New to me '78 C15 F44 with no rear brakes

[MotoRod]

First and foremost, this is my first post, been lurking a bit and realized y'all are way smarter than I am. I recently bought a '78 GMC C15 with the F44 option. When I picked it up, it only had front brakes engaging. Got her home and opened up the rear drums to find 2" shoes, which looked like they had a ton of meat left on them, but not adjusted at all. So I adjusted them and still no pedal. The RPO codes have F44 listed. Is it me, or should I have the 2 3/4" shoes? I'm not wondering if someone put the wrong shoes in. Having a hard time finding any specs on backing plates as well to determine if the backing plate is for a 2.75" shoe. Any help or suggestions you guys/gals have is greatly appreciated.

 

[Bextreme04]

J55 heavy duty power brakes would have been included in the F44 HD chassis RPO.

Top of page 28 here: https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1978-Chevrolet-Truck.pdf

Do you have an SPID sheet in the glove box? Do you have pics of the axle and brakes? It looks like maybe the smaller 11x2" brakes were for up-to 5600lb GVWR and the 2-3/4" ones were for 6,000lb.

 

[MotoRod]

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unfortunately it doesn't show the brake code

 

[Mr Clean]

Would the backing plates be different? When I redid the brakes on my 82 1/2 ton, it asked for the option of the 2" or the 2 3/4" brakes. I got everything for the wider brakes, drum, brakes, spring kit, and wheel cylinders. It all bolted in they were just wider, not longer.

Have you tried bleeding the brakes to see if is getting brake fluid to the rear? Could be a Master cylinder, Proportioning valve, you could have a pinch in the line somewhere. There is only one line that goes to the back.

 

[PrairieDrifter]

Would the backing plates be different? When I redid the brakes on my 82 1/2 ton, it asked for the option of the 2" or the 2 3/4" brakes. I got everything for the wider brakes, drum, brakes, spring kit, and wheel cylinders. It all bolted in they were just wider, not longer.

Have you tried bleeding the brakes to see if is getting brake fluid to the rear? Could be a Master cylinder, Proportioning valve, you could have a pinch in the line somewhere. There is only one line that goes to the back.

I was chasing a brake drag issue, and I couldn't figure out if it was the wrong brakes or not. This led to me to getting the small brake stuff and trying to install, I found that it seems the backing plates are indeed different, im not 100% certain, but with just the shoes on and installing the shoe clip guys to mount them to the backing plate, the shoes didnt have the right geometry to contact the center pin shaft thing. Plus did not seem to have the correct geometry on the mounting points. So I think the backing plates for the smaller brakes would be maybe and inch smaller diameter.

This is just MY theory anyways.

 

[Mr Clean]

I was chasing a brake drag issue, and I couldn't figure out if it was the wrong brakes or not. This led to me to getting the small brake stuff and trying to install, I found that it seems the backing plates are indeed different, im not 100% certain, but with just the shoes on and installing the shoe clip guys to mount them to the backing plate, the shoes didnt have the right geometry to contact the center pin shaft thing. Plus did not seem to have the correct geometry on the mounting points. So I think the backing plates for the smaller brakes would be maybe and inch smaller diameter.

This is just MY theory anyways.

That's why I was asking. Maybe I just lucked out and had the 2 3/4 brakes and I thinking I had the smaller brakes. It was some time ago, and I've slept a time or two since. What you are saying makes sense.

 

[AuroraGirl]

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what shape drum do you have? the width of the shoe is significant and the height of the shoe is not nothing, meaning it has to have a different curvature id think

The adjusters are different
The shape of the lever is different
The cable wire is shaped different
the wheel cylinder is different
The primary and secondary shoe is held down with different springs(?)
The hardware kit beyond what mentioned is the same

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I would say does your drum match up to the wheel cylinder? Does your adjuster setup appear the same

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you can see the one shoe has a lot more rivets and is narrower. Shape is also different, holes in diff spots. Basically you should be able to determine if its all "congruent" and like eachother. if anything oddly stands apart, you know you have an issue

 

[PrairieDrifter]

That's why I was asking. Maybe I just lucked out and had the 2 3/4 brakes and I thinking I had the smaller brakes. It was some time ago, and I've slept a time or two since. What you are saying makes sense.

From what I've seen the bigger brakes are way more common. All four of my half ton axles have the big brakes.

 

[Rusty Nail]

I think the backing plates for the smaller brakes would be maybe and inch smaller diameter.

Hold on a second.
Stop here - wait.

What am I thinking about- isn't an 11 inch drum vs. a 13 inch drum a thing?
Trying to recall Jb7 vs Jb8..there IS such a thing as 13 inch squarebody drums which certainly would have different backing plates ya..it is directly related to wheel cylinder diameter when making the jump to ONE TON?

Hope this helps!

 

[Grit dog]

Hold on a second.
Stop here - wait.

^This.
@MotoRod, @AuroraGirl posted the 2 different drums. That will tell you which brakes you have, and I'd bet money that 2" wide pads ride fine in a 2-3/4" setup, but not the other way around, as the backing plate would need a different profile or axle length/protrusion would be different to accommodate for the additional or lesser brake width. My money is on backing plate is different, logically and mathematically, don't have squarebody brake parts memorized.

However regardless of whether the 2" shoes are sitting in the wider drums or not, 99% likely the first problem has nothing to do with that.
You said "no pedal" and know that only front brakes are working. Yes front brakes only and air in the rear lines or tripped prop valve or bad master cyl, or any number of things result in a spongy pedal with decently working front brakes.
Find out why you're not getting hydraulic pressure to the rear first. IMO good luck

 

[Strick]

What am I thinking about- isn't an 11 inch drum vs. a 13 inch drum a thing?

13's should be on 14SF, 14FF or Dana 70; all 8-lug axles. There is a width difference on the 10 & 12 bolt 1/2 ton 5 & 6 lug axles.

Strickland