This is info I post on a lot of places whenever I see references to brake upgrades to disc. I feel it is important enough to bear repeating time and again.
You my already be aware of this, but in case you are not (or others reading this thread), it bear bringing this forth. The Master Cylinder (MC) for disc brakes is not the same as the MC for drum brakes, whether front disc or all disc. The first difference is the bore size of the MC is not always the same between the drum and disc brake systems. Second, the MC brake fluid reservoir chamber is larger for the disc brakes as the volume of brake fluid needed in the disc calipers is much greater than the volume of brake fluid in drum brake wheel cylinders. And most arguably the important difference is the Residual Pressure Check Valve (RPCV) needed for drum brake hydraulic fluid outlet(s) in the MC is NOT needed or wanted for disc brakes. Finally, never add any petroleum fluids to the brake system's hydraulic fluid, as it will cause the rubber seals and hoses to swell, requiring all hydraulic parts to be rebuilt or replaced. An aside, never let brake fluid get on painted surfaces, as it is an excellent paint remover.
If a brake system has its front drum brakes upgraded you need to make certain to remove the RPCV, otherwise the disc pad will be constantly dragging on the discs under pressure, which overheats the pads and rotor, causes excess wear of the pads, causes warping of the rotors, and impedes performance of the vehicle as there is a constant drag of the brake pads under pressure 100% of the time. If in doubt, replace the MC. The RPCV units are installed under the brass seat(s) of the brake fluid outlet port(s) if it/they are present. They are the little "flapper valves" made of rubber. If you have drum brakes the RPCV is critically needed.
Also, use only DOT2 or DOT3 brake fluid. Do not mix newer silicone based brake fluid with older DOT2 or DOT3 fluid. Never use DOT1 fluid as its boiling point is too low for use with disc brake systems.
Feel free to look at the attached file for more info, to include a graphics showing the correct orientation of the single piston brake calipers to ensure the brake system can be properly bled. If the calipers are installed on the incorrect side of the vehicle they will "fit," but you will never be able to bleed all the air from the calipers, and the brake pedal will feel mushy.
