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I have a 67 chevy 3/4 ton truck. I have swapped in a 72 front end with disc brakes. I have also added discs to the rear. Now I need a booster assembly and master cylinder. I have a 79 booster (from a similar chevy truck) and master cylinder, will it work? Or should I try and find something different.

Nick

You will need a MC for a Disk/Disk system.

You can usually tell if the master cylinder is for drum or disc by looking at the volumetric size of the unit. Disc require more fluid to operate so the size of the resivoir will be larger. Also, the piston is larger on the disc type M/C. The piston size of 1-1/8" or 1-1/4" is about normal for a disc setup when drums pistons are usually 1" or less in order to build higher pressures that drums need. You also need to to do the length of pedal calculations so you don't over pressure your disc and put your 'sweet thing" through the windshield. Good luck.

Thank you... You just helped out one of my problems. I didn't know there were different masters.

I've found many advertise their m/c fits drum-drum, drum-disc, or disc-disc. This is confusing to me. All my research says Beltfed is exactly right, so why the universal fit advertising? Are they making up the volume difference with an adjustable proportioning valve? Sorry for the hijack, but I'm facing the same problem and can't afford to do this twice. I've also read not to use a booster smaller than 8" for a disc-disc setup. Can someone educate me ? Thanks

Disc brakes don't actually require more fluid to operate. If you look at drum brakes, you will always see an adjuster on the brake shoes. Disc calipers have no adjustment for wear of the pads. As the pads wear down, there is more and more fluid sitting in the caliper when the brakes are not applied. The reservoir is larger so that there is no danger of the fluid running low during the life of the pads. If you keep a disc reservoir completely full, you will have to remove fluid when you change the pads. That doesn't happen with drums when you change the shoes.

Piston size is not necessarily bigger on disc brake MC. Size is more of a factor of power booster or non-power. If you look at most older drum/drum brakes they are non-power. With some exceptions, disc/drum and disc/disc have a power booster. Larger MC pistons require more input pressure for the same line pressure as a smaller piston. The booster allows less pressure from your foot to overcome the need of the larger piston.
Look at a disc/drum MC. The bore diameter is the same for the disc portion and the drum.
The web site below has as much info as any.

http://www.mpbrakes.com/technical-support/configurations.cfm

The info you need has more to do with pressure than volume.

Non-power MC's have a deep hole for the brake rod, so the rod doesn't fall out when you take your foot off the pedal. Most power MC's have a shallow hole because the rod in front of the booster is short. I say most, as there are some that use a longer rod.

Very good information Richard, thank you.

"Disc brakes don't actually require more fluid to operate." Richard, how about 'over their operational life'? Are we just "splitting hairs"?
I went down this road with a somewhat heavy Packard with a drum/drum with booster setup. Using the larger size piston would not stop the car in a panic scenario, but then I don't have the leg muscles of a USC fullback. I found the solution by going to a 7/8" piston from a 1-1/8". It was somewhat hard to fine a smaller piston M/C that would fit up to a booster setup.

As long as I have a couple of knowledgeable guys here, I have another question. I have a drawing from eci.hotrodbrakes and a drawing from mpbrakes.com. Both show a typical disc/disc configuration. One uses a combination valve and one does not. One says an adjustable proportioning is an option, the other never mentions one. Now you see why I'm confused. As near as I can figure to properly run a disc/disc system with power boost, I need a 1 1/8" master with at least a 7" dual diaphram booster for proper pedal action and travel. Two 2 lb. residual should also be used. Does this sound correct?

Do you need a proportioning valve on disc/disc? I would say there is no easy answer to that. You have a 3/4 ton. What did the rear discs come off of? If you have the same braking ability, front and rear, the rears will lock up you when you jump on them. You don't want either to lock up, but if the rear does first, the truck will swap ends.
Residual valves?
http://www.mpbrakes.com/technical-support/configurations.cfm
One more little click. http://www.mpbrakes.com/uploads/documents/residualvalves.pdf
Booster?
http://www.mpbrakes.com/technical-support/configurations.cfm
Another click. http://www.mpbrakes.com/uploads/documents/optomize.pdf
My '67 3/4 weighs 4600 lb. I used a booster from a '80s 3/4 Chevy van because they can be mounted flush to the firewall. The pickup boosters sit out about three inches. A 7" might be on the small side.

beltfed, Going from 1 1/8" down to 7/8" would increase the pedal travel. I would have looked at the pedal ratio http://www.mpbrakes.com/uploads/documents/pedalratiopdf.pdf and then maybe looked at a bigger booster for a car that big. If it works and the pedal travel is OK, then good enough.

I only used MP's web page because it is easy to find what you're looking for. They don't really have answers, but a good place to start and to see what you can expect with different changes.

My rear discs are on a 2003 s10 blazer rear end. I have this intstalled in my 37 Bolt. After visiting MP's page, I'm beginning to wonder if it dose'nt have the low drag calipers. That would change everything. Guess I got a little more research before I buy a M/C.

I have to agree with Richard on the proportioning valve. This same subject came up this very weekend while bench racing with two very experienced car builders. One ('34 Ford with 350ci) said the valve was unnecessary on that car, the other ('48 International panel) said they were a good thing. I would think a proportional valve that is pressure adjustable might be a good idea for 'dialing' in the correct pressure. As far as residual valves go...I think we all use them here. My M/C is low under the foorboards and they are recommended for that type of setup. Many M/C have residual valves mounted in the body of the M/C. I remember reading somewhere that you can take a stick like a wooden qtip and run it inside the ports. If it goes in just a little then the M/C has the valves inside the body.
In all fairness to Richard. He hit my problem right on the head. I had no more room in the frame area for a booster bigger than 7 inches. I had to give up pedal travel for pressure. If I could have gone with say...a 9 incher, the smaller piston may have not been necessary.

MP is very expensive compared to the others.