Here are some thoughts I have saved in a file. The info may or may not be applicable to you. But, if you are using disc brakes be sure to heed the info re: the Residual Pressure Check Valve below.
When replacing front and/or rear drum brakes with disc brake components on a First Generation Mustang or Cougar you do not want to go cheap. This is a fairly serious, even if fairly easy, operation. Use high quality parts, preferably in a kit designed to do a full upgrade. I like Wilwood, a reputable brand with a lot of years in this end of the business. Here are the most important things to be aware of:
• Replace the Master Cylinder with one designed for Disc Brake systems. Often the MC bore size is different compared to Drum Brake system MCs. Also, for Drum Brake MCs the fluid outlet port(s) for drum brakes has/have a Residual Pressure Check Valve (RPCV) under the outlet port’s brass seat. The RPCV is needed for drum brakes to keep fluid pressure on the wheel cylinder cup seals so they do not suck air into the cylinders when the fluid cools off. But, for Disc Brake systems such a valve puts needless pressure on the caliper pistons and causes the pad to drag on the rotor under pressure. That results in reduced vehicle performance, overheating of the front pads and rotors, and rapid wearing of the front pads.
https://www.youtube.com/watch?v=WxT9vLO6pOc (00:42)
• Flush the old brake fluid and replace it with fresh DOT3 Disc Brake Fluid. DOT3 is totally compatible with Drum Brake hydraulic components, and differs from DOT2 Drum Brake Fluid in its boiling point is higher – which is needed for Disc Brake systems. Do not mix newer brake fluid types with DOT3 fluid. And never use petroleum fluids (oil of any type) - ever.
• Get a Power Brake Booster, as the Disc Brakes are not a self-actuating design like a Drum Brake system is. I have driven Disc Brake systems without a power booster before. No fun. They work, but power boosted Disc Brake systems are much better.
• Never use copper lines for hydraulic brake systems (Nickel-Copper alloy metal lines made for brake systems are fine). The copper metal alone is too soft and the lines can and will rupture if enough pressure is applied. Use only properly designed steel or Copper-Nickel alloy lines with double flare endings.
• Do not go cheap on brakes, suspension, or tires, ever.
For front disc calipers, be sure to put the calipers on the correct side, otherwise you will never be able to properly bleed the air from the hydraulic system. Refer to the attached graphic file, apparently created by Richard Ackerman (I got it from Dennis Keegan), to see how the calipers ought to be oriented. Many thanks to Richard and Dennis for allowing me/us to share this excellent graphic.