Can a disk/drum MC be used on a drum/drum setup?

Heath M

Heath M

Member
I couldnt find an answer to this anywhere.

I have a 72 Scout ll 2x4 that the master cylinder went out(manual brakes). I have a new master cylinder and power booster I bought as one unit. This unit is for a disk/drum setup; I have a 1979 parts Scout ll 4x4, that I am swapping in the axles, transfer case, etc into the 72...I am not ready to do that yet, but had the mc/pb, so I thought I would use it.

Can I use this mc/power booster on my drum/drum setup until I can get the front disk axle in, in the future some time? Would I still be able to use the same block (attached to fender well), or would I need a proportioning valve like the one on the 79 Scout?:confused5:
 
I went the other way kinda when I swapped in a booster and a master from a 75 disc brake truck into my 72 drum brake one and it worked fine until I swapped in disc front axle. I used the disc everything and it stopped fine. Wouldn't hurt to try. If it doesn't work that May motivate you to swap at least the front axle in a little sooner:). Then when you get bored swap in a hydro-boost! That'll put you through the windshield!
 
qdawg said:
I went the other way kinda when I swapped in a booster and a master from a 75 disc brake truck into my 72 drum brake one and it worked fine until I swapped in disc front axle. I used the disc everything and it stopped fine. Wouldn't hurt to try. If it doesn't work that May motivate you to swap at least the front axle in a little sooner:). Then when you get bored swap in a hydro-boost! That'll put you through the windshield!
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Thanks for the input. It should work fine.
I believe when I get the disk axle in, that I have to switch out the warning light switch with the combination valve(proportioning valve) as shown in the service manual.

The brakes aren't working properly and I bled the system until all air was purged out. maybe there is something else I am supposed to do?
 
It almost doesn't stop, but the pedal is completely different than before. Short stroke, and soft in that short stroke.
 
You swapped it over to the power brake unit from the newer truck correct? Or just trying to fix the old manual/drum setup?
 
I have yet to hear if the proportioning valve was installed too. Basically you're mixing apples and lemons together at the moment. If you have the disc brake axle on hand, I would install it now. It all works together as a package.
 
the brakes aren't working properly
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Mc's for a drum/drum are different from a disc/drum. The drum circuits use a residual pressure valve in the master cylinder to hold about a 5 psi back pressure on the brake hydraulics. This keeps the system "charged" for immediate application of the brakes at the slightest pedal movement. The shoe return springs pull the shoes and the wheel cylinder pistons to home when the pedal is released, but only enough to release them from drum contact because of the residual press. Valves.

Caliper cylinders don't have return springs, and the hydraulics don't have the residual valves. Depending how close the shoes are adjusted, without residual pressure on the circuit; using a master like your saying, one or the other axle is gonna do a majority of the braking and probably why it don't stop so good for ya.

When you do go to a disc/drum setup, the proportioning valve is a must because it will balance your braking forces front and rear and it is a totally different part from the warning switch dealy Bob you've got now. A combo valve is a proportioning valve with a warning light switch if you elect to get one of those.
 
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scooter said:
I have yet to hear if the proportioning valve was installed too. Basically you're mixing apples and lemons together at the moment. If you have the disc brake axle on hand, I would install it now. It all works together as a package.
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I did not install the proportioning valve. The master cylinder and power booster are all I installed.

The disk brake axle wont match the current rear gearing. I was planning on swapping the matching rear off my parts Scout, however, when I pulled the rear dif cover off and drained it, to investigate,it had a lot of metal chunks in it. That postponed my whole axle swap.

Today I tried bleeding the system again with a pressure hand pump, after an hour of pumping tons of bubbles, I decided to get one of those tall and skinny closet mirrors. I propped the mirror up to where I could see all wheels except the driver side front. I then was able to see any bubbles coming from the tube in the bottle. Doing all the wheels this way took a lot less time than the hand pump on one wheel.

I drove it around today while I was working, I tried to take note of how the pedal felt, however, I have no frame of reference as I have always had a sasquatch leg for use with my manual brakes, which used to go all the way to the floor most times. I think, after today's driving, I would consider the new brake pedal soft. Oh, and it does not come to a complete stop.

Hopefully all this makes sense.
 
heath m said:
I couldnt find an answer to this anywhere.

I have a 72 Scout ll 2x4 that the master cylinder went out(manual brakes). :
Click to expand...

heath m said:
I did not install the proportioning valve. The master cylinder and power booster are all I installed.

The disk brake axle wont match the current rear gearing. I was planning on swapping the matching rear off my parts Scout, however, when I pulled the rear dif cover off and drained it, to investigate,it had a lot of metal chunks in it. That postponed my whole axle swap.
Click to expand...
Kinda conflicting with what you're saying. Just trying to get things correct and on the same page, so we can help properly. Your first post mentions that you have a 2x4 - which tells me that you have a two wheel drive and but the front spare axle with disc brakes came out of a 4x4 Scout. Are you converting your 4x2 into a 4x4? If the front axle with disc does not have matching gear ratio to your current rear end, then why not rebuild the front end with gears to match?

Greg r went more indepth on the brake systems between the two types you are dealing with. As it stands now, your current braking is totally unsafe. Until you get it correct don't drive it. Seriouse injury will occur. You will need to install the proper proportioning valve to match the master cylinder/booster combo. Providing that you have disc in the front. Otherwise you will continue having the soft feal on the pedal along with lack of stoping.
 
I appreciate the help. The item in the picture; the power booster, mc and non-proportioning valve do not go together with a drum/drum system.

So, I have a disk front matching the gear ratio of the back that I swapped in to the 2x4, along with transfer case and what not. Along with this, I replaced the non-proportioning valve with a proportioning valve.

So the power booster/mc/proportioning valve I have now, works with my disk/drum setup. If I was to stay with a drum/drum setup I would have had to switch out the particular mc shown in the picture.

I believe this makes sense and clarifies it a little better? Sorry for the confusion.
 
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