77 c20 'heavy' Brake Drum question

[WALYS60]

Hi All,

New guy here. I’ve been on the 60-66 GMC forum for several years as I’ve been working on my Granddad’s (Wally) 60 GMC. Now my Dad passed last June and my brother and I have inherited his c20 ’77 square body. Ultimately it will be my brothers since I have the GMC but I’m borrowing it until I get the old one back on the road.

Anywho, that is the background. Now to the issue. I drove it 350 miles to my house with very little trouble. The power steering pump was going out but we changed that out without any trouble. Also, the rear left brake would drag and ultimately I lost most rear braking as the back reservoir of the master cylinder went dry. I thought maybe the two issues were related but I identified a leaky wheel cylinder on the drinker’s side and the driver’s side drum is just SHOT. Plenty of pad but at some point in its life it had to have run well beyond pad and probably into springs before it was looked at. The grooves feel like you’re running your fingers over banjo springs.

So here is the question…

It has some ‘rare’-ish ¾ ton ‘heavy’ brake setup on it. It’s the 14 bolt full floating single wheel type, 13 inch (pictured). I have found it on this forum in a few conversations but they were all about converting them. That is AWESOME…and I’m sure my brother will eventually want to do that. However, it ain’t gonna be mine ultimately, and I don’t want to put that kind of money into it! lol. I just want to make it relatively safe and usable. I plan on going to the classic car salvage lot to see if there might be a drum I can use…but otherwise I just plan on doing what, apparently, Dad did and throw some new shoes and wheel cylinder at it and call it good.

The guy at the part store is willing to work with me but we can’t seem to identify it in the system. Is there something I can call it that they can use to find the shoes and cylinder, or would someone have a part number for it, or the internals (shoes, etc)?

Thanks VERY much in advance,
Ron S.

[Dead Parrot]

Can it be turned? Normally OEM drums will have stamped a min thickness or max diameter spec to guide the shop.

[Keith Seymore]

What is the GVW of the truck?

K

[cadillac_al]

Usually the diameter and width of the shoes will get you the right brake parts. I would spring for the hardware kit and adjusters too. I have to do my 76 K20 this summer.

[THI]

The 13" brake drum would be the JB7 option on brakes, it takes a 2.5" wide shoe and the drum needs to be pressed on and off the hub. If the drum is as grooved as you say it is, you will have to replace it and should do the same to the other side at the same time.

I would not be in a rush to convert to disc brakes as these old 13" drums work really well! I currently have a 1978 C20 (Chevy) and 1978 K25 (GMC) and have owned several square bodies over the years with the same setup - never had an issue with the rear brakes not being adequate even carrying a slide in camper! Most who convert to rear disc brakes do so for off-roading - mud cleans off of disc brakes better than drum brakes.

[WALYS60]

Thanks all! Great info. I will respond to each…

“Can it be turned? Normally OEM drums will have stamped a min thickness or max diameter spec to guide the shop.” – I am going to take it in to see for sure but I’m guessing it is beyond turning with as deep as the grooves are. That did make me go look closer for stamps on it though and I found ‘GM 331442’ in addition to what I had already found …13.09 max dia, and F187 which was no help at all.

“What is the GVW of the truck?” – 6400

“Usually the diameter and width of the shoes will get you the right brake parts. I would spring for the hardware kit and adjusters too. I have to do my 76 K20 this summer.”- Roger, okay, if I can’t ID them I’ll just go with the 13” shoes and see if they work!

“The 13" brake drum would be the JB7 option on brakes, it takes a 2.5" wide shoe and the drum needs to be pressed on and off the hub. If the drum is as grooved as you say it is, you will have to replace it and should do the same to the other side at the same time.” Awesome, okay. Any idea where I can find these drums, other than a junk yard? I can’t seem to find it with JB7 or with the SN I found on em.

[K25alive]

I think the GM p/n for the drum is 6260588, for 74-78 CKGP-2, 3 w/JB-7 (exec M.H.C., Dana axle). The Raybestos p/n is 8027.

[Robert Crandall]

The chart in the photos in this thread may help identify the system, if that is what is needed.

https://67-72chevytrucks.com/vboard/...d.php?t=843188

[THI]

Should be able to get those drums through any auto parts store - I got my last set through NAPA for my C20 and will soon be getting another set for my K25

[Warrens69GMC]

Rockauto has them, use their part number and have them cross reference .

[WALYS60]

Wait a second...are you saying the drum in this picture is the drum and that the 'hub' is attached via the lugs?

[Warrens69GMC]

You have to take off your hub, by pressing out your wheel studs and transferring your hub to the new drum.

[WALYS60]

I'm an idiot. Of course! I swear I don't see the seam but obviously just grime covered. (sigh) oh well, not the first time I had to be slapped in the face to see the obvious answer.

Thanks all!