Identify which rear drum brakes

[Tja1161]

I have a 1985 K2500 with 6.2 Diesel and need to order rear brakes. I don't see an RPO code that tells me which ones I

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have. Any help is appreciated

 

[Tja1161]

forgot to mention rear is a 14 bolt Semi float

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[Turbo4whl]

The easiest way to make sure you get the correct size, pull the wheel and the drum. Measure the drum diameter and measure the brake shoe width.

These squares all had rear brakes sized for the many different GVW's. The GM dealer's could usually match them well back in the day. Today parts houses still have some info, but they will ask you what is the drum diameter, how wide is the brake shoe pad.

 

[Keith Seymore]

Do you know your GVW?

Since it is a big axle I assume it is 8600 lbs. That would make the truck a JD7 and the rears would be 13"x2.5".

That's assuming nothing has been changed since it was built (like it's the rear axle it came with).

K

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[Tja1161]

Thanks, Turbo and Keith for your replies. Turbo I will take your advice. Keith per the VIN decode it says 6001-7000

 

[AuroraGirl]

Thanks, Turbo and Keith for your replies. Turbo I will take your advice. Keith per the VIN decode it says 6001-7000

just go take a photo of the drums from under the truck
this is a 14 bolt ff but the reason i ask is usually if there is more than one brake option it may visually be apparent, so it could help
heres a larger k25 brake on 14bolt ff (1980)

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here is the same differential with the smaller brakes (1976 but the diameter would be the same)

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[sickchev]

Measure the inner diameter of the drum. Mine was a little over 11 inches on my 1979 14 bolt FF. 11 inch rear brakes will be the JB6 front brakes. I have all the part numbers for a drum and front brake rebuild in the build thread if you are looking for specifics. https://www.gmsquarebody.com/threads/1979-c20-long-bed.37512/#post-778873

 

[Bextreme04]

You really don't even have to measure anything... The drum will almost always have a min or max measurement cast into the drum or rotor that you can't machine down past without compromising the drum/rotor. I have an 8600GVW K25 and it has vacuum brakes with the big 13x2.5 rear drums on a 14BFF. The drums had a "13.125 max" cast into them, so it was pretty obvious they weren't the 11" drums

 

[tophat36]

You really don't even have to measure anything... The drum will almost always have a min or max measurement cast into the drum or rotor that you can't machine down past without compromising the drum/rotor. I have an 8600GVW K25 and it has vacuum brakes with the big 13x2.5 rear drums on a 14BFF. The drums had a "13.125 max" cast into them, so it was pretty obvious they weren't the 11" drums

Is this what you mean? I really didn’t want to pull the drum and all that just to have to put it all back together while I order the parts. This looks to me to say max 13”. Does this other stamp on the drum mean anything? Appears to say H219. Should I just order 2 sets of different size shoes just in case since there’s really no guarantee to know before hand? Front disk were 13”, and as far as I can tell the 8600 GVW are JB7 brake option, is this just in reference to the front brake setup? I have the 14 bolt full float w/ hd chassis if it matters.

 

[Keith Seymore]

Is this what you mean? I really didn’t want to pull the drum and all that just to have to put it all back together while I order the parts. This looks to me to say max 13”.

Yes

 

[Keith Seymore]

Does this other stamp on the drum mean anything? Appears to say H219.

No. That's a date code.

 

[Keith Seymore]

far as I can tell the 8600 GVW are JB7 brake option, is this just in reference to the front brake setup?

JD7 in your case ("D" = hydroboost, or diesel. "B" = vacuum).

That defines the total brake system, not just the front.

K

 

[HotWheelsBurban]

Is this what you mean? I really didn’t want to pull the drum and all that just to have to put it all back together while I order the parts. This looks to me to say max 13”. Does this other stamp on the drum mean anything? Appears to say H219. Should I just order 2 sets of different size shoes just in case since there’s really no guarantee to know before hand? Front disk were 13”, and as far as I can tell the 8600 GVW are JB7 brake option, is this just in reference to the front brake setup? I have the 14 bolt full float w/ hd chassis if it matters.

I enlarged the picture and that actually says 13.09". Most OEM drums have.090" room before they become oversized.
You have the 13"x 2 1/2" drums. I think this is standard on the later 14 bolt semi floating axle equipped trucks.

 

[Bennyt]

If it was me, I'd also order a full hardware kit and possibly wheel cylinders. It's all fairly cheap online for high quality stuff and you won't have to look at again for 100k miles.

 

[Bextreme04]

JD7 in your case ("D" = hydroboost, or diesel. "B" = vacuum).

That defines the total brake system, not just the front.

K

No, she's right. She has JB7, just like my 1980 8600GVW K25 with a 350.

Is this what you mean? I really didn’t want to pull the drum and all that just to have to put it all back together while I order the parts. This looks to me to say max 13”. Does this other stamp on the drum mean anything? Appears to say H219. Should I just order 2 sets of different size shoes just in case since there’s really no guarantee to know before hand? Front disk were 13”, and as far as I can tell the 8600 GVW are JB7 brake option, is this just in reference to the front brake setup? I have the 14 bolt full float w/ hd chassis if it matters.

Yes, for our year when you look up the brake hardware there are two 11" options and only the single 13" option. You will need the 13" ones. You have the exact same axle and equipment I do. I rebuilt the entire rear axle, brakes, hubs, etc... about 2 years ago. If you are doing the brake shoes, you might as well have your local oreilly's turn the drums and also replace all of the bearings/seals in there as well. If I remember correctly it was about $20-30 per drum to get them turned. The whole axle and drum has to come off to access the brakes. Here is all of the parts I used to rebuild mine(all from rockauto.com). You will need everything except for the Turn signal switch and Differential Rebuild kit in this list to do a full rebuild of the hubs/brakes.

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