Air Pack Questions

My 1970 M35A2C had been sitting for a few years until I replaced the hydraulic head and injector lines. Now I have turned my attention to the brakes. The master cylinder was dry, and when I filled it I could see where it was leaking out of the front inside the boot. I replaced the mc, no more leaks, at least not externally. I installed a remote reservoir in the cab which is great because fluid level is easy to see now and easy to add. I gravity bled the top of the long type air pack and topped off the reservoir. Now a week later the level is down about 3/8 inch and there are drips on the air pack skid plate on both ends and on the rear of the air pack cylinder. The master cylinder exterior is dry. None of the line connections are wet. The brake switch in front of the air pack is not wet. Much of the air pack and lines has a layer of wet dirt on it. So my question is, do air packs leak brake fluid from seams or somewhere I cannot see? Should I replace the air pack? I don't want to try rebuilding it. There are a number of sources for new ones all about the same $600 price and Big Mike's is advertised as new manufactured. Any advice would be greatly appreciated.

I did some more research today and found the old thread about upgrading the hydraulic switch to the air side switch. It mentioned the switch can leak brake fluid so I will check that tomorrow as a possible source of my leak. It sounds like upgrading with Big Mike's kit would be a good idea regardless if my switch is leaking or not.

GunInstructordotcom said:

I did some more research today and found the old thread about upgrading the hydraulic switch to the air side switch. It mentioned the switch can leak brake fluid so I will check that tomorrow as a possible source of my leak. It sounds like upgrading with Big Mike's kit would be a good idea regardless if my switch is leaking or not.

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Yes, I agree with that. Eliminating yet another potential hole in in your one-circuit brake system is a good plan. FWIW, you can use any aftermarket air pressure brake switch; the plumbing is simple and the electric connection from the hydraulic switch gets re-used.
You will see that the hydraulic switch has a bakelite component crimped inside the metal hull and this thing appears to crack/leak over time.

cattlerepairman said:

Yes, I agree with that. Eliminating yet another potential hole in in your one-circuit brake system is a good plan. FWIW, you can use any aftermarket air pressure brake switch; the plumbing is simple and the electric connection from the hydraulic switch gets re-used.
You will see that the hydraulic switch has a bakelite component crimped inside the metal hull and this thing appears to crack/leak over time.

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Looking again at the air pack, I have lost another 1/4" of brake fluid in the remote reservoir and the bottom of the air pack canister was wet again. The leak is definitely coming from the canister area. Considering that I have owned this deuce for over 20 years now and I ignorantly never oiled the canister, I am not surprised. An air pack is on the way from Big Mike's. I'm doing the air brake switch conversion too since it will be so easy. At the rate I am going I will probably refurbish the entire brake system before I am done. Thanks, Garrett, for those videos. They are real gems if one has never worked on a deuce before. Between the videos and the various pictorial how-to threads on S.S., my life is much easier.

Does anyone know if the hydraulic side of the air pack is leaking, that this is a typical failure of old air packs?

I changed the gear oil in my deuce differentials. One of them hissed slightly when I removed the fill cap. Does that mean the breather is clogged?

GunInstructordotcom said:

I changed the gear oil in my deuce differentials. One of them hissed slightly when I removed the fill cap. Does that mean the breather is clogged?

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Yes. Hissing when you remove the drain plug means that you have something blocking your vent.
Possibility of small flying creatures building in the end of the vent...

Mullaney said:

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Yes. Hissing when you remove the drain plug means that you have something blocking your vent.
Possibility of small flying creatures building in the end of the vent...

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I suspected so. I'll check all the vents and possibly replace them. Thank you for the response.

GunInstructordotcom said:

I changed the gear oil in my deuce differentials. One of them hissed slightly when I removed the fill cap. Does that mean the breather is clogged?

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If you have the stock vents that came on the truck then yes it should have had a slight positive pressure buildup. Inside the vent is a spring and a small piece of rubber. They were designed like that to keep a small positive pressure inside the housing so when they forded the truck it would help keep water out of the differential. People have changed them out to open air vents but I never worried about it. And on the side note the other differential should have hissed a little positive pressure too so that meant that one could be the bad one. I've had some axles do it just a little when they're cold but usually it happens when the Axel has been hot that day. You can check those by unscrewing them and blowing through the backside and we clean them when they're installed by just twisting the top.

Thank you for the detailed information. This was the middle differential. I don't recall the front and back differentials hissing but I may have missed it. I'll check the vents out. In the 23 years I have owned this truck I have never touched them. I'm catching up on much deferred maintenance.

By the way, I plan on filling the transfer case and transmission with Redline MT-90 GL4 75W-90 synthetic gear oil. It has done wonders in my Toyota manual transmissions. I guess that is one more contribution to the never-ending debate about what oil the tranny and transfer should take. That decades-long debate reminds me of the .45 ACP versus 9x19 Parabellum caliber debate. It never ends and no one gets convinced by the arguments of others.

Floridianson said:

If you have the stock vents that came on the truck then yes it should have had a slight positive pressure buildup. Inside the vent is a spring and a small piece of rubber. They were designed like that to keep a small positive pressure inside the housing so when they forded the truck it would help keep water out of the differential. People have changed them out to open air vents but I never worried about it. And on the side note the other differential should have hissed a little positive pressure too so that meant that one could be the bad one. I've had some axles do it just a little when they're cold but usually it happens when the Axel has been hot that day. You can check those by unscrewing them and blowing through the backside and we clean them when they're installed by just twisting the top.

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I replaced the vents on my truck last year. Got the best price from armyjeepparts.com. I called them to ask about the vent style and the guy got confused when I mentioned it was for an M35 The M38A1 uses the same vents, maybe the M151 too?

Anyway when I replaced my vents the rear axles had a loud hiss to them. There was also gear oil in the hollow stud and therefore the hub and outer bearings, so I knew there was a problem with the vents and maybe seals. So if you have to do axle seals, I'd clean or replace the vents too!

Before truck season begins for the year I now have "check the axle vents" on the prep list!

HDN said:

I replaced the vents on my truck last year. Got the best price from armyjeepparts.com. I called them to ask about the vent style and the guy got confused when I mentioned it was for an M35 The M38A1 uses the same vents, maybe the M151 too?

Anyway when I replaced my vents the rear axles had a loud hiss to them. There was also gear oil in the hollow stud and therefore the hub and outer bearings, so I knew there was a problem with the vents and maybe seals. So if you have to do axle seals, I'd clean or replace the vents too!

Before truck season begins for the year I now have "check the axle vents" on the prep list!

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Axle seals is on my list. Brake system first, but then the two systems overlap when it comes to slave cylinders. Probably will be next year, as I am burned out from replacing the hydraulic head, brake master cylinder, and air pack this year, as well as installing the remote brake reservoir and replacing all the fluids. Incidentally, when my air pack arrives this week, I was thinking of using a stopper like a cork to plug the master cylinder to air pack line so I don't have to drain all that expensive Dot 5 fluid. Slightly loosen the master cylinder end, remove the air pack end and plug it. Sounds right in theory. Has anyone done this?

Floridianson said:

If you have the stock vents that came on the truck then yes it should have had a slight positive pressure buildup. Inside the vent is a spring and a small piece of rubber. They were designed like that to keep a small positive pressure inside the housing so when they forded the truck it would help keep water out of the differential. People have changed them out to open air vents but I never worried about it. And on the side note the other differential should have hissed a little positive pressure too so that meant that one could be the bad one. I've had some axles do it just a little when they're cold but usually it happens when the Axel has been hot that day. You can check those by unscrewing them and blowing through the backside and we clean them when they're installed by just twisting the top.

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Sometimes twisting the top isn't enough. I've had to take them off and using the backend of a drill bit or drift punch press a stuck rubber seal loose before.

GunInstructordotcom said:

Axle seals is on my list. Brake system first, but then the two systems overlap when it comes to slave cylinders. Probably will be next year, as I am burned out from replacing the hydraulic head, brake master cylinder, and air pack this year, as well as installing the remote brake reservoir and replacing all the fluids. Incidentally, when my air pack arrives this week, I was thinking of using a stopper like a cork to plug the master cylinder to air pack line so I don't have to drain all that expensive Dot 5 fluid. Slightly loosen the master cylinder end, remove the air pack end and plug it. Sounds right in theory. Has anyone done this?

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I'm careful about not calling the wheel cylinders "slave cylinders" on these trucks since the hydraulic part thats connected to the airpack is known as the slave cylinder in the TMs.

I am about to install the air brake light switch conversion kit. The white plastic sleeves that go over the crimp-on connectors may not fit over the metal connectors if I crimp the round rear part of the metal connector. It appears that crimping would oval the metal and the plastic sleeve would then not fit over the crimped area. Has anyone installed those crimp-on connectors? From what I can find, no one mentions a problem with the white sleeves going over the metal connector after crimping. The black rubber sleeves are the third layer that go over the white plastic sleeves.

Are you doing all new wiring? I just pulled the wires off the old hydraulic switch and put them on the new air one? Or maybe I already had an air activated switch and I just had to replace it when it blew apart . . . yeah, I think that's what it was. In that case, they make a crimper that crimps them round (forgot what it's called) and thats what a lot of people use when rewiring these trucks.

I am not familiar with a round crimper. I will do some research.

I found an old thread that describes the Balmer AF-8 crimper and how to modify it so the male connectors will pass through it. I found one on eBay so I'm set once it arrives. I may very well be replacing other wiring on my deuce, so having some connectors and the tool on hand will be handy. Tomorrow I install the new air pack and hope I can bend the line to the elbowed air side switch adapter without breaking it.

By the way, Dot 3 brake fluid dribbled out of the new air pack while I was switching over all the fittings. I know it was Dot 3 because it was smelly and it ate a bit of the paint off the bottom of the air pack. I am using Dot 5 so I hope that is not a problem.

GunInstructordotcom said:

I found an old thread that describes the Balmer AF-8 crimper and how to modify it so the male connectors will pass through it. I found one on eBay so I'm set once it arrives. I may very well be replacing other wiring on my deuce, so having some connectors and the tool on hand will be handy. Tomorrow I install the new air pack and hope I can bend the line to the elbowed air side switch adapter without breaking it.

By the way, Dot 3 brake fluid dribbled out of the new air pack while I was switching over all the fittings. I know it was Dot 3 because it was smelly and it ate a bit of the paint off the bottom of the air pack. I am using Dot 5 so I hope that is not a problem.

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With that small amount, and as much fluid as you're going to move through it when bleeding it, you won't have any problem.

I thought that would be the case. For all of those inexperienced guys like me, here is a tip. If you have the 3/8" line instead of the 1/2" line for installing the air side brake switch, I realized the fitting screwed into the air pack is 1/2" pipe x 3/8" compression. So i went to the local Peterbilt truck dealer and obtained a 1/2" male pipe x 1/2" female pipe elbow. Then I screwed the fitting removed from the air pack body into the elbow, and the line is now being threaded back onto the same fitting it came off. Easy-peasy and commonly available. I got the Peterbilt source fitting because I knew it would be DOT specs. All of $6.00, a bargain in these Bidenomics times. Additional note: I just installed the air pack. Because the added elbow moves the fitting over and extends it a bit, you will have to bend the brake line and align it before attaching. Check for clearance with the skid plate before mounting the skid plate. I had to bend my line up and towards the air pack to clear the skid plate. Just humble suggestions from a novice deuce mechanic.