12 Bolt Drum Brake Upgrade

[aotte1]

Researching is it possible to upgrade early ( 2" shoes) 12 bolt drum brakes to the newer heavy duty brakes ( JB 5 brake option) found on later 12 bolts. Believe, the backing plates would need to be changes out. Specifications for 1979 K10, with the JB5 option for 12 bolts were provided to me by member Keith Seymore, below. Thanks Keith. Thinking of using the '79 specifications, because the front axle will be a '79 Dana 44 with the heavy duty/JB5 brake option. A more general question previously on 12 bolt brakes, has been asked in the 67 to 72 forum, by Lostmy65 in an older thread..

Les

[aotte1]

So directly from the '79 brake chart for a K10 JB5:

Drum - 359492; size is 283.2mm x 69.8mm
Wheel cylinder - 18004890 (diameter is 23.8mm)
Master Cylinder w/Boost - 18004252 (Delco Moraine, 28.57 mm dia); optional Bendix M/C = "KP"; booster = tandem vacuum 18003681
Combination Valve - 18002887: metering = 517 kPa, split point 1379 kPa, slope = .41
Backing plate:
Flange plate & brake asm LH = 14009889
Flange plate & brake asm RH = 14009890

The 69.8 mm is 2.75 inches. And 283.2 is about 11.15 inches.

[racecougar]

Yep. I'm in the process of doing this myself. As you guessed, you'll need the later backing plates, but the brake offset (distance from axle flange to housing end) is the same.

[aggie91]

Rod is correct. Just get the later model backing plates, then everything else is available at the parts store. This is a simple and effective upgrade. I might even have a spare set of those backing plates.

[aotte1]

Rod and Carl thanks for the conformation, that this swap works. Sounds like you have done it with good results.



Les

[PGSigns]

What would you do for drums and 6 lug? be a nice upgrade on my 66. Would you use a K10 drum?
Jimmy

[franken]

Almost all braking (75-80%) is done by the front brakes. Changing the rears is more cosmetic than functional.
Fade under continual use is another story.

[racecougar]

Originally Posted by PGSigns

What would you do for drums and 6 lug? be a nice upgrade on my 66. Would you use a K10 drum?
Jimmy

Yep.

[aotte1]

Jimmy,

As Rod said, will use the K10 drums.
Their may be certain Suburbans up to 91 that have this brake package also, would have to confirm this. Have a '89 Suburban "C10" complete front suspension , including brakes and crossmember that have JB5 brakes. Could check these later Surburbans and see if their rear drums are the same part number as '79 K10.

And as said above doing this as a whole brake system upgrade to the optional heavy duty "JB5" brake option, as this is a super braking package, GM designed.

L



Les

[aggie91]

Les

I replied to your PM, just saw it sorry it took so long to respond.

Karl

[aotte1]

Continuing to research this upgrade, and found the brake shoe frame design chaned in '78. The size of the lining and drum stayed the same. However, '77 and older used a different brake shoe frame design, along with likely different internal parts too. The holes in the brake shoe frame are different between '77 and older, and '78 and up.
Still researching if their are differences in backing plates between '72 and Earlies 80s, along with C and K vehicles.

Jimmy, using K drums for 6 lugs.

L

[racecougar]

Interesting and good information!

[aggie91]

Originally Posted by aotte1

Continuing to research this upgrade, and found the brake shoe frame design chaned in '78. The size of the lining and drum stayed the same. However, '77 and older used a different brake shoe frame design, along with likely different internal parts too. The holes in the brake shoe frame are different between '77 and older, and '78 and up.
Still researching if their are differences in backing plates between '72 and Earlies 80s, along with C and K vehicles.

Jimmy, using K drums for 6 lugs.

L

I looked into this some more too. All the parts of the brake system are the same part numbers EXCEPT for the shoes. The wheel cylinders, drums and hardware kits are all the same. Here is a pic of the backing plates with shoes and hardware that I believe are from a 1977 pickup.


Best as I can tell the '78 up shoes are missing the two smaller holes seen in the shoe frame in the pic above. All the hardware attachment holes appear to be the same. The parts store did not have the '77 shoes in stock, but had the '78 shoes and here is a pic of those:

[SCOTI]

Originally Posted by franken

Almost all braking (75-80%) is done by the front brakes. Changing the rears is more cosmetic than functional.
Fade under continual use is another story.

Improvement is improvement. I've done this upgrade on my 68 & 64 after realizing how easy the swap was. I would also upgrade the front brakes on a 81-87 if it only had the 1" vs 1.25" thick rotors.

Common place to find the correct backing plates is under Blazers or burbs that are getting parted out.

[racecougar]

Finished up my swap today. Those are the "JB5" 11-5/32" x 2-3/4" drums on a narrowed '65 GM Truck 12-bolt. I used C10 rather than K10 drums, as I went to a 5 on 5" bolt pattern.

[nds1968]

I've done this upgrade as well. Truck isn't on the road so...

[aotte1]

Scott, Rod, and NDS,

Thanks for the comments. Rod like the true pizza cutter wheels, had not thought of them, good idea, sort of substitute for a lift to work on underside stuff.

Les

[racecougar]

Thanks. Those are a stand-in to help determine the final tire size.

[aotte1]

Karl,

Opened box today and everything is great! Thank you.

Les

[aggie91]

you are welcome. Glad I could help.

[aotte1]

A few photos of the backing plate, for reference. Did not see any part number on them.

Karl thought the backing plates are from a 1977 truck with a 12 bolt and heavy duty, JB 5 brake option. The last photo show a '79 brake shoes set on them, and looks like these shoes should work fine. FYI, these brake shoes were used from '78 to at least 87, maybe longer,

The difference in the brake shoe frame holes are not an issue, as Karl had speculated, for '77 and down vs '78 and up from what I see.

Les

[aggie91]

Those cleaned up nice.

How did the new shoes compare to the one old set I sent you with the backing plates? Can you get some side by side photos of them from both sides?

[aotte1]

Karl,

Good idea, will get some photos.

Yes they are cleaning up nice, thanks.

Les

[ERASER5]

This is on my list of things to do, once I have the '70 back on the road. I plan to load it heavy from time to time, so better rear brakes will be an asset.

[jjzepplin]

So I guess the backing plates are offset more to the inside and the shoes are 3/4" wider as well as the drums?