Brake Plunger Mystery

[Oski1042]

Working on fixing a non-functioning front left brake on my 1995 M1078. I have confirmed I have air to the chamber and it is functioning properly. From there I pulled apart the plunger assembly. It was interesting because it was packed with grease that looked new. Anyway ... when I pulled the wedge assembly out of the wedge housing I found one of the rollers out of it's brackets. When put it back in .... I noticed it is asymetric

Is that what it should look like?

Also I am left wondering how this would work. It seems to me when it is depressed the wedge assembly pushes out against the two adjusting plungers. But it would seem to me the rollers would have to be larger than they are.


What am I missing?

 

[Oski1042]

Follow up ... finding the part number in the TM's I found the Meritor Catalog and this picture ..

Clearly the roller brackets are not asymetrical. They must have gotten torqued.

I still wonder though how these push out far enough (when air compresses this unit) to move the adjustable anchor plungers.

 

[Oski1042]

Went ahead and ordered one from finditparts dot com

 

[Ronmar]

If memory is working, shoe to drum clearance should be .020-.040” a roller should only have to move 40 thousandths of an inch to push the shoe into contact with the drum. I think if you put a calliper across the wedge at minimum and maximum thickness you will find its difference greater than 80 thousandths .080” required(40 tho per side/shoe)...

 

[Oski1042]

If memory is working, shoe to drum clearance should be .020-.040” a roller should only have to move 40 thousandths of an inch to push the shoe into contact with the drum. I think if you put a calliper across the wedge at minimum and maximum thickness you will find its difference greater than 80 thousandths .080” required(40 tho per side/shoe)...

I always learn a lot from these discussions ... thanks. I didn't initially understand how the plunger works. I thought the whole unit is compressed. Now I understand it is just the rollers that press out against the 14 degree sloped sleeves. It is amazing how such a small movement can develop such force.

 

[m-35tom]

ramps. just like levers. The wedge is between the rollers, the rollers come out sideways as they also move inward creating enough clearance for the roller holder to fit between the shoe piston. Just what I see in the pictures, I may not be right!

 

[Oski1042]

So it turns out the plunger was not the problem. When I put it all back together and took it out for a test drive ... it was still not working. So now I am wondering if there is a problem with the air chamber. I know that air is getting to the chamber. But when I took the chamber off and reattached the air lines. There was no movement in the piston inside the chamber. So I was hoping to learn:

• Do front brake air chambers go bad?
• Can I open it up and look for a problem?
• Do they have any killer springs like I have read about on the rear brakes?
• Any other tips you guys might have...

Thanks

 

[Ronmar]

So it turns out the plunger was not the problem. When I put it all back together and took it out for a test drive ... it was still not working. So now I am wondering if there is a problem with the air chamber. I know that air is getting to the chamber. But when I took the chamber off and reattached the air lines. There was no movement in the piston inside the chamber. So I was hoping to learn:

• Do front brake air chambers go bad?
• Can I open it up and look for a problem?
• Do they have any killer springs like I have read about on the rear brakes?
• Any other tips you guys might have...

Thanks

Yes chambers go bad, but they usually leak air. In this case air applied to the activation port would pass on thru a damaged diaphragm and exit the vent port.
yes you can open it up.
The front chambers have no mechanical springs.
are you sure you have the air lines attached to it correctly? If you were feeding air to the vent port it wont work. The activation port is the port farthest away from the plunger and brake drum. The vent port is the one nearest to the plunger and brake drum. You should also be able to remove the actuator and test it. Slide something down the tube and apply compressed air and a blowgun to the actuator port and the object you put down the tube should move...
When assembled, if you don't see the shoes moving when air is applied there is no point driving it...

 

[Oski1042]

Yes chambers go bad, but they usually leak air. In this case air applied to the activation port would pass on thru a damaged diaphragm and exit the vent port.
yes you can open it up.
The front chambers have no mechanical springs.
are you sure you have the air lines attached to it correctly? If you were feeding air to the vent port it wont work. The activation port is the port farthest away from the plunger and brake drum. The vent port is the one nearest to the plunger and brake drum. You should also be able to remove the actuator and test it. Slide something down the tube and apply compressed air and a blowgun to the actuator port and the object you put down the tube should move...
When assembled, if you don't see the shoes moving when air is applied there is no point driving it...

Thank you so much. The air lines are hooked up correctly. I will open it up tomorrow and take a look. Do you happen to know if there is a rebuild kit for these chambers? I looked in the TM's and around the web ... but couldn't find anything.

Again ... thank you for the primer!

 

[Floridianson]

https://www.axletech.com/at-admin/resources/Manuals/mm-0153.pdf

 

[Oski1042]

Thank you so much ! I can't tell you how useful this is.