84 C20 brake question
 

I bought an 84 GMC C20 single cab long bed that my 13 yoa son has been wanting for over a year. I got a really good deal on it and there is no rust/rot on it anywhere. The title shows it is a C10 and the vin matches the cab. So it looks like it is a C10 cab swap on to a C20 frame. It has disc/drum brakes with a master bolted to the firewall. There is a booster in the floor of the cab and the brake pedal goes to the floor, absolutely no brakes.

I am going to replace the entire brake system minus the hard lines. My question is as long as I replace everything and go with the JB6 or JB7 brakes does it matter what is on it now?

If it does matter what brakes are on it now how can I tell if it has the JB6 or the JB7 brakes on it now?

Re: 84 C20 brakes
 

I'm a C10 guy, but found this info when searching for parts to upgrade my front brakes from JB1 to JB3.

From RockAuto and on Chevy specification literature. Hope it helps:

JB6 --12.5" rotors, 11” rear drums, dual diaphragm booster
JB7 --12.5" rotors, 13” rear drums, dual diaphragm booster

I suspect rear backing plates for 11" and 13" drums are different

JB6 and JB7 front brakes appear to use different calipers but exact same 13" rotors that are 1.29" thick.

Master cylinders are also different.
JB7 uses a 1-1/4" piston bore for 13" drums
JB6 uses a 1-1/8" piston bore for 11" drums

JB6 and JB7 use different boosters P/Ns, but both are shown as 10.75" diameter.

Re: 84 C20 brakes
 

Thanks for the info. So I need to figure out what rear drums are on the truck and go from there.

Re: 84 C20 brakes
 

Might want to look into replacing the hard lines, too. I have bled my 84 C10's system at least a couple times, but the fluid in the reservoir doesn't stay clear for long. I suspect there is corrosion inside the hard lines. At the very least flush them 100% when you get the new M/C installed.

Re: 84 C20 brakes
 

Ok I have a question on the rear drums. There shows to be 2 different 13 inch drums for my truck. 1 drum is 5.1 inches and the other is 6 3/8 both show to take the same brake shoes.

The question is whats the difference between the 2 drums and can they be interchanged without any issues.

Rockauto part numbers ACDELCO 18B147 6 3/8 inch drums and ACDELCO 18B111 5.1 inch drums.

I have a 14 bolt rear end.

Re: 84 C20 brakes
 

I was going to flush the lines but I may just change them all. Haven't decided yet. Thanks

Re: 84 C20 brakes
 

bump