67-72 Brake booster m/c guide

[Southedisto]

Alright, I have 72 Jimmy 4wd with disc front/disc rear (D60 front and 14b rear with 3/4ton calipers). 3/4t master cylinder, stock booster and p valve. I bench bled the master, pressure bled the calipers and guess what? I cant move the pedal!!! I removed the master cylinder and looked at the booster but cant see anything wrong. When I tried to reinstall the MC, it would go. So I loosened one front and one rear bleeder and it went on semi smoothly. What gives? Is my push rod to long? MC bad (brand new from partsAmerica) I was getting great flow with my pressure bleeder, but now..................I feel like tossing all of it and going to hydroboost!!! I have been battling this thing for a while now, motor running= nothing. Motor off= nothing!!! The booster check valve is good (mouth suck test)

[Livrat]

If you are using the stock Jimmy 1/2 ton booster the rod is too long for a 3/4ton master cylinder.. You will need the 3/4 ton booster instead..
The 1/2 ton master cylinder uses a rod the goes about 2 inchs into the piston and the 3/4ton master cylinder rod is almost flush with the end of the piston..

[Southedisto]

Originally Posted by Livrat

If you are using the stock Jimmy 1/2 ton booster the rod is too long for a 3/4ton master cylinder.. You will need the 3/4 ton booster instead..
The 1/2 ton master cylinder uses a rod the goes about 2 inchs into the piston and the 3/4ton master cylinder rod is almost flush with the end of the piston..

Great! Now I have to buy more stuff.

So do I need the mount for the booster also? Or will the booster itself bolt to my bracketry?

[Livrat]

There are three different boosters for our trucks, I have been told that all the brackets are different too, but have never compared them side by side.. I have several of both types I can compare them for you when I get to the shop today and post my findings this evening if you like.

[Southedisto]

Please do, I dont want to waste any more money.

[Livrat]

Okay guys.. Here's what I know...

The first picture is of a 3/4 ton Disc Brake booster and MC.. it is a Delco Moraine booster..

The second picture is of a 1/2 ton Drum brake booster and MC.. you can usually identify this one by the Brass distribution block and the master cylinder is smaller than a disc brake one... this booster is a Bendix..

The third picture is of a 1/2 ton Disc brake booster and MC.. you can tell by the "cast iron" porportioning valve and the MC has larger resevoirs than the drum one

Both Drum brake and disc brake 1/2 tons use the same Booster..

[Livrat]

I have found two different styles of disc brake MC's.. both will interchange with each other as long as 1/2 ton stay with 1/2 ton and so on..
The first two pictures are of 1/2 ton MC's
The third picture is of the two different 3/4 ton ones..

[Livrat]

The first picture is of the back of a 3/4 ton disc brake MC.. You can see how shallow the hole in the piston is for the rod..

The second picture is of a 1/2 ton drum brake MC.. you can see how the rod goes into the piston much further than the 3/4 ton one..

The third picture is of the 1/2 ton Bendix booster (On the left) and 3/4 ton Delco Moraine on the right..

[Livrat]

Now on to the mounting brackets.. Yes they are different between 1/2 and 3/4 ton!!..

The first picture is of the 3/4 ton Delco Moraine booster... the mounting studs are approx 3- 3/4 inches apart...

The second picture is of the 1/2 ton Bendix booster.. the mounting stud are approx 3 1/2 inches apart...
The third pic is of the 1/2 ton Bendix booster with the correct brackets..

The fourth pic is of the 1/2 ton Bendix booster with the 3/4 ton Delco Moraine brackets..

You can see they don't quite fit.. but I suppose if one is "creative" and with a "round" file you may be able to line things up..

[Livrat]

Next I found the there is a slight difference in the lenght of the rod from the booster to the bracketry...
the 1/2 ton Bendix booster has a rod approx 4 3/4 inches long and the 3/4 ton Delco Moraine has a rod just over 5 inches long...