Bad airpack?

[M109a3RV]

Trying to improve a soft brake pedal and have bled the entire brake system. Noticed a slow, steady stream of large air bubbles coming up into the master cylinder reservoir when the air tanks are pressurized. Is this the sign of a bad airpack?

 

[V8srfun]

That is interesting you may be able to rebuild the air pack for much less than replacing it. But it really can’t be anything but the air pack there is no other source for pressurized air to get in the system.

 

[nchittendon]

You can find rebuild kits out there for a fraction of what it costs to get a different airpack. There are some great write ups with pictures on here too. I only mention it because I think I need to tear mine apart as well.
My brakes are always great in the summer. Good pedal, good feel, great stopping power. Once it gets cold out, I get about half of the response/pedal travel than i normally get. Yesterday I had zero pedal. I do 90% of my slowing with my transmission anyways, but needed the park brake to come to a complete stop. I don't have leaks anywhere, but i've never changed fluid or bled the system either. Regardless, with all the trouble I see these airpacks giving MV owners, I plan to rebuild it and give the whole system an update.

Get a rebuild kit, and have the peace of mind that you KNOW it's good.

 

[M109a3RV]

That is interesting you may be able to rebuild the air pack for much less than replacing it. But it really can’t be anything but the air pack there is no other source for pressurized air to get in the system.

Thanks for the reply. Definitely getting air through the airpack into the reservoir . Looks like I’ll be rebuilding an airpack soon.

 

[M109a3RV]

You can find rebuild kits out there for a fraction of what it costs to get a different airpack. There are some great write ups with pictures on here too. I only mention it because I think I need to tear mine apart as well.
My brakes are always great in the summer. Good pedal, good feel, great stopping power. Once it gets cold out, I get about half of the response/pedal travel than i normally get. Yesterday I had zero pedal. I do 90% of my slowing with my transmission anyways, but needed the park brake to come to a complete stop. I don't have leaks anywhere, but i've never changed fluid or bled the system either. Regardless, with all the trouble I see these airpacks giving MV owners, I plan to rebuild it and give the whole system an update.

Get a rebuild kit, and have the peace of mind that you KNOW it's good.

Definitely the airpack leaking. Talked to Big Mike and will be ordering a rebuild kit. Going to replace the old brake lines while everything is apart.

 

[M37M35]

Trying to improve a soft brake pedal and have bled the entire brake system. Noticed a slow, steady stream of large air bubbles coming up into the master cylinder reservoir when the air tanks are pressurized. Is this the sign of a bad airpack?

Check the exhaust vent line from the airpack going up under the hood and see if it's plugged somehow. It sounds like your airpack is leaking air internally, but it should vent out the exhaust line before forcing it's way into the brake fluid side.

 

[M109a3RV]

Check the exhaust vent line from the airpack going up under the hood and see if it's plugged somehow. It sounds like your airpack is leaking air internally, but it should vent out the exhaust line before forcing it's way into the brake fluid side.

Here’s what I noticed today:
The truck was not running, zero air pressure. No bubbles in the m/c reservoir.
Started the truck and let air pressure build, bubbles began to rise up in the m/c.
I never touched the brake pedal at all.

I assume the underhood vent is for when you release the brake pedal after applying the brakes.

 

[M37M35]

Here’s what I noticed today:
The truck was not running, zero air pressure. No bubbles in the m/c reservoir.
Started the truck and let air pressure build, bubbles began to rise up in the m/c.
I never touched the brake pedal at all.

Yeah, that definitely points to something going on with the airpack.
It sounds like there's two issues here...
One is the valve in the airpack leaking.
Second is the leaking air finding it's way into the brake fluid.

I just rebuilt my airpack awhile back, and offhand I can't think of any way leaking air would get into the brake fluid without the vent being blocked, or the brake fluid seals being so bad it dumps fluid like crazy.
I'm almost positive your vent is blocked.

Let's try something... I bet if you remove the cap and open the valve on the service gladhand at the rear of the truck (the one on the drivers side) and try the above again, that you won't have the bubbles...

I assume the underhood vent is for when you release the brake pedal after applying the brakes.

Yes. It vents air pressure from the airpack when you release the brakes. It also vents the air from the trailer brakes, if you're pulling a trailer.

 

[M109a3RV]

Yeah, that definitely points to something going on with the airpack.
It sounds like there's two issues here...
One is the valve in the airpack leaking.
Second is the leaking air finding it's way into the brake fluid.

I just rebuilt my airpack awhile back, and offhand I can't think of any way leaking air would get into the brake fluid without the vent being blocked, or the brake fluid seals being so bad it dumps fluid like crazy.
I'm almost positive your vent is blocked.

Let's try something... I bet if you remove the cap and open the valve on the service gladhand at the rear of the truck (the one on the drivers side) and try the above again, that you won't have the bubbles...


Yes. It vents air pressure from the airpack when you release the brakes. It also vents the air from the trailer brakes, if you're pulling a trailer.

Doesn’t the airpack have a plunger with brake fluid on one side and air on the other? When you apply the brakes the plunger moves and regulates how much air is applied to the air piston?

 

[M37M35]

Doesn’t the airpack have a plunger with brake fluid on one side and air on the other? When you apply the brakes the plunger moves and regulates how much air is applied to the air piston?

Sort of...
The air pressure and brake fluid don't share the same piston. Trying to explain it the best I can from memory...

When you push the brake pedal, fluid from the master cylinder pushes on two plungers in the airpack. One pushes fluid out to the wheel cylinders, the other pushes on the air valve to close the vent line and send air to the air plunger in the airpack and to the trailer brakes. When air pushes on the air plunger, a rod from it pushes on the plunger that pushes fluid to the wheel cylinders. When you release the brake pedal, the air valve closes the air coming in, and opens the valve to the vent line to release the air from the air plunger and the trailer brakes.

The back sides of the air plunger and the plunger that pushes on the air valve I believe are supposed to be open to the vent all the time so no pressure builds up there. I think what's happening with your airpack (among other things) is that passageway or vent line is blocked and air pressure is building up on the back sides of those plungers and forcing it's way around the seals into the brake fluid side.

I think your airpack still needs to be rebuilt because it shouldn't be leaking air to begin with, but try what I said in the last post and see if the bubbles in the MC stop.

Also, remove the vent line from the airpack (the one that runs up under the hood) and blow compressed air through it to make sure it's clear. When you do that, remove the cap from the fuel tank. Some trucks have the vent line for the fuel tank tied into that line and you don't want to pressurize your fuel tank.

 

[M109a3RV]

Sort of...
The air pressure and brake fluid don't share the same piston. Trying to explain it the best I can from memory...

When you push the brake pedal, fluid from the master cylinder pushes on two plungers in the airpack. One pushes fluid out to the wheel cylinders, the other pushes on the air valve to close the vent line and send air to the air plunger in the airpack and to the trailer brakes. When air pushes on the air plunger, a rod from it pushes on the plunger that pushes fluid to the wheel cylinders. When you release the brake pedal, the air valve closes the air coming in, and opens the valve to the vent line to release the air from the air plunger and the trailer brakes.

The back sides of the air plunger and the plunger that pushes on the air valve I believe are supposed to be open to the vent all the time so no pressure builds up there. I think what's happening with your airpack (among other things) is that passageway or vent line is blocked and air pressure is building up on the back sides of those plungers and forcing it's way around the seals into the brake fluid side.

I think your airpack still needs to be rebuilt because it shouldn't be leaking air to begin with, but try what I said in the last post and see if the bubbles in the MC stop.

Also, remove the vent line from the airpack (the one that runs up under the hood) and blow compressed air through it to make sure it's clear. When you do that, remove the cap from the fuel tank. Some trucks have the vent line for the fuel tank tied into that line and you don't want to pressurize your fuel tank.

Thanks for your detailed reply. I’ve already removed and disassembled the airpack, waiting for rebuild kit to arrive. I will definitely confirm that the vent line is clear tomorrow.

 

[M109a3RV]

Sort of...
The air pressure and brake fluid don't share the same piston. Trying to explain it the best I can from memory...

When you push the brake pedal, fluid from the master cylinder pushes on two plungers in the airpack. One pushes fluid out to the wheel cylinders, the other pushes on the air valve to close the vent line and send air to the air plunger in the airpack and to the trailer brakes. When air pushes on the air plunger, a rod from it pushes on the plunger that pushes fluid to the wheel cylinders. When you release the brake pedal, the air valve closes the air coming in, and opens the valve to the vent line to release the air from the air plunger and the trailer brakes.

The back sides of the air plunger and the plunger that pushes on the air valve I believe are supposed to be open to the vent all the time so no pressure builds up there. I think what's happening with your airpack (among other things) is that passageway or vent line is blocked and air pressure is building up on the back sides of those plungers and forcing it's way around the seals into the brake fluid side.

I think your airpack still needs to be rebuilt because it shouldn't be leaking air to begin with, but try what I said in the last post and see if the bubbles in the MC stop.

Also, remove the vent line from the airpack (the one that runs up under the hood) and blow compressed air through it to make sure it's clear. When you do that, remove the cap from the fuel tank. Some trucks have the vent line for the fuel tank tied into that line and you don't want to pressurize your fuel tank.

Vent line was clear. Hopefully the rebuild will do the trick. The seal on the air piston shaft looked a little suspicious.