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

cory321 · 04-07-2007, 04:20 PM

I was looking at brakes today at autozone for the 78 how do i tell if it is jb7 or jb8 brake system.It i a one ton 4wd

Srubydo1986 · 04-07-2007, 08:05 PM

i think its a rotor sizes might say in your info in the dash.

boxrodz · 04-10-2007, 12:37 AM

The JB7 system runs a vacuum boost with 1.28" front discs and 2.5" rear drums whereas the JB8 system uses the hydroboost setup with larger discs (1.53") and larger drums (3.5").

Ivan D. · 04-10-2007, 01:54 AM

True that, however I guess somewhere in the '80s things changed cause my hydroboost module that came off an '80s 1-ton diesel has to use a JB7 master cylinder (for a '79 C30), the JB8 one that's supposed to fit it has a bore in the piston that is too deep for the hydroboost rod. I'd suggest just pulling a wheel and measuring yer current rotors, they will be thinner than the brand new ones but they will still give you an idea of what to get. Check the rear drums too, just to make sure.

cory321 · 06-02-2007, 01:50 PM

does it make a differance in telling that it has a dana 60 in the front.Also whne measuring the rear brake does 3.5 inches the meauserment of the shoe width or the drum width

Ivan D. · 06-02-2007, 01:58 PM

dunno about the front axles I'm running the JB7 hydroboost master cylinder for a 1-ton with my half-ton rotors and calipers without any issues. The 3.5" should be shoe width, you will need to pull drums for that and that can be pretty nasty depending on yer axle's rust developments. Really best way would be to pull a front wheel and measure the rotor thickness, if it's about 1.5" you should have the JB8, and if it's around 1.25" it's the JB7, but even then I'd check with the actual part dimensions at AutoZone cause stuff gets mislabeled sometimes. As I said, I have to run a '70s JB7 master cylinder on my '80s JB8 hydroboost system, and I only figured that out cause I had measured the bore depth of the original hydroboost master cylinder in the junkyard.

cory321 · 06-02-2007, 03:29 PM

Thank you I'll measure the front rotors tonight

04-07-2007, 04:20 PM	#1
cory321 Problem Child Join Date: Sep 2005 Location: Kettering Ohio Posts: 1,248	brakes I was looking at brakes today at autozone for the 78 how do i tell if it is jb7 or jb8 brake system.It i a one ton 4wd __________________ 1972 gmc sierra grande 350/350 a/c soon`to get a new 350 I Survived Norwood

04-07-2007, 08:05 PM	#2
Srubydo1986 The Crazy Machanic Join Date: Jan 2007 Location: Nokesville Va Posts: 1,437	Re: brakes i think its a rotor sizes might say in your info in the dash. __________________ 1986 Sliverado longbed 400 small block mean as hell, 700r4 trans (5 speed coming one day) 1970 chevelle 2dr sedan Latest update June 24 2007. *1957 4 door stationwagon - pink all original needing a good restore (body great, floors bad) newest additions 1965 shortbed fleet great granddads. Been told im nuts to bring it back from the dead (still going to happen) plans-all stock with a turbo 292 inline 3 speed column shift. and thats it big block 427 and 5 speed trans and ford 9 inch theyhave no home yet thinking what to put them in and a 79 c60 trash truck um no idea i have one

04-10-2007, 12:37 AM	#3
boxrodz Until Seventy Times Seven Join Date: Feb 2005 Location: Rocky Mountain Front Range Posts: 1,301	Re: brakes The JB7 system runs a vacuum boost with 1.28" front discs and 2.5" rear drums whereas the JB8 system uses the hydroboost setup with larger discs (1.53") and larger drums (3.5"). __________________ boxRODz 1983 C10 SWB Fleetside When two halves is gone, there's nuthin left. Two nothings is nuthin. That's mathematics son. You can argue with me but you can't argue with figures. Two half nothings is a whole nuthin...

04-10-2007, 01:54 AM	#4
Ivan D. glamoros piece o' lowlife Join Date: Mar 2007 Location: Redford, MI Posts: 1,164	Re: brakes True that, however I guess somewhere in the '80s things changed cause my hydroboost module that came off an '80s 1-ton diesel has to use a JB7 master cylinder (for a '79 C30), the JB8 one that's supposed to fit it has a bore in the piston that is too deep for the hydroboost rod. I'd suggest just pulling a wheel and measuring yer current rotors, they will be thinner than the brand new ones but they will still give you an idea of what to get. Check the rear drums too, just to make sure. __________________ The more I drink, the more I drink, the more I drink... diesel, reg cab, long bed, 4 across the rear, single stack, wooden stakes, and lotsa lights - the Hay Express

06-02-2007, 01:50 PM	#5
cory321 Problem Child Join Date: Sep 2005 Location: Kettering Ohio Posts: 1,248	Re: brakes does it make a differance in telling that it has a dana 60 in the front.Also whne measuring the rear brake does 3.5 inches the meauserment of the shoe width or the drum width __________________ 1972 gmc sierra grande 350/350 a/c soon`to get a new 350 I Survived Norwood Last edited by cory321; 06-02-2007 at 01:50 PM.

06-02-2007, 01:58 PM	#6
Ivan D. glamoros piece o' lowlife Join Date: Mar 2007 Location: Redford, MI Posts: 1,164	Re: brakes dunno about the front axles I'm running the JB7 hydroboost master cylinder for a 1-ton with my half-ton rotors and calipers without any issues. The 3.5" should be shoe width, you will need to pull drums for that and that can be pretty nasty depending on yer axle's rust developments. Really best way would be to pull a front wheel and measure the rotor thickness, if it's about 1.5" you should have the JB8, and if it's around 1.25" it's the JB7, but even then I'd check with the actual part dimensions at AutoZone cause stuff gets mislabeled sometimes. As I said, I have to run a '70s JB7 master cylinder on my '80s JB8 hydroboost system, and I only figured that out cause I had measured the bore depth of the original hydroboost master cylinder in the junkyard. __________________ The more I drink, the more I drink, the more I drink... diesel, reg cab, long bed, 4 across the rear, single stack, wooden stakes, and lotsa lights - the Hay Express

06-02-2007, 03:29 PM	#7
cory321 Problem Child Join Date: Sep 2005 Location: Kettering Ohio Posts: 1,248	Re: brakes Thank you I'll measure the front rotors tonight __________________ 1972 gmc sierra grande 350/350 a/c soon`to get a new 350 I Survived Norwood



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