M35A2 Air Pack Questions???

[WhoMe08721]

Hey guys,

Was working on my buddy Deuce and a Half the other day. We where working on the brake system and replace every thing. New brake pads, springs, and clips all around, New master cylinder, and all new wheel cylinders. The only thing we did not replace is the (air pack) & the lines. The line all visually look good no leaks or bends. We bled master cylinder on the work bench before installing, We adjusted all the brake pad from the back of the wheel as the TM manual describes. We power bled the air pack and then all the wheels. I also adjusted the brake peddle throw and oiled the air pack. But the brake peddle is not as hard as I would think it should be. The only thing left is the air pack. Which I really don't know any thing about. So here are my question 1) If the air pack is bad can the truck still stop? 2) Is the air pack like a brake booster where it help you push the peddle or dose the air pack actually give you more stopping power. 3) What air pressure is needed for the air pack and what the max psi? 4) What the difference in long or short air pack? 5) How do you test a air pack? 6) Is there any good diagrams explaining how the air pack work & How air over hydraulic brakes work?

Thanks

 

[ToddJK]

Hey guys,

Was working on my buddy Deuce and a Half the other day. We where working on the brake system and replace every thing. New brake pads, springs, and clips all around, New master cylinder, and all new wheel cylinders. The only thing we did not replace is the (air pack) & the lines. The line all visually look good no leaks or bends. We bled master cylinder on the work bench before installing, We adjusted all the brake pad from the back of the wheel as the TM manual describes. We power bled the air pack and then all the wheels. I also adjusted the brake peddle throw and oiled the air pack. But the brake peddle is not as hard as I would think it should be. The only thing left is the air pack. Which I really don't know any thing about. So here are my question 1) If the air pack is bad can the truck still stop? 2) Is the air pack like a brake booster where it help you push the peddle or dose the air pack actually give you more stopping power. 3) What air pressure is needed for the air pack and what the max psi? 4) What the difference in long or short air pack? 5) How do you test a air pack? 6) Is there any good diagrams explaining how the air pack work & How air over hydraulic brakes work?

Thanks

How soft is the pedal exactly? Does the truck still stop when the brake is applied? The air tanks drained of any water?
From driving three different deuces, all I can say is, each is their own animal when it comes to the brakes. One the pedal was soft but it stopped and stopped well every time. A buddies had a very hard break pedal, didn't stop very well and from what I knew, he was in the process of changing the air pack out. My deuce had a very firm pedal, stopped fine, but now since the brakes have worn in a bit, the pedal is a bit softer but stops fine and has been this way for a while. On your questions specifically, I can't answer those. How much of an effect does the air pack have on pedal firmness, not sure. I do know, if there's water in the air tanks, the pedal feels mushy and sinkers further to the floor. Since you just did the brakes and stuff, I would assume it's due to those changes. If the trucks fails to stop easily with firm pressure, that air pack may be in need of rebuilding or replaced.

 

[WhoMe08721]

Thanks for the replay Toddjk. The pedal goes all the way down to 1in off the floor before stopping. The truck does stop when the brake are applied and stops very well. The pedal is really soft and there is no resistance. Also the pedal sinks to the floor. There is no water in the air tank. Tanks are drain after every time the truck is ran. But the brake pedal is NOT firm at all. I also found this on the web about (AIR-OVER-HYDRAULIC BRAKE SYSTEM) Construction Mechanic, Basic, Volume 2 PDF on (Page 7-44)

https://www.militarynewbie.com/wp-content/uploads/2013/11/US-Navy-course-Construction-Mechanic-Basic-Volume-2-NAVEDTRA-142731.pdf

 

[ToddJK]

Thanks for the replay Toddjk. The pedal goes all the way down to 1in off the floor before stopping. The truck does stop when the brake are applied and stops very well. The pedal is really soft and there is no resistance. Also the pedal sinks to the floor. There is no water in the air tank. Tanks are drain after every time the truck is ran. But the brake pedal is NOT firm at all. I also found this on the web about (AIR-OVER-HYDRAULIC BRAKE SYSTEM) Construction Mechanic, Basic, Volume 2 PDF on (Page 7-44)

https://www.militarynewbie.com/wp-content/uploads/2013/11/US-Navy-course-Construction-Mechanic-Basic-Volume-2-NAVEDTRA-142731.pdf

Whew, that was a long read! Just read chapter 7 on the brakes. From what I've read, perhaps the rod or the lever for the pedal that goes to the air pack is out of adjustment or there's too much travel for the pads before reaching the drums. It did say though 1" off the floor should be the max travel distance. It's hard to say really, I wasn't there and I'm far, far, from being an expert on drum brakes, let alone the deuce brakes, lol. Disc brakes, I've done a lot of, on normal cars, but that's still a different brake system. What are you guys gonna try to do?

 

[ldmack3]

I had similar trouble with mine. After doing the measurement adjustment per the TM you spin the wheel and adjust to where it just starts making noise. I mis-adjusted one of mine and it gave me similar trouble. Mine would pump up though. After readjusting them they worked fine. Hard pedal and one inch of travel before engaging.

 

[GopherHill]

Try a major and minor brake adjustment. It's kind of irritating until you have done it before. See operator's manual.

 

[WillWagner]

In my experiences, if the pedal stops then slowly goes to the floor, it is usually a master cylinder issue. Did you install ChiCom master and wheel cyls? If so, I know in the past, the cups are junk and I have actually seen a wheel cylinder cup split. The rubber is crap, like Harbor Freight tires. Repair components can still be had from Midland, Wagner for both the master and wheel cylinders. Other issues, Chinese or not, the bleed hole in the master can be plugged.

 

[WhoMe08721]

Updated on brakes. So a few weeks ago I order a new Air pack & rebuild kit. Finally got time this weekend and installed the new air pack and bleed the brakes again. Finally now the brake pedal is perfectly firm and the trucks stop on a dime. I am amaze how good the brakes are in this old truck. I always through the brake where not that great in these trucks from all the ones I have driven. but after doing a lot of research on air-over hydraulic brakes system. I now understand how they work and know now that the Air pack plays a big part in this type of brake system and is not for assist. But is used to stop the truck. The problem I had was condensation in the air pack making the cycle not able to move properly casing bad brakes. Also, I do add air oil once in a while to the back of the air pack. But I think there was to much condensation at this point. When thing slow down here I am going to open and rebuild the old air pack.

 

[cattlerepairman]

@WhoMe08721 kudos for not only going through the brake system with a fine tooth comb and making it better than new, but also for trying to understand how everything works. Never mind a 25,000 lbs truck being slow off the line - the 25,000 lbs need to reliably stop in order to be safe in today's traffic, complete with ignorant and de-brained drivers.

 

[ToddJK]

Updated on brakes. So a few weeks ago I order a new Air pack & rebuild kit. Finally got time this weekend and installed the new air pack and bleed the brakes again. Finally now the brake pedal is perfectly firm and the trucks stop on a dime. I am amaze how good the brakes are in this old truck. I always through the brake where not that great in these trucks from all the ones I have driven. but after doing a lot of research on air-over hydraulic brakes system. I now understand how they work and know now that the Air pack plays a big part in this type of brake system and is not for assist. But is used to stop the truck. The problem I had was condensation in the air pack making the cycle not able to move properly casing bad brakes. Also, I do add air oil once in a while to the back of the air pack. But I think there was to much condensation at this point. When thing slow down here I am going to open and rebuild the old air pack.

That's makes sense and glad you got it fixed. I always wondered what kind of issues having water in the air system can cause other than rusting the air tanks. Water affecting the air pack is probably a lot more common than previously thought and is a common issue among deuces if the water isn't bled from the air tanks after each operation.

 

[ToddJK]

@WhoMe08721 kudos for not only going through the brake system with a fine tooth comb and making it better than new, but also for trying to understand how everything works. Never mind a 25,000 lbs truck being slow off the line - the 25,000 lbs need to reliably stop in order to be safe in today's traffic, complete with ignorant and de-brained drivers.

Agreed. What scares me are the truckers and others operating large vehicles stopping at the last minute. Countless videos online of truckers and such losing brakes and wham! I'll take all the finger flippers, I'll slow down and coast my way to a stop, even with a long line of cars behind me. Murphy doesn't take sick days!

 

[Mullaney]

Agreed. What scares me are the truckers and others operating large vehicles stopping at the last minute. Countless videos online of truckers and such losing brakes and wham! I'll take all the finger flippers, I'll slow down and coast my way to a stop, even with a long line of cars behind me. Murphy doesn't take sick days!

.
Yes Sir!

I generally will pull over and let the line pass me if there is a safe spot to do so. Otherwise, like you said @ToddJK - rolling down the road, I can ignore all the salutes with the finger they want to send my way. I am really tempted to get myself a bumper sticker that says I May be Slow, But I am Ahead of You. But I figure there isn't any reason to antagonize... Or maybe a bumper strip with if you wanted to be ahead of me, you should have started sooner.

Driving with a little common sense and remembering how big and heavy these truck are really is a good plan!

 

[rustystud]

The oil your suppose to use in a "airpack" has water repelling quality's and rust repellents in it. It is mostly used in Aircraft hydraulics. I have a gallon on hand, but it is pricey. Another thing I have successfully used in the past is "silicone grease" on the "airpacks" piston and seal and cylinder walls. You will need to disassemble the unit though to apply it.

 

[ldmack3]

The oil your suppose to use in a "airpack" has water repelling quality's and rust repellents in it. It is mostly used in Aircraft hydraulics. I have a gallon on hand, but it is pricey. Another thing I have successfully used in the past is "silicone grease" on the "airpacks" piston and seal and cylinder walls. You will need to disassemble the unit though to apply it.

Are you talking about the red MIL5606?

 

[rustystud]

Are you talking about the red MIL5606?

Yes I believe that is the number. It is "red" and the MIL number is either 5606 or something really close.
It's about $50.00 a quart. Though I got a break on buying a gallon of it. There are several companies selling it now in smaller quantities. You used to have to buy it in fifty-five gallon drums.

 

[ldmack3]

It's gotten cheaper but still pricy compared to NAPA air tool oil.
1 qt about $35
12 qts $210/$17 qt.

 

[rustystud]

It's gotten cheaper but still pricy compared to NAPA air tool oil.
1 qt about $35
12 qts $210/$17 qt.

I would not use air tool oil in the airpack. It does not have the anti-rust/corrosion inhibiters like the MIL spec oil does. It would basically "flush" out the cylinder as that is it's job in an air tool.

 

[ldmack3]

What do you recommend? 5606 or disassemble and use silicone grease?

 

[rustystud]

What do you recommend? 5606 or disassemble and use silicone grease?

Well the best option would be to use the MIL spec oil. The problem there is the cost since it requires you to add oil on a regular basis, as a small amount of oil is expelled on every brake application. The "Silicone" grease lasts years though. I have two airpacks on my Deuce and have silicone grease in both right now. Works great. There is a slight pause in application since the grease does not allow the piston to move as quickly as the oil would, but it is very negligible.

 

[WhoMe08721]

Is this the 5606 aircraft hydraulic fluid you guys are talking about.