M923a2 air question

[Coffey1]

I also learned from another thread that the brake treaddle valve will leak air by to stack a sticking valve in it. So when truck is parked 30 min later no air

 

[Artisan]

When you RELEASE the pedal, you will lose air, yes.

If you're just pressing down and holding steady, the needle should settle and not leak down more than 1 psi per minute.

If you have a leak down problem while firmly holding the pedal down (steadily), have someone go underneath the truck to see where the air is leaking (listen at each cyl)

Do a search for COLA air brake test.

• C = Air compressor cuts in.
• O = Air compressor cuts out.
• L = Low pressure warning.
• A = Air leakage.

______________________________________________________________________

This has changed, it is now C.O.L.A.S.


Cut Out Low Air Spring

Governor Cut Out no more than 130PSI***
Governor Cut In no less than 100PSI***
Low Air Warning Light and Buzzer comes on between 55-75 PSI
Air Leakage 4/60 Applied test, engine off, tractor and trailer brake valves pushed IN, let settle, now place foot on
service brake pedal and hold firm for one minute, no less than 4 PSI lost.
...and the newest addition is
Spring Brakes, continue to fan down pressure till tractor and trailer valves pop out, should be between 20-45PSI

I just passed the Class A CDL Examiners test at the DMV two days ago, I aced the walkaround and the skills

***BUTSOMEHOWEVER, if you read the manufacturers poop sheet on the governor it says the Cut in
pressure is usually 25PSI below whatever the Cut Out pressure is set at.... PUNT! Check it out!

 

[FFRCobra]

Hope this isn't a thread hijack...

So after reading this I tried the COLA test today and during the air leak down (engine off, press hard on brake pedal, count to 60) test it dropped from 120+ to about 93 immediately. I counted to 60 and there was no movement on the gauge. The parking brake was on though. I think the proper procedure may be for the wheels to be chocked and parking brake off? Was the near 30 psi drop normal for a HARD brake application with engine off or did the parking brake effect things?

 

[Jason O]

FFR Cobra- Your air pressure drop is due to the volume of air required to apply all of the brakes hard. If it held steady after the initial drop, you are likely OK

 

[dilligaf13]

Agreed. A little much but the pressure holding is the key.

 

[Artisan]

Air Leak Down test, I call it the 4/60 Applied test. You want to make sure you
do not loose more than 4 pounds in one minute after you set the truck up
for the test. Of corse the semi is a 10 Speed w/ clutch and tractor and
trailer parking brakes. The M939 trucks have a reserve of air so when
you press that air button it gives you just enough time to push
the truck off the road before all air is exhausted again and the
brakes set (So one dead truck in a convoy can be pushed off a cliff
or a road if it brakes down so as not to hold up the whole convoy)

Put truck in 1st gear
Shut off engine
Let out clutch
Release tractor and trailer brakes, let settle for maybe 15 seconds
Apply a firm uniform light pressure to the brake pedal, NOT HARD.
Note the primary and secondary tank PSI readings.
Start 1 minute test...
No more than 4 PSI drop allowed in 1 minute...

I guess I need to learn how to do a COLA test to a M925, I bet the TM
tells how to do it...

Probably something like this;

Chock wheels
Release parking brake
Apply a firm uniform light pressure to the brake pedal, NOT HARD.
Note the primary and secondary tank PSI readings.
Start 1 minute test...
No more than 4 PSI drop allowed...

Best to go se a brake specialist

 

[Moosemcnally]

So if my primary tank is the only one bleeding down while I hold the peddle down and I can hear the air rushing in the stack, is this more likely the Q2 valve or the diaphrams in the rear? This thread kinda went down the testing (colas) path and got off the fixing the problem that already was known to exist path.

 

[MtnSnow]

Moose, Do the Q2 valve as that is most likely your issue

 

[Moosemcnally]

Anybody know the napa equivalent part number?

 

[doghead]

Please do not cross post.

Please update both threads that you're using now.

 

[MtnSnow]

Moose, I posted to the other thread the Napa site's Bendix branded LQ2 valve. or you can get them on the big E site for much less.

 

[Ford Mechanic]

Bump to the top,

I'm having the same issue at the moment. Mine is venting the secondary tank with the brake pedal depressed, engine off into the stack. I thought I could hear it at the stop sign, so I tried it with the engine off when I got home. It might be just frozen being it's currently 23*F. So I'll let it warm up above freezing to recheck before replacing.

Anyone else having winter time freezing issues with the air brakes?

Pic of my frozen truck after riding today.

We'll they wouldn't attach to SS so I'll post them this way and correct them at work where I have better service.

 

[simp5782]

Bump to the top, I'm having the same issue at the moment. Mine is venting the secondary tank with the brake pedal depressed, engine off into the stack. I thought I could hear it at the stop sign, so I tried it with the engine off when I got home. It might be just frozen being it's currently 23*F. So I'll let it warm up above freezing to recheck before replacing.

Anyone else having winter time freezing issues with the air brakes?

Add brake antifreeze. Doesnt take much cold weather to cause chaos. My one truck wont blowoff at the dryer atm

Sent from my SM-G860P using Tapatalk

 

[Jericho]

Yes , issues, When the cold air goes below -34, last night , the air pressure wont build above 70 to 80 psi . escaping air heard in stack. Worst part is it doesn't have enough pressure to engage the front axle drive when I need it to go down my driveway (every day in the winter) have added brake dry and it does work for a while , drain tanks every day, changed dryer filter, and dryer heating element (wasn't working) , freed the transfer case lock in valve , continue to have a problem , still working thru it , learn a lot here every day !

 

[simp5782]

Yes , issues, When the cold air goes below -34, last night , the air pressure wont build above 70 to 80 psi . escaping air heard in stack. Worst part is it doesn't have enough pressure to engage the front axle drive when I need it to go down my driveway (every day in the winter) have added brake dry and it does work for a while , drain tanks every day, changed dryer filter, and dryer heating element (wasn't working) , freed the transfer case lock in valve , continue to have a problem , still working thru it , learn a lot here every day !

Did you try closing your primary tank to let the secondary build more pressure with the extra volume to see it would climb to let you engage the front axle and then open your primary?

Sent from my SM-G860P using Tapatalk

 

[wheelspinner]

Front axle engage is drawn from accessory air system. This is not fed until the Pressure Protection Valve (PPV) is happy. These get nasty and freeze up all the time. Cheap to replace, fairly easy to take apart and clean. If frozen can be defrosted quite easily with a hair dryer. Just look at the valve on the front of the air tank you can see by looking between the RS tool box and air drains. Not making more than 70 and air bleeding into stack is most likely a frozen check valve also.

 

[Jericho]

AH HA . all good information. Now its forecasted to be 45 on Wednesday ! better get a flu shot! Hair dryer, can by one I guess, don't have enough hair to use one, but for a truck YUP