M35A3 brake issue Air hisses out master cylinder front when braking

[1 Patriot-of-many]

When I depress the brake pedal air hisses out the front of the master cylinder past the boot, when fully depressed it stops. Brakes stop the vehicle. When i release the brake pedal from a stop, it hisses air like a brake chamber on an airbrake equipped vehicle. How much of an issue is this to drive? I don't really understand the air over hydraulic. If i lose the air does the mechanical hydraulic still work? I've looked all over the site and web but can't find a rebuild kit for the master cylinder. Any leads or advice?

 

[HDN]

When I depress the brake pedal air hisses out the front of the master cylinder past the boot, when fully depressed it stops. Brakes stop the vehicle. When i release the brake pedal from a stop, it hisses air like a brake chamber on an airbrake equipped vehicle. How much of an issue is this to drive? I don't really understand the air over hydraulic. If i lose the air does the mechanical hydraulic still work? I've looked all over the site and web but can't find a rebuild kit for the master cylinder. Any leads or advice?

If you lose air, you still have brakes, but it's much harder to stop as you might imagine. I'd get the issue fixed so you don't find yourself surprised if you don't test the brakes before actually braking for a stop. If it were me I wouldn't drive it until it's fixed. I don't think that would even pass an inspection, in NY anyway.

The air acts as a booster just as vacuum or hydraulics are used in cars. Think about cutting your car's power while braking and multiply the trouble by 4 or so for what it would be like for your deuce!

It sounds like the master cylinder can use a rebuild or replacement. Did you check the maintenance TMs for troubleshooting and repairing this issue?

 

[USAFSS-ColdWarrior]

RED LINE the vehicle until repaired.

The liabilities fully justify removing this vehicle from service until it is returned to serviceable condition. Lives are on the line here.

In my humble opinion.

 

[HDN]

I looked in the TMs regarding the master cylinder. It seems that there are no air lines going to it, only hydraulic lines. I'm thinking the air is coming from somewhere else. Can you please post a picture of where you think the air is escaping?

Maybe there is air in the hydraulic side of the brake system and it is actually escaping past the boot? If that's the case, it sounds like the brakes need to be bled, and a new master cylinder may be needed because everything in the braking system should be tight and not leaking anything (at least anything constituting a class 3 fluid leak per the TMs).

 

[1 Patriot-of-many]

If you lose air, you still have brakes, but it's much harder to stop as you might imagine. I'd get the issue fixed so you don't find yourself surprised if you don't test the brakes before actually braking for a stop. If it were me I wouldn't drive it until it's fixed. I don't think that would even pass an inspection, in NY anyway.

The air acts as a booster just as vacuum or hydraulics are used in cars. Think about cutting your car's power while braking and multiply the trouble by 4 or so for what it would be like for your deuce!

It sounds like the master cylinder can use a rebuild or replacement. Did you check the maintenance TMs for troubleshooting and repairing this issue?

Yeah I've driven while it been doing that around the block and even checked the brakes before the air pressure even built up definitely have brakes. been looking for a rebuild kit for the M/C, new one for the A3 costs $1000!

 

[1 Patriot-of-many]

I looked in the TMs regarding the master cylinder. It seems that there are no air lines going to it, only hydraulic lines. I'm thinking the air is coming from somewhere else. Can you please post a picture of where you think the air is escaping?

Maybe there is air in the hydraulic side of the brake system and it is actually escaping past the boot? If that's the case, it sounds like the brakes need to be bled, and a new master cylinder may be needed because everything in the braking system should be tight and not leaking anything (at least anything constituting a class 3 fluid leak per the TMs).

On the A3? It felt like it was coming from the boot. Maybe it's actually coming from the airpack under the M/C?...... I'll have to crawl back under there and make sure. Thanks guys for the input.

 

[HDN]

On the A3? It felt like it was coming from the boot. Maybe it's actually coming from the airpack under the M/C?...... I'll have to crawl back under there and make sure. Thanks guys for the input.

I encourage you to look through this PDF as the split brake system on the M35A3 is very similar to those fitted to USAF M35A2s, just with some revised brake system component placement. For maintenance info on the brake system of the M35A3 specifically, consult TM9-2320-386-24-1-1, WP 0154.

 

[1 Patriot-of-many]

Thanks HDN, I've looked through the TM's can't seem to find any troubleshooting for this problem. Gonna have to crawl under there and remove the skid plate under the drivers side airpack to see if that's where it's coming from.

 

[glcaines]

When I depress the brake pedal air hisses out the front of the master cylinder past the boot, when fully depressed it stops. Brakes stop the vehicle. When i release the brake pedal from a stop, it hisses air like a brake chamber on an airbrake equipped vehicle. How much of an issue is this to drive? I don't really understand the air over hydraulic. If i lose the air does the mechanical hydraulic still work? I've looked all over the site and web but can't find a rebuild kit for the master cylinder. Any leads or advice?

Is this something new happening? I would recommend letting the air pressure build up and then shutting down the engine. Then press the brake pedal and see where the air is coming from. With the engine running there is a lot of noise. On my M35A3 you can hear the air packs operating when you push the brake pedal. You can also hear air being discharged when the brake pedal is released. You need to ascertain that your brakes are working properly before driving again.

 

[frank8003]

Aattach a compressor service air line to the rear gladhand .
With truck engine OFF have soldier B operate the brake pedal while you do your crawl inspection,
actually lot if it is sitting up. get as lot of light.
It is much more safe and much quieter to do it that way. Fix it.

 

[1 Patriot-of-many]

I removed the skid plate, guess what I found? LMAO. The brake switch? is blown apart on the drivers side airpack. Guess what I happen to have on hand in my collection of stuff? At least i think that's what I have, the numbers are faded on the bag. Talk about serendipity! Thank you Lord.

 

[frank8003]

I removed the skid plate, guess what I found? LMAO. The brake switch? is blown apart on the drivers side airpack. Guess what I happen to have on hand in my collection of stuff? At least i think that's what I have, the numbers are faded on the bag. Talk about serendipity! Thank you Lord.

We are old ...................................
As this age of You and Me and Her just replace both of them switches.

Was very common in "the old days" for oil pressure "switch" to blow out and destroy engines, same technology.
Switch leaked while engine running and blew and put entire output of crankcase onto the ground, Red light comes on and 20-30 seconds later She slips a main bearing. Not everybody remembers.

Very common, in GM cars, all flavors, and a few different Fords too.

Duel circuit brakes MC rebuild is just a few o-rings, not like M35A2 single brakes actuators.

 

[HDN]

I'm glad you found the air leak! Best of luck getting your truck back on the road soon

 

[1 Patriot-of-many]

Thanks guys, amazingly I actually had the part on hand in the top of my pile of stuff! Talk about serendipity! Couldn't even see the part number on the bag it was so faded, but it looked the same, all fixed works properly now.