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Interchangability of master cylinders/brake boosters

Discussion in 'Lifted & Offroad Suspension' started by Crawdaddy, Nov 20, 2010.

  1. Crawdaddy

    Crawdaddy ↑↑↑ Has no life Staff Member 5+ Years 1000 Posts Platinum Contributor

    Well, I'm getting close to finishing my 1-ton Suburban conversion, but I think I need a 1-ton brake booster and master cylinder. For about 3/4 of the brake pedal travel, the brakes don't seem to do anything at all. From what I understand, the brake boosters have the same mounting patterns that bolt onto the firewall. I just wanted to double-check this before I go ordering a new brake booster and master cylinder that won't fit without modification on the truck. Thanks for the help.
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    What is it you need? Measurements from master & vac booster mounts from any 1 ton approximately '91, or smth else? I have '88 C-3500 1 ton, that I can measure. Will this help? Were they the same through those years? Are Burb & Silverado going to be same booster/master cyl?
  3. Crawdaddy

    Crawdaddy ↑↑↑ Has no life Staff Member 5+ Years 1000 Posts Platinum Contributor

    If you have the booster off the truck or can easily access the bolts under the dash, please measure all the characterstics about the booster you can. More data=better. And your year of truck is a perfect replacement to fit my truck. Thanks!
  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    Hey Christopher, booster & master are all assembled, but I think I can get mount measurements etc. I will measure today.
  5. stephan

    stephan Rockstar 4 Years 5000 Posts

    Ok Christopher, I was able to measure everything you should need.
    Booster to firewall mounts seem to be on a perfect square.
    Side to side, studs meas. 3 7/16" center to center.
    Top to bottom studs are 3 7/16" center to center. These are + or - 1/32"
    Booster diameter max o.d. 10 1/2"
    Booster reduction diameter 8 5/8"
    Booster depth, firewall to m.c. mount 5 3/4"

    M.C. mounting studs on booster are 3 7/16" c. to c.
    Master cyl. is 8 1/4" oal
    Max dia. 1 11/16" so best guess on pistion dia. is around 1' to 1 1/4"

    I am surprised with the size of this booster. I seem to remember the boosters on my '80 Z-28 & my '85 IROC were much bigger, maybe 12" or even 14" diameter & those cars weighed 1700 lbs less. I can't fault the ability of this booster though, as it will lock up all 4 wheels (no abs lol)
  6. Crawdaddy

    Crawdaddy ↑↑↑ Has no life Staff Member 5+ Years 1000 Posts Platinum Contributor

    Awesome. I'll check your measurements against my engine compartment, see if it'll bolt up. As for the boosters being so huge in those old cars, it might have been during the time when they were only using single diaphragm boosters, where we have dual-diaphragm boosters now.
  7. stephan

    stephan Rockstar 4 Years 5000 Posts

    If your booster bolt pattern is larger, you should still be able to mount it, as this is a large mounting area, & you should be able to redrill for a larger spread pattern on booster bolts.
  8. Crawdaddy

    Crawdaddy ↑↑↑ Has no life Staff Member 5+ Years 1000 Posts Platinum Contributor

    My booster is on a bracket that holds it away from the firewall by a couple inches. I don't know how that's going to affect me. Another thing I need to figure out is shaft lengths going from the booster to the brake pedal. Too much or little shaft will cause the brake pedal to be located wrong. Ugh, just another thing to figure out...
  9. stephan

    stephan Rockstar 4 Years 5000 Posts

    ok, I didn't know about your bracket, but that gives you more options for mounting & rod adjustment.
    I have been wondering if maybe your 3/4 of pedal travel before having any brakes might be a brake adjustment issue. You put a complete 1 ton rear axel assembly in it right? Maybe that puppy needs the brakes adjusted. They should be self adjusters, but they will only self adjust if you are backing up & hit the brakes. If they are way loose, this could require doing it 10 or 15 times. Mine were never working properly (mine were actually getting too tight & dragging, & hurting mpg) so I defeated the self adjusters, & just manually adjust them once a year now. I don't think swapping axles should require you to get a new booster. Your booster & master cyl. is matched to your pedal stroke & rod, so all should be good as is if the brakes themselves are adjusted & bled.
  10. Crawdaddy

    Crawdaddy ↑↑↑ Has no life Staff Member 5+ Years 1000 Posts Platinum Contributor

    Does anyone know anything about what a Pedal Rod ID is? I'm looking at 2 different boosters. The JB5 booster is what I currently have, and autozone is listing it's pedal rod ID as C14. The JB7 has a pedal rod ID of C17. What in the world is a Pedal Rod ID, and what are the differences?

    Just as an FYI, the RPO code JB5 stands for JB5 : BRAKE, POWER, DISC/DRUM, 6400 LBS. The JB7 RPO code stands for JB7 : BRAKE, POWER, DISC, DRUM 8400 L85. There is a RPO code with a higher rating, the JB8, listed as JB8 : 8RAKE, POWER, DISC, DRUM 10000 LBS, but since it's not listed for any application I can think of, it's probably for a real old 4500 or 5500 class truck, and I'd think they might not be compatible.
    Last edited: Nov 24, 2010

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