79' K20 with 1-Ton rear brakes from factory??

[Blue hauler]

I'm restoring a 79' K20 with the 10.5" dia. ring gear 14 bolt.
It has the 13" dia. x 3-1/2" wide rear drum brakes. I'm wondering if this was an option on 3/4 ton trucks? I have only seen these on 1 ton trucks.

[400/400]

Do you have your SPID?

I believe it’s option JB7 for HD brakes.

[Blue hauler]

Not sure what the SPID is. Is that the option sheet in glove box? I don’t have that.

[Keith Seymore]

Originally Posted by Blue hauler

Not sure what the SPID is. Is that the option sheet in glove box? I don’t have that.

If you are super interested in how your truck was equipped you can contact the GM Heritage Center archive with your VIN and they can provide the original invoice, complete with option content, pricing, build date information and the selling dealership.

If your truck was built in Canada, or in the US for Canadian consumption, GM of Canada would also have the info. https://www.vintagevehicleservices.com/

K

[Blue hauler]

Keith,
Thank you so much for the extensive information. I will definitely pursue the vehicle information through the GM Heritage Center. The options decal was no longer in the glove box when the truck was purchased, and I'm wondering if reproduction option decals are available from somewhere. It is a fairly rare truck with the E81 wood bed option. In looking more closely at the VIN tag I realized it is a 78', for some reason I had 79' stuck in my head. I'm guessing someone installed the brakes from a 1-ton onto this truck. The spring perches and shock mounts are factory and not re-welded.

[Keith Seymore]

Once you have the detail contact site member Chipflyer and he will make an authentic looking SPID for you.

Jeff at https://outintheshop.com/

K

[Keith Seymore]

Originally Posted by Blue hauler

Keith,
Thank you so much for the extensive information. I will definitely pursue the vehicle information through the GM Heritage Center. The options decal was no longer in the glove box when the truck was purchased, and I'm wondering if reproduction option decals are available from somewhere. It is a fairly rare truck with the E81 wood bed option. In looking more closely at the VIN tag I realized it is a 78', for some reason I had 79' stuck in my head. I'm guessing someone installed the brakes from a 1-ton onto this truck. The spring perches and shock mounts are factory and not re-welded.

'78 built in Janesville.

I was going to comment if it was a '79 built in Flint I might have seen it go down the line!

K

[Dead Parrot]

Years ago, I worked with a guy that special ordered a 75 3/4 ton with a 1 ton rear axle because he was going to set it up with a 5th wheel hitch for his travel trailer. Possible yours was ordered or modified for similar duty. That was a time when you could do a lot of special options on a custom build truck.

[Keith Seymore]

I will check the brake charts on Monday.

I'm at the race track this weekend.

K

[Honest Bob]

1 ton brakes are JB8 and are hydroboost.

There are a few things that I could see happening.

1) Maybe it was ordered that way. These trucks seemed to be nearly completely customizable from the factory.

2) Someone swapped the rear axle with one from a 1 ton truck. The spring perches would have to be moved (I think the shocks have different mounting locations on 3/4 vs 1 ton as well.) to fit on a 3/4 ton truck.

3) Someone may have upgraded to jb8 rear brakes. I suspect this could be easily done by swapping the entire axle backing plate setup along with shoes and drums and wheel cylinders. I would think there would be brake proportioning valve and master cyl differences as well which if not have been done would cause the rear brakes to lock up well before the fronts.

[Blue hauler]

Thank you for the info!
I wonder if I need to go with hydro boost set up for the large brakes?
Truck came with vacuum power brakes.

[Keith Seymore]

Wow.

The '79 brake charts are really oddly formatted and therefore hard to read. And - conveniently charted in metric, so that you have to convert to english.

Short answer: No. The K20 was JB7 only; JB8 not available from the factory.

K

[Keith Seymore]

Originally Posted by Dead Parrot

Years ago, I worked with a guy that special ordered a 75 3/4 ton with a 1 ton rear axle because he was going to set it up with a 5th wheel hitch for his travel trailer. Possible yours was ordered or modified for similar duty. That was a time when you could do a lot of special options on a custom build truck.

Originally Posted by Honest Bob

1 ton brakes are JB8 and are hydroboost.

There are a few things that I could see happening.

1) Maybe it was ordered that way. These trucks seemed to be nearly completely customizable from the factory.

2) Someone swapped the rear axle with one from a 1 ton truck. The spring perches would have to be moved (I think the shocks have different mounting locations on 3/4 vs 1 ton as well.) to fit on a 3/4 ton truck.

3) Someone may have upgraded to jb8 rear brakes. I suspect this could be easily done by swapping the entire axle backing plate setup along with shoes and drums and wheel cylinders. I would think there would be brake proportioning valve and master cyl differences as well which if not have been done would cause the rear brakes to lock up well before the fronts.

While quite customizable from a trim or comfort/convenience standpoint, they were not infinitely customizable from a brake standpoint.

The combinations would need to be tested and validated for every model series, wheelbase, CG location, tire option and GVW weight rating, which is why we had the various brake systems defined and released in buckets (ie, JB1, JB3, JB5 etc).

We wouldn't be able to sell a product that had not been fully validated especially after the FMVSS standards were put in place.

K

[cadillac_al]

I have a 77 with the rotten wood bed option. The only truck I ever had that the bed floor disappeared. I'm not a fan. They look cool on show trucks though.