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So I have a 66 mustang with drum brakes on all 4 and they’re terrible. I wanna put disc brakes on the front, but would they work with the stock single pot master cylinder? I just don’t eanna go through the trouble of making that a project. I just wanna jack up the front, swap out the drums to disc’s, bleed em and be done. Would the single pot master function correctly with the discs? Would they work better or would they just be kinda the same as the drums with the stock master?
 

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No because it has a residual check valve and lacks the fluid capacity as the pads wear and the pistons move in. The popular master cylinder is the one for early 70's Mavericks with manual disc brakes. Has the residual check valve for the rear drums and a 15/16" piston.

When I still had drums on my 66 and had to rebuild them when I bought the car. I noticed there was only one drum listed for 65-73 cars and for both small block and big block application. The small block used a 2.25" wide shoe while the big block used a 2.50" wide shoe. So I installed the wider big block 2.50" shoe. Along with the finned aftermarket drums that cool better then the stock non finned drums, this set up worked very well on my 66! I never had any issues. In fact when I put 72 discs on my car, I really didn't notice much if any improvement. I'm not saying this is a replacement for disc, just saying what I did IMO, worked much better then stock drums.

Here's a little reading too. Again just giving this info if for some reason you stick with drums and trying to make the most out of them. There are also several good factory reproduction KH disc brake kits that will have everything you need. These are very good brakes and they are pure bolt on and work with existing 14" steel wheels

 

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Actually there was a factory single pot master cylinder for disc brakes in '65/'66. It is not the same one that was used for drum brakes. The main noticeable difference is the clip-on cap vs. the screw-on cap. The disc brake option also included an inline proportioning valve for the rear brakes.
 

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The drums brake are not that bad. They will lock up all four tires and have a nice hard pedal. Thy work fine unless your going racing. If yours won't don't lock up the wheels and stop straight we need to find out if there is something wrong with your brakes before you condemn them.
 

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There is no quick and easy drum to disc swap. Even if you get factory original parts there's no guarantee that the braking will be much better than the 4 wheel drum setup. If you're expecting a 66 to stop like an 80, you're better off selling the 66 and buying an 80.
 

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So I have a 66 mustang with drum brakes on all 4 and they’re terrible. I wanna put disc brakes on the front, but would they work with the stock single pot master cylinder? I just don’t wanna go through the trouble of making that a project. I just wanna jack up the front, swap out the drums to disc’s, bleed em and be done. Would the single pot master function correctly with the discs? Would they work better or would they just be kinda the same as the drums with the stock master?
Properly installed drum brakes could easily lock the wheels at 70 mph. Something is wrong with yours. These cars stop very well. There's a Mustang in a shop near here that has a car now that came in with a similar complaint. Turned out the wheel cylinders were mostly frozen with corrosion. New wheel cylinders are being installed, and when done it will stop on a dime.

Disc brakes require a disc brake master cylinder, and a proportioning valve controlling the rear brakes. Without the proportioning valve, the disc brakes will not function. Disc brakes require something like four times the line pressure than drums. I have seen discs installed as you suggest. The result is disastrous. The rear drums end up doing all the stopping, while the disc brakes don't even get warm, i.e., they do nothing. I have driven disc brake cars with proportioning valve failure. Basically, a car in that condition is a death trap.

The best thing to do is fix the brakes you have. At some point, if you want to convert to front disc, fine, plan it out, get an OEM style kit, and do it. But do it because the type of driving you do requires it. Do you live in the mountains? Tow a trailer? Drive in autocross events? Or are you just nutz like? If yes, start saving. If no, well, then, we just saved you a ton of money.
 

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The factory type Kelsey-Hayes front disc brakes are one of the best mods I ever did to my car, but that included the correct master cylinder, all stainless hard lines, braided stainless flexible lines (painted black -wink-), new rear cylinders, rear shoes and hardware, and all new fluid. Brakes are worth doing RIGHT. My son's car still has 4-wheel drum and they function great, but we may change them to disc at some point.
 
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No, you cannot use your single bowl drum MC in a disc/drum application, but as others have said, there is a single bowl MC for cars that came with discs/drums. However, at that point, if you're buying a MC anyways - which you will need to - then you might as well swap to a 1974 Maverick dual bowl MC, which will work great with a disc/drum application and add some extra safety. It is really not that much of a pain to swap out a MC once the system is drained anyways.

To convert from your single to a dual reservoir system, all you will need to do is:
1) Attach the old brake line on the MC side to the rear bowl (which goes to the front brake circuit). Leave the other end attached to the distribution block as original.
2) Remove rear brake line where it goes into the distribution block, and plug the outlet hole for the rear brakes in the distribution block (so it only distributes fluid to the front brake circuit)
3) Acquire a proportioning valve
4) Plug the old rear brake line (which previously went into the bottom outlet of the distribution block) into the outlet of the prop valve
5) Route one new line from the bowl for the rear brakes to the inlet of the prop valve

If you want to be really fancy (and more correct) about it, you could install a residual valve in the rear line as well. But many people do not.

Braking is not one of those things that you want to get wrong. I'm about to redo my brakes because I'm unhappy with them, and I'd rather shell out the money now than pay for a new front end when I inevitably can't stop in time for someone slamming on the brakes in front of me.
 
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