265
622
113
Location
Mesa, AZ
Reference post 23: If you have the airlines that go between a towing vehicle and the dead horse you are towing: hook up service to service and emergency to emergency open all 4 valves and motor. The towed truck will act like a trailer and follow the towing vehicle's brakes. No need to cage brakes as towing vehicle will maintain air pressure to keep them released.

If you are going to air up the malfunctioning vehicle and drive it, hook up emergency to emergency, open 2 valves and charge air system in second vehicle, when same is charged up, closed 2 valves and disconnect line and proceed with the bottled air. If you don't use the brakes too much you can go considerable distance before recharging the brake reservoir, unless you have a leak. And you can still stop the truck in an emergency or it will stop on its own when the air pressure leaks down to about 40 pounds. With a 5 ton you, as post 23 states, introduce air from either end.
 

Slate

Well-known member
Steel Soldiers Supporter
682
377
63
Location
Ozona Texas
Reference post 23: If you have the airlines that go between a towing vehicle and the dead horse you are towing: hook up service to service and emergency to emergency open all 4 valves and motor. The towed truck will act like a trailer and follow the towing vehicle's brakes. No need to cage brakes as towing vehicle will maintain air pressure to keep them released.

If you are going to air up the malfunctioning vehicle and drive it, hook up emergency to emergency, open 2 valves and charge air system in second vehicle, when same is charged up, closed 2 valves and disconnect line and proceed with the bottled air. If you don't use the brakes too much you can go considerable distance before recharging the brake reservoir, unless you have a leak. And you can still stop the truck in an emergency or it will stop on its own when the air pressure leaks down to about 40 pounds. With a 5 ton you, as post 23 states, introduce air from either end.
Makes sense to me.

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