E-brake different from a 78 to a 80? .also proportioning valve question

[sircrunch]

I'm swapping the frame on a 1978 v8 big 10 shortbed. The donor frame is from a 1980 6 cyl shortbed.

So I'm finishing up and I noticed something strange when trying to put the e-brake cable back together. The ebrake cable part that hangs out of the firewall looks like it mounts to the chassis in a different location on this frame. You can see where the 80 model mounted, but the cable that comes out of the firewall on my 78 cab does not reach this bracket. Any advice?
Also, I'm keeping my Big 10 style brakes front and rear (heavier duty). I'd like to use the same brake lines including proportioning valve and everything from the 6 cyl regular duty brakes. Are they the same?

[sircrunch]

nobody knows either of these questions?

[Keith Seymore]

Sorry - didn't see this until now.

Yes - e brakes are different. Perhaps you could make a little "jumper" connector, out of wire (or with a mechanical bracket of some type), to bridge the extra distance.

We might need more detail on the prop valve/brake cylinder questions. I have the brake engineering release charts at home and will check when I get there, but might need to know (or figure out) JB1/JB3/JB5/JB6 for both the donor and recipient.

K

[Keith Seymore]

Turns out I don't have the '80 brake chart, although I have the '78, '79 and '81. I can use either the '79 or '81 chart as a sub, or - if they are the same - demonstrate the difference on the '78 chart.

Originally Posted by Keith Seymore

We might need more detail on the prop valve/brake cylinder questions. I have the brake engineering release charts at home and will check when I get there, but might need to know (or figure out) JB1/JB3/JB5/JB6 for both the donor and recipient.

But - we need more detail. The '78 Big Ten is pretty straightforward, with a GVW of either 6050 or 6200 lbs. Either of those result in a JB5 brake system.

The 80, however, spans from 4900 lbs to 6000 lbs GVW, which runs the full range of JB1, JB3 and JB5, which makes a pretty big difference in rotors, calipers, foundation brakes, apply system and prop valve.

Any way to determine the GVW rating of the '80 frame you have?

K

[sircrunch]

Neither one of my trucks have the stickers for GVW or RPO codes but I can tell you that the 1980 donor truck was just a very basic 2wd 6 cylinder truck with no air.... auto trans... light duty Springs and such as that it was a regular C10 ...it did have power brakes. Hope tthishelps

[Keith Seymore]

Originally Posted by sircrunch

Neither one of my trucks have the stickers for GVW or RPO codes but I can tell you that the 1980 donor truck was just a very basic 2wd 6 cylinder truck with no air.... auto trans... light duty Springs and such as that it was a regular C10 ...it did have power brakes. Hope tthishelps

Power brakes eliminates JB1, and drops it to either JB3 or JB5.

I'll take a look in a few minutes to see what effect that has.

K

[sircrunch]

I'm crossing my fingers that either proportioning valve will work. That thing looks like a booger to pull out and every single time I touch a brake line the hex head strips on the fitting.

[Keith Seymore]

Well - the 81 chart converts all the dimensions to metric so I ended up comparing the 78 and 79 charts.

Although the brake line routing and front calipers are the same between a JB3/JB5 1979/1980 model year and the 79 Big 10 (JB5 system) pretty much everything else is different.

Meaning - the master cylinder diameter and part number is different, the Big 10 has bigger rear brakes with smaller wheel cylinders, and the prop valve meter/split/slope is different as well.

Difficult to tell on paper what the combination of all those changes would do. My recommendation would be to forge ahead and see how it feels/if you like it. If not you could go to an aftermarket adjustable prop valve and tune it where it needs to be.

K

[sircrunch]

keith, thank you very much for your help and research. Right now I'm looking forward to getting the truck finished. The truck was smacked hard in the front. I've replaced the frame, core support, hood, one fender, one inner fender, the grill, the bumper, and more. Its been a lot of hard work, but its about to be drivable. I'm crossing my fingers that the trans is good. The truck starts and runs great. I can't put it in gear yet though because my brakes are not hooked up yet For what its worth, I found out the hollow bolt that goes into the caliper from the rubber brake line in the front is a bigger diameter on my big 10 than on the standard. Little unexpected things.