84 C20 brakes - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TazmanB · 02-29-2020, 11:22 PM

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?

**MikeB** · 03-01-2020, 05:14 AM

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.

TazmanB · 03-01-2020, 01:53 PM

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

**MikeB** · 03-03-2020, 11:35 AM

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.

TazmanB · 03-06-2020, 08:01 PM

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.

TazmanB · 03-06-2020, 08:02 PM

Quote:

Originally Posted by MikeB

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.

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

TazmanB · 03-08-2020, 05:15 PM

Quote:

Originally Posted by TazmanB

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.

bump

02-29-2020, 11:22 PM	#1
TazmanB Registered User Join Date: Jan 2011 Location: Arlington Texas Posts: 144	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? __________________ If you ask the question. Make sure you are ready for the answer. 69 C-10 Longbed (In Progress) Build thread http://67-72chevytrucks.com/vboard/s...d.php?t=568171 Last edited by TazmanB; 03-01-2020 at 01:28 AM.

03-01-2020, 05:14 AM	#2
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,429	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. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! Last edited by MikeB; 03-03-2020 at 11:30 AM.

03-01-2020, 01:53 PM	#3
TazmanB Registered User Join Date: Jan 2011 Location: Arlington Texas Posts: 144	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. __________________ If you ask the question. Make sure you are ready for the answer. 69 C-10 Longbed (In Progress) Build thread http://67-72chevytrucks.com/vboard/s...d.php?t=568171

03-03-2020, 11:35 AM	#4
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,429	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. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

03-06-2020, 08:01 PM	#5
TazmanB Registered User Join Date: Jan 2011 Location: Arlington Texas Posts: 144	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. __________________ If you ask the question. Make sure you are ready for the answer. 69 C-10 Longbed (In Progress) Build thread http://67-72chevytrucks.com/vboard/s...d.php?t=568171 Last edited by TazmanB; 03-06-2020 at 08:59 PM.



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