M35A2 will not maintain air pressure

I need some quick help please. Our fire department has a 63 White M35A2 and the air system will not hold pressure unless you hold the breaks. If the breaks are released then air comes out of the crankcase tube. We need this truck up and running for the storm this weekend and this problem showed up today. Any help on what could be possibly wrong would be outstanding. Email would be better because of the time frame involved. jreintges@gmail.com Thank you

I have been going through the manuals and I cant find anything on the air system that shows a layout of the piping or even talks about why the air is fed into the breather pipe. I am at a loss here and have even considered capping the pipe off so the air doesn't bleed out of it.

Only works when you hold breaks? Hmm..stop taking (tea or cofee) breaks then!. If you mean brakes. Ok you should check the one way valves in the braking system. ( or at the compressor) I went to check my TMs only to find the deuce ones are out on loan.
But as memory serves, I seem to recall that the service line goes from the compressor outlet to the regulator on the firewall to the 1st service air tank to the power brake cylinder. There will be check valves in the line. With the brakes released see if you can hear air escaping from the air pack. (Chock the vehicle). Build air to the maximum anyway you can, switch off and have personel imiediatly listen for air leaks. If leaking apply brakes, stops leak? and re-release to ascertain where the leak is. Sorry I lent out my TMs. But the air scematics are on this site. (and) perhaps other members will pitch in here. Post up your progress.

We checked the lines and with the brakes applied there is no air leaking. when the brakes are released the air comes out of the crank case vent tube and this is what has me lost.

The master cylinder is usually vented to the fuel tank vent which is vented to the slobber tube on the engine. There shouldn't be compressed air going to the master cylinder. The airpack has compressed air of course but it shouldn't be connected to the slobber tube. I'd check to see if something is piped in a strange way. Maybe there's a member near you who could take a look at it.

Flyingvan911 said:

The master cylinder is usually vented to the fuel tank vent which is vented to the slobber tube on the engine. There shouldn't be compressed air going to the master cylinder. The airpack has compressed air of course but it shouldn't be connected to the slobber tube. I'd check to see if something is piped in a strange way. Maybe there's a member near you who could take a look at it.

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Yep, that's pointing to the air pack. which as said is vented. On the return side. Could be a leak in there. Look in the Tm.s. and get the schematic tor the air system. (as) Im almost sure the compressor dump valve exits air to the vent lines as well.

Time to rebuild the air pack.

After that, remove the exhaust port from the fuel line breather.

All good things to check thank you. This truck had the engine replaced recently and the shop who did it isn't familiar with these trucks so bad piping or wrong connections are possible. What manual would have the air system diagram in it. I have downloaded all the ones under the M35 folder and didn't see it in them.

Looking at the 5ton tms there is no link to the master cylinder. But there apears to be a vent to the exhaust of the air pack. Air in the crankcase..leading to the slobber tube!?..Take off the oil filler cap and check there. Most odd. Im checking Tm Further. for a deuce schematic.

It sounds like there is confusion how the air is getting into the slobber tube, there is an air line that connects about 1/2 way down and that is where the air is getting into the tube. I have thought of capping off the air line for now but not sure if that will damage the system or not. is it possible that the air regulator could be the issue and is venting pressure into the tube?

Does the line that is plumbed into the slobber tube go up to a hard line that comes up the firewall near the air regulator?

In the older trucks, the fuel and airpack was connected to the same vent- that vent went to the slobber tube like you describe.

The fuel vapors screws with the airpack seals, so there is a WMO to separate the vents.

Rebuild your airpack and split the vents.

TB58 said:

Does the line that is plumbed into the slobber tube go up to a hard line that comes up the firewall near the air regulator?

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Thats the MWO, it sounds like his truck doesn't have it.

the line runs down to the frame rail and disappears in a mass of tubing and wires and hoses so I am not sure where it goes from there but the hydraulic side of the brakes still work. the problem started when it was taken for a test drive and we were on the side of the road in the rain trying to check it out. I will be at the garage tomorrow and see if I can figure out exactly where the line goes to.would it be safe to cap the vent so we can get through the weekend?

Eh I am certainly no expert on this brake system but if it is loosing large quantities of air out the vent that would mean that seals inside the air pack are bad. Who knows how long until they completely give up. Swapping out an air pack is fairly quick if you have a bleeder pot. Especially with this being a rescue vehicle I would think the risk of it losing its brakes would not be worth it.

TB58 said:

Eh I am certainly no expert on this brake system but if it is loosing large quantities of air out the vent that would mean that seals inside the air pack are bad. Who knows how long until they completely give up. Swapping out an air pack is fairly quick if you have a bleeder pot. Especially with this being a rescue vehicle I would think the risk of it losing its brakes would not be worth it.

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The only problem is that we do not have a spare air pack laying around and would not get one shipped on time for tomorrow. I was thinking of just capping the air to the air pack and just go manual brakes.

You will be best to deadline the truck till its fixed right.

If you plug the line, you will soon realize that you cannot stop the truck. Don't waste anymore time with this, you have other things to worry about.

THIS TRUCK WILL ONLY CAUSE PROBLEMS IF USED THIS WEEKEND. Might even put first responders in harms way.

Thanks for everyone's help. This truck will be sidelined and the parts will be on order tomorrow. How would you guys recommend routing the relief/ vent when I split them?

It looks like both Eastern Surplus and TNJ Murray are about 4 hours drive from Charlottesville, VA if you had someone to wake up early and make a road trip...


I just purchased a new air pack (its going into my truck tomorrow) from White Owl. They had the best price I could find.


Good luck with the truck and the storm. Stay safe!

jreintges said:

Thanks for everyone's help. This truck will be sidelined and the parts will be on order tomorrow. How would you guys recommend routing the relief/ vent when I split them?

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The vent line should be on the pass side frame rail and the "T" should be near a crossmember. Just pull the brake vent off and extend it up to the air filter area. Make sure you plug the hole left in the original vent tube, and that should be it.

The reason the air-pacs and fuel tanks where vented to the slobber tube was for deep water fording.