Master cyinder- Brake booster replacement - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Ehodg66** · 10-19-2021, 09:06 AM

I'm looking to replace my old (and worn out) Master cylinder and Brake booster. I plan on using a newer model master cylinder but have a few questions.

The PO's who did the frame off rebuild on my 82 truck had an 87 donor truck, they used the interior and the bed, from what I've been told. They also installed an 89 suburban 373 rear end w/ larger brakes. I'm not sure if they used the master cyl and booster from the 87 or possibly the suburban.

When I look in side my glove box, the info label has J5 next to brakes but via info from a few sites, the master cylinder I have is JB7-JD7. My booster is 11" diameter. (pic attached of my setup)

I do not claim to be a brake guru and want to make sure I'm installing the proper booster. Is there anything I should know regarding replacing the booster because of the 89 rear end? Does it matter?

Lastly, I've been looking everywhere for a zinc coated or painted NEW booster and the only one I've found is on CJ pony for 242.99. (pic attached) Any site suggestions would be much appreciated.
thank you

**MJN** · 10-19-2021, 10:43 AM

Not sure what truck its going on, assuming 1982 C-10, but Rockauto has more than a few boosters listed for that application. Looks like the booster thickness is spec'd and not the diameter.

**Ehodg66** · 10-19-2021, 11:21 AM

Yes, it's for my 82. The only one that is new and "looks" as if its painted is SK5471069. JB5. I do not see any specs on thickness there but
Using that part number on other sites, it says it will fit the 82 and will fit an 89 suburban. So I guess my larger, suburban rear end isn't an issue.
Certainly cheaper than 242.99

Honest Bob · 10-21-2021, 11:20 AM

I would verify why kind of brakes you have front and rear and try to match the master up from there. If your brakes worked great at one time with the JB7 master you could of course just use that.

JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm

JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm

JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost

I suspect the boosters between jb5 and jb7 might be interchangeable but I am not 100% sure.

If you can identify what rear axle you have you can assume some things.

If its a semi float 14 bolt its probably got jb5 rear brakes.

If its a full float 14 bolt then its probably got jb7 rear brakes.

A one ton 14 bolt does not have the same spring perch width as a 1/2 or 3/4 ton so you probably do not have jb8 rear brakes.

10-19-2021, 09:06 AM	#1
Ehodg66 Junior Member Join Date: Dec 2019 Location: fort lauderdale Posts: 48	Master cyinder- Brake booster replacement I'm looking to replace my old (and worn out) Master cylinder and Brake booster. I plan on using a newer model master cylinder but have a few questions. The PO's who did the frame off rebuild on my 82 truck had an 87 donor truck, they used the interior and the bed, from what I've been told. They also installed an 89 suburban 373 rear end w/ larger brakes. I'm not sure if they used the master cyl and booster from the 87 or possibly the suburban. When I look in side my glove box, the info label has J5 next to brakes but via info from a few sites, the master cylinder I have is JB7-JD7. My booster is 11" diameter. (pic attached of my setup) I do not claim to be a brake guru and want to make sure I'm installing the proper booster. Is there anything I should know regarding replacing the booster because of the 89 rear end? Does it matter? Lastly, I've been looking everywhere for a zinc coated or painted NEW booster and the only one I've found is on CJ pony for 242.99. (pic attached) Any site suggestions would be much appreciated. thank you Attached Images

10-19-2021, 10:43 AM	#2
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,958	Re: Master cyinder- Brake booster replacement Not sure what truck its going on, assuming 1982 C-10, but Rockauto has more than a few boosters listed for that application. Looks like the booster thickness is spec'd and not the diameter. __________________ '51 South Bend Model "9A" '56 Chevy 3600 NAPCO '74 Chevy Cheyenne Super K20-restored on 37's '72 GMC Sierra Custom K2500- in progress '08 GMC Envoy Denali '12 GMC Sierra 2500HD Denali '17 GMC Yukon XL VCCA member #58596 http://www.nohrco.com

10-19-2021, 11:21 AM	#3
Ehodg66 Junior Member Join Date: Dec 2019 Location: fort lauderdale Posts: 48	Re: Master cyinder- Brake booster replacement Yes, it's for my 82. The only one that is new and "looks" as if its painted is SK5471069. JB5. I do not see any specs on thickness there but Using that part number on other sites, it says it will fit the 82 and will fit an 89 suburban. So I guess my larger, suburban rear end isn't an issue. Certainly cheaper than 242.99

10-21-2021, 11:20 AM	#4
Honest Bob Registered User Join Date: Jan 2017 Location: Easton PA Posts: 212	Re: Master cyinder- Brake booster replacement I would verify why kind of brakes you have front and rear and try to match the master up from there. If your brakes worked great at one time with the JB7 master you could of course just use that. JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost I suspect the boosters between jb5 and jb7 might be interchangeable but I am not 100% sure. If you can identify what rear axle you have you can assume some things. If its a semi float 14 bolt its probably got jb5 rear brakes. If its a full float 14 bolt then its probably got jb7 rear brakes. A one ton 14 bolt does not have the same spring perch width as a 1/2 or 3/4 ton so you probably do not have jb8 rear brakes. __________________ 1975 GMC C35 - Junk yard survivor. 1986 Pampered Private - M1009



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