Do Q brakes require drum replacement when shoes replaced?

belfert · September 20, 2025, 02:22:20 PM

I need to replace the shoes on my drive axle brakes. My understanding is I have Q brakes on my Dina.

I have heard that Q brakes require new drums at every shoe change. Is this correct?

luvrbus · September 20, 2025, 05:45:20 PM

Yea you do Q brakes only work good when the thickness of the drum is right and your geometry is going to be off with new shoes and old drums, Q brakes use a higher lift S cam they are not as forgiving as the older style brakes

belfert · September 20, 2025, 10:10:03 PM

Damn. That means my bus may very well be parked until 2027 to come up with the funds to fix the brakes.

Drums were $600 each plus truck freight in 2006. They were also hard to find back then. I imagine the price with shipping will be over $1,000 per drum, if not even more. I expect with shoes, misc parts, and labor I could be looking at upwards of $5,000 to replace the brakes on my drive axle.

I also need a new rooftop A/C unit, and I really should replace the steer tires in 2026. I replace the steer tires every five years and rotate them back to the tag axle. The steer tires I expect will cost a minimum of $1,500 mounted.

thomasinnv · September 21, 2025, 07:13:56 AM

My MCI service manual has a section on Q brakes and it shows a table with maximum allowable drum diameter for re-use. It states the drum can be re-used as long as the diameter falls within the specification.

luvrbus · September 21, 2025, 11:41:06 AM

Quote from: thomasinnv on September 21, 2025, 07:13:56 AM
My MCI service manual has a section on Q brakes and it shows a table with maximum allowable drum diameter for re-use. It states the drum can be re-used as long as the diameter falls within the specification.

Must be a MCI thing,my Bendix Q training manuals (2001) say to replace drums because of such a close tolerance you cannot turn one, on the Q-2 system you replace all pins and the high dollar S cams,the Q brake is a good system I don't why the clearances had to be so close.I didn't read it but the instructor told us if a new drum was over 0.020 out of round don't use it

belfert · September 21, 2025, 03:01:31 PM

The post mentioning the service reminded me to check my service manual to see what it says about the brakes.

thomasinnv · September 22, 2025, 08:12:48 AM

Quote from: belfert on September 21, 2025, 03:01:31 PM
The post mentioning the service reminded me to check my service manual to see what it says about the brakes.

When all else fails, rtfm lol.

belfert · September 23, 2025, 05:56:11 PM

Quote from: thomasinnv on September 22, 2025, 08:12:48 AM
When all else fails, rtfm lol.

The manual says nothing about if drums should be replaced when the shoes are replaced.

thomasinnv · September 24, 2025, 06:15:48 AM

Quote from: belfert on September 23, 2025, 05:56:11 PM
The manual says nothing about if drums should be replaced when the shoes are replaced.

Service manual doesn't give you any specs for allowable diameter of the drum? I wonder if 'Q brakes' are the same across different manufacturers? Are they just like a brand name or type like meritor? I can snap a picture of the q brake section in my manual if you're interested. Maybe luvrbus can chime in on wether q brakes are the same on dyna and mci?

luvrbus · September 24, 2025, 07:27:30 AM

Quote from: thomasinnv on September 24, 2025, 06:15:48 AM
Service manual doesn't give you any specs for allowable diameter of the drum? I wonder if 'Q brakes' are the same across different manufacturers? Are they just like a brand name or type like meritor? I can snap a picture of the q brake section in my manual if you're interested. Maybe luvrbus can chime in on wether q brakes are the same on dyna and mci?

They can be different from OEM to others they are sized for weight and the DOT required stopping distance, The drums I replaced on the schools Dina where 16.5 x 7 inches, all the replacement drums I saw for the Q brakes have a "Q" in the parts number. The Dina is a good tough bus designed for the Mexicans roads and highways and you never know what they have, I never under stood the Roto chambers unless it was for stopping distance those chambers are expensive they cost more than the DD-3's

belfert · September 24, 2025, 01:25:51 PM

My rotochambers were replaced with standard brake chambers years ago.

luvrbus · September 24, 2025, 06:27:13 PM

Quote from: belfert on September 24, 2025, 01:25:51 PM
My rotochambers were replaced with standard brake chambers years ago.

I know I remember telling you could change those when you were bummed about the price

belfert · September 25, 2025, 01:59:46 PM

Quote from: thomasinnv on September 24, 2025, 06:15:48 AM
Service manual doesn't give you any specs for allowable diameter of the drum? I wonder if 'Q brakes' are the same across different manufacturers? Are they just like a brand name or type like meritor? I can snap a picture of the q brake section in my manual if you're interested. Maybe luvrbus can chime in on wether q brakes are the same on dyna and mci?

My understanding is that Q brakes are a style of air brakes originally made by Rockwell International (Now Meritor/Cummins.) I think others make Q brake parts now.

Yes, the manual has allowable tolerances for drum sizes. Of course, I don't have the correct tool for measuring the inside diameter of a cylinder.

thomasinnv · September 25, 2025, 03:06:13 PM

Quote from: belfert on September 25, 2025, 01:59:46 PM
My understanding is that Q brakes are a style of air brakes originally made by Rockwell International (Now Meritor/Cummins.) I think others make Q brake parts now.

Yes, the manual has allowable tolerances for drum sizes. Of course, I don't have the correct tool for measuring the inside diameter of a cylinder.

Any machine shop should be able to measure them for you. Or any shop that turns rotors and drums.