1977 C20 Heavy duty with 11.15 drums?

[klunker797]

Hey guys,

I have a 1977 c20 sierra with a 10.5” 14. Bolt FF differential. I want to replace the master cylinder but I need to know if the drums are 11.15 or 13”. I want to avoid removing the drums to measure internally.
From everything I read, they should be a 13” drum ( especially being that it’s a 10.5 FF) but it’s GVWR is only 6400 and by measuring from underneath the truck, the internal drum seemed to be approximately 12”. There is also no mention of JB6 or JB7 on the RPO code sheet.Seems simple, but I don’t want to install the wrong MC.

Thanks for the help!

 

[HotWheelsBurban]

That drum does seem a bit small....my one ton has that rear axle but hydro boost instead of vacuum booster, and mine definitely has the 13" drums. Got a full floater out of a one ton square body in the shop, and I'm 99% sure it has the 13" too.

 

[legopnuematic]

especially being that it’s a 10.5 FF

Not really a factor in this case, until the introduction of the 9.5 14b SF all 3/4 tons were 10.5 full floats.

J55 is the RPO code for HD brakes.

You have the JB6 brakes and 11" drums. JB6 master reservoir walls are straight up and down, JB7 has a bulge in-between the bails.

 

[Shorty81]

Wow, that is a sharp truck!!!

 

[Old Guy Bill]

My ‘78 K20 has the 11 5/32” drums on a 14 bolt FF.
You need to pull a drum to verify…

 

[AuroraGirl]

My ‘78 K20 has the 11 5/32” drums on a 14 bolt FF.
You need to pull a drum to verify…

no you dont, if you have a 14 bolt you simply need to look at the back of the wheel where the drum is. the 11 inch drum will have lots of room

The 13 will fill up the gap

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[bucket]

Yep, it's a very obvious size difference between the two of them. Anybody that has seen both drum sizes can tell by looking at them. And now thanks to @AuroraGirl posting pictures, anyone that has read this thread can now know what size they have, without finding a tape measure.

 

[AuroraGirl]

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Heres some more differences in the 1973-1987 or so 14 bolt FF brakes (11 to 13)
I have no idea what the slip on era (88-01?) look like 11vs 13 or if they even have both. figured this would be good reference too kinda like bucket said

 

[klunker797]

Thanks everyone. Very helpful, that’s why I love this group!

 

[HotWheelsBurban]

no you dont, if you have a 14 bolt you simply need to look at the back of the wheel where the drum is. the 11 inch drum will have lots of room

The 13 will fill up the gap

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I can vouch that on my OBS one ton with 13" brakes on a 14B FF and 16" factory steelies, the space is a bit tight....as in the upper picture @AuroraGirl posted. Just enough room to get a wheel retaining strap between the wheel rim and drum.
Yes you can put this big of a truck on a flatbed wrecker, if you're careful.... literally about 6-7" of ramp left, the truck's brush/grille guard was almost touching the lightbar.

 

[Radiohead]

I love the photodramatic views of 11" & 13", excellent reference. My helpful(?) hint was do what I did and pull off a wheel and read the max diameter value stamped into the drum. But, duh...

The pictures are a great indicator of what to look for. Thanks AuroraGirl, good looking out!

 

[HotWheelsBurban]

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Heres some more differences in the 1973-1987 or so 14 bolt FF brakes (11 to 13)
I have no idea what the slip on era (88-01?) look like 11vs 13 or if they even have both. figured this would be good reference too kinda like bucket said

I think the later style OBS has slip on drums. Pretty sure my '97 does because the shop didn't charge for pulling the axles and refilling the diff oil when they changed the wheel cylinders last fall. But some of our guys on the OBS/400 forum with older one tons have the square body style drums where they have to pull the axle shaft out. And I think all the 14B FF axles in our series trucks have the 13" diameter drums. Difference on our trucks, is the semi floating axle in the 2500 uses a 13 x 2 1/2" and the FF uses the 3 1/2" wide shoes.
Either way it's a big heavy drum if you're running the 13".....

 

[jwroblesk]

I have a 1974 c20 and it has GVW of 6400lbs with 11” drums. When I had them off they were so heavy that I couldn’t carry them upstairs to the sandblaster. I can’t imagine trying to deal with the 13” drums. Joe

 

[HotWheelsBurban]

Here are some pictures I took of a C20 custom deluxe stepside in the storage place I use. Has a FF axles and 16" steel wheels; those drums look little in the wheels.... truck has a fo fiddy fo in it too. Hopefully someone gets ahold of it to put it back running, this is a kinda rare truck, long bed stepside, and it's just sitting there, rusting away....

 

[AuroraGirl]

have some new info:
there were TWO sizes of 13 inch drums, apparently the backing plates/shoes/drums are different, and the 73-80 and 81+ split on rear park brake cable design.
I only have a 1980 so it wouldnt help anyone 81+ with the park brake appearance on the back, but I discoverd after searching the service manual (1980 Chevrolet) that there is 13 inch x 2.5 and also a 13 x 3.5
The 3.0 is reserved only for hydroboost JB8 which also has larger front rotor thickness, which means to me it would probably have the same caliper, possibly pads, and the thickness is for heat dissipation/service life

I am inferring I have JB8 because of C6p, my RPO sheet doesnt actually include a brake RPO.

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here is the bendix mini-master as described by the service manual, the one pierce hinged latch was delco

This is also a large 13 inch booster, tandem (dual diaphram)