Power Brake Booster - Which Setup Would Be Correct

Hello all, I am getting ready to start running brake lines on my CJ5 rebuild and am doing some planning ahead,
Here's my original setup.
1973 CJ5, 360 V8 (original 304) swapped in, all stock. T15 trans. Had factory power steering and power brakes.
Last year I had the front axle rebuilt because it was an absolute mess. During the rebuild they changed it over to front disks from a 1979. Same Dana 30 front axle, they swapped from the knuckles out. I still have the original 11 rear brakes. I did save the original brake booster which would have to be rebuilt if I use.
I know I am going to have to put in a proportioning valve for disk/drum. Can use my original (rebuilt) booster and master cylinder setup or can I buy a newer booster/master setup that would fit a 1979, which are much easier to find. I wasn't sure about the pedal arrangement and firewall differences from the intermediates to the 1976 and newer. I am going to add a hydraulic clutch if that makes a difference as far as fitting everything else. Thanks for the info.

 

Typically, drum/drum master cylinders do not displace enough fluid for the front (disc) brakes. Drum brakes operate on much less fluid volume being moved. So yes, you need a later model disc/drum master ideally matched to the calipers you're going to use.

 

After thinking it over my main target should probably be getting the correct master cylinder. Instead of trying to reconfigure the entire pedal/master cylinder/booster, it might be easier and cheaper just to have the original booster rebuilt and use that. At least I know it fits properly. I would still need the proportioning valve though. Thanks.

 

It's not so much the volume per pump of the foot but overall reservoir capacity and residual pressure valves.

 

while technically you are correct about volume, many of us have run drum brake master cylinders with disc brakes. I converted to 4 wheel discs using GM 1/2 ton (large caliper pistons) with the original master cylinder for 9” drum brakes for a LOT of years and while the pedal travel was a little more, it was not bad and not unsafe. I finally went with a dual reservoir setup from R&P meant for disc brakes for safety. While the pedal was a little higher operation was just about the same.
I would always recommend a disc master cylinder for disc brakes but a drum unit can in the right circumstances work ok. There are other considerations though like residual pressure valves, proportioning valves, metering valves, etc.

 

Do some of the master cylinders have the residual valves built in?
When I bought the jeep it had a brand new master cylinder. I don't recall seeing any part numbers on it. I guess I could always try it and see what happens.

 

Yes they do.

 

Many if not most drum brake master cylinders have the residual pressure valves behind the brake tube seats in the master cylinder.

 

Can you tell by looking at them? I'm not sure what they would look like. I am guessing it would be internal and I would have to take the piston out of the master cylinder and look at the construction of it.

 

Check out this thread, lots of info with pictures here.
http://www.earlycj5.com/xf_cj5/index.php?threads/front-disc-brake-tutorial.116749

 

In my experience not necessarily externally. Many disc brake master cylinders have a larger reservoir for disc brakes and a smaller reservoir for drum brakes due to the difference in fluid movement but not all. The tube seats can be removed with a wood screw carefully then reinstalled after the residual pressure valve has been removed then the needed rpv installed inline.

 

I'd be inclined to use a later disc/drum CJ master with an inline proportioning valve in the rear circuit.

 

I agree. I dug through all of may parts boxes and found the master cylinder that came with the jeep when I bought it. The brake fluid chambers are the same size, for the drum/drum system. Even though it's new, I will get the right one when the time comes. Thanks all for the advice.

 

Correct.
Drum brake wheel cylinders pressure out till the brakes contacts the shoes, then return. With the correctly adjusted shoes, the master cylinder moves about the same amount of fluid to the wheels, which returns to the MC when the pressure is released.

Disc brake pistons continuously move out as the pads wear. This means the MC is continuously "loosing" fluid to the calipers, which is why the MC runs a larger reservoir. Many modern disc MC's run a float which doubles as a wear indicator for the calipers. When the pads are worn enough to lower the fluid to a specific height, the light will come on, popping a warning light which triggers an inspection.

 

He he, that float might also keep a Subaru from starting if the level gets too low.