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3/4 ton brake booster mod.

AddictedOffroad

AddictedOffroad

1/2 ton status
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Feb 5, 2008
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Windsor, CO
Note to self: When testing the new mod, buckle up. :confused:


I just about put myself through the windshield. :D:D:D

Swapped the 1/2 ton Blazer booster for a 3/4 ton Burban tonight. Huge difference in braking power. Front discs, rear drums. Can lock up 37s with no issue. The old ones felt like manual brakes. New ones feel better than my 2000 Tahoe.

Anyway, this is a great mod, and is a must for anyone running the stock booster. You don't know what you're missing! One 9/16" wrench and 45 minutes and you're done.
 
I've been thinking about doing something like that for years. My dads '72 3/4 ton has way more boost than my '71 Jimmy. I've often wondered if swapping in a 3/4 ton booster would make life a little better. Thanks for posting that.
 
What year was the 3/4 ton booster off of? Has anyone else done this with 4 wheel discs?
 
yes more info. I have an 87 JIMMY I'd like to do that to as I am having break problems now. EJS is a week away. HELP! Part Numbers would be helpful too
 
AJs72K5 said:
What year was the 3/4 ton booster off of? Has anyone else done this with 4 wheel discs?
Click to expand...


I did the mod on my Jimmy with 4 wheel disc and it will lock up the 37's with very little effort. My setup is off of a 72 3/4 truck. There are several different styles of boosters and brackets.

There is some good info in this post
 
I've also done it with the 4wheel discs. Just ask for a replacement booster for the same year, but ask for the 3/4 ton instead of 1/2 ton.

I had to do some adjustment of the master cylinder pushrod though as it was too long after the swap. Making things shorter is pretty easy though.
 
some use corvette MCs too since they are more setup for 4 wheel disks. I traded a guy a complete hydroboost setup off a early 90 CC (straight axle) 1-ton truck just need him to get them here so I can swap. I also removed the factory porportioning valve for a wilwood adjsutable one and set it so my brakes locked a bit more equally which helped (I have 4 wheel disk already)
 
So....here's what's under my hood. 3/4 ton booster? I can't readily identify master cylinder. I expected a lot more grab out of the disc conversion.

IMG_2430.sized.jpg
 
I think the difference is in the length of the booster. It does kinda look like a 3/4 ton booster, but it's hard to tell without the smaller one to compare with.

The master cylinder looks like a stock 1/2 ton. It looks identical to mine.

If it's not grabbing as much as you'd expect, there's a good chance it's not bled all the way. I had a super hard time bleeding the rears after installing the 4disc proportioning valve. Depending on which calipers you have, they need to be bled off of the bracket since the bleeder valve doesn't point upward like it needs to.

Since it looks like you're using the stock prop. valve, there's some mods you can do make it work better with 4wheel disc if you have an auxillary proportioning valve installed.

http://coloradok5.com/atrondiskbrakes.shtml
 
I ordered a 72' 3/4 ton Suburban 4x4 booster. Even gave them the 1/2 ton core back for the refund.

It really is a huge difference. With the 1/2 ton, I had to apply a great amount of pressure to get the rig to stop. Now, its perfect.
 
bp71k5 said:
If it's not grabbing as much as you'd expect, there's a good chance it's not bled all the way. I had a super hard time bleeding the rears after installing the 4disc proportioning valve. Depending on which calipers you have, they need to be bled off of the bracket since the bleeder valve doesn't point upward like it needs to.

Since it looks like you're using the stock prop. valve, there's some mods you can do make it work better with 4wheel disc if you have an auxillary proportioning valve installed.

http://coloradok5.com/atrondiskbrakes.shtml
Click to expand...

I'm just running 3/4 ton front calipers...not the Caddys. I think we got them bled out fairly well...but that doesn't mean they're perfect.

I'm 90% sure there's no auxilliary proportioning valve on the rear, but I'd have to look to make sure. The mod to the stock prop valve looks pretty easy. I'll pick up an aux valve for the back and do the mod to the stock prop valve. If it still needs more oomph I'll check into the booster.
 
I actually just used the 4wheeldisc combo\prop valve from inline tube. It works pretty well in that my rear end does not lock up before the front. Although, I've only locked up the brakes once. That was in Moab heading down a steep rock face which I remember at the time second-guessing myself on whether I'd bled everything really well. I did.
 
bp71k5 said:
For reference, here's the web page for the valve I used.

Click "proportioning valve" on the left side and scroll down to "1971-77 GM Prop Valve - Inline Tube Exclusive"

www.inlinetube.com
Click to expand...

Thanks for pointing me in the right direction. Just ordered one of their 4 wheel disc prop valves. Will update when I get it on the truck.
 
P1010005.jpg


Here is my 1ton 72 4x4 Master/Booster combo I put in. Notice the lines to the P-valve are plumed opposite of a 1/2 ton setup. I bought thoes small lines from Inline Tube and they fit perfect.
The stoping power is 10times better :D
 
i need to do the same. I've been a little disappointed in my 4 wheel discs. it stops ok, but cant lock'um up like they should....... I used a 77 corvette mastercylinder and the proper aftermarket (inline tube) p-valve, but left the stock 1/2 tone booster. It was hard to buy a new booster when the one i had was only a few months old.
 
I did both a new master and booster, I can't remember if mine were for a 3/4 or one ton.:o, But I also put in a wilwood adjustable proportioning valve on the line to the rear, it is mounted to the inner fenderwell and I have a residual pressure valve so the rear disc don't back off too much.

It stops very well and the line to the rears is almost closed. I did numverous stops from 25 mph and kept adjusting with somebody watching my wheels to see what locked up first.

To bleed we took the calipers off the brackets and used Oak spacers that were machined to the same size as the rotors. so we could get the bleed done right.

*** I ran into a problem, not sure if it was the jimmy vs blazer thing or what but I had to find brackets for the back of the booster that were shorter. seemed the 1/2 tons stuck out further than the 3/4 tons did. So went to the junk yard and found the brackets I needed and they bolted right up, just a couple inches shorter from the firewall.
 
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