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If I fit a disc conversion kit front only to a 56 3100 do I have to upgrade the master cylinder or fit a booster as well

No you don't "have " to. Will need proportioning valve and original MC bore may be too small. Among other things.

What else would be involved?

I agree with Bartamos. My experience is that you can certainly try it that way, but I think that you will discover that doing it that way is a rather large step backwards in regards to the amount of pedal pressure required to get your truck to stop and it is likely that your rear drums are going to lock up fairly early in the process. Be prepared $$$ to install a boosted MC kit with a proportioning valve.

When you have only drum brakes, the brake fluid returns to the reservoir when you release your brake pedal. You release pressure on your brake pedal, the brake return springs causes the shoes the retract, which cause the wheel cylinder to retract, which forces brake fluid back into the system.

With disc brakes, that does not happen. When you apply the brakes, fluid fills the calipers and pistons compress the pads. When you release your brake pedal, the fluid does not return to the reservoir. As your brake pads wear, you need more fluid to apply the same amount of pressure.

That is why on a master cylinder for a disc/drum application, the disc reservoir has a larger capacity than the drum reservoir.

I think I've go that right, but if I don't, some knowledgeable Bolter will correct me.

The very worst part of the drum brake system on these trucks is the stock master cylinder. I certainly would not attempt to use it with discs. I have installed a booster and upgraded MC with stock brakes on both cars and trucks before disc brakes were available and they operated much better. Do it right, use a dual diaphram booster, dual MC, and a proportioning valve and you will have great brakes.
The last booster/MC unit I purchased was under $300, well worth the expense.
Fred

as said above, buy a booster/master cyl kit as well. You can get one that goes in the stock spot and works with your stock pedal. You can get it with a remote reservoir that you can mount under the hoos where you can get to it to check and fill it.

Drum brakes can do plenty of stopping- - - -otherwise 18 wheelers wouldn't still use them. Getting 40 tons hauled down from highway speeds repeatedly isn't an easy task. Disc brakes are really a giant leap backwards from drums as far as stopping ability goes. They use a tiny contact patch of the pad on the rotor, and extremely high clamping force to create the friction necessary to stop the vehicle. Vacuum-boosted drum brakes worked for years before discs became popular. The only advantage to discs is the fact they're less prone to "fade"- - - -where the drum or disc absorbs so much heat it loses its ability to provide braking effect. Discs cool faster than drums, due to the hollow spaces between the swept surfaces, and the long sweep after leaving the pads before the friction is applied again. Bigger diameter and/or wider drum brakes with a vacuum booster on the master cylinder would be as good as discs, maybe better, up to the point where fade begins.
Jerry

The stock master cylinder is NOT disc brake compatible. Disc brakes take about twice the pressure to operate.

The disc conversion needs a new master and power unit is optional but a good idea.

When you order the master cylinder they will ask if your disc front and rear or dics front and drum rear and the master cyclinder will be set up for those brakes.

There a ton of info on choosing a master cylinder............

http://www.classicperform.com/tech_articles/PDF/Dems_DA_Brakes_3.pdf

http://www.classicperform.com/quickstop.htm

[img]http://www.classicperform.com/Store/1947_59_Trucks/Images/4954CBK-6HK.jpg[/img]

Disc may or may not be as "good" as shoes but, probably are better. Pad surface area is greater and there aren't a dozen or more springs, clips, pins, nails, adjuster wheels, wheel cylinders, bars........to screw with. Much better idea if you can afford the swap. Pretty sweet with power assist and dual MC. Rear brakes don't need disc on our trucks. As said, disc better for heat dissipation and water throw off. The big trucks are switching from drum to disc to meet a "new", mandated, shorter stopping distance requirement.

Excerpts from Dec. 15, 2005, notice of proposed rulemaking
(NPRM) to improve the stopping distance performance
of truck tractors, the National Highway Traffic Safety
Administration (NHTSA)

Increased Use of Air Disc Brakes
While already highly utilized in Europe and in select
North American applications, air disc brakes are
experiencing continued growth in popularity on North American highways with
commercial vehicle fleets and owner-operators. Previously considered a niche
technology for bus, coach, fire trucks, and other special applications, air disc brakes are
becoming increasingly sought out for over-the-road applications.
Bendix experienced a nearly 120 percent increase in demand for these advanced
braking systems from 2009 to 2010. The company added a new assembly line in 2011
to meet increasing demand. In early 2012, Bendix Spicer Foundation Brake reached the
quarter-million milestone on the brake, which it began producing in 2005. The company
surpassed the 400,000-unit mark in early 2013, an increase of more than 60 percent in
just one year.
Industrywide, air disc brakes represent only a 10 percent market share, with drum
brakes encompassing 90 percent. Their acceptance is clearly growing, however,
as evidenced by the ever-increasing output at Bendix. The industry is increasingly
demanding air disc brakes and the higher performance and safety they offer, as well as
their reduced maintenance demands.

I agree with Bartamos. Disks are a huge improvement. I haven't heard of anyone "upgrading" their factory disks to drums! A BIG problem with drum breaks is getting the shoes arked to the same curvature as the drums. If the drums are turned and the new shoes don't make full contact then you'll have awful brake performance. Happily that isn't a problem with disks. Brake shops used to have the machine to grind the proper ark on the new shoes but good luck finding those machines now as they produce asbestos dust.

I disagree that running the stock master cylinder would work in a satisfactory manner. You need a m/c made for disk /drum applications. As for the need for a boosted m/c,
I am running a non boosted master cylinder on my disk /drums and the pedal feel is nice.

David

....................think about being SAFE. What is it worth to you and your family?