What was the first year that disc brakes came out on impalas?
Would the combo valve and master cylinder be similar if the car weights were close?
My 64 has factory power brakes could the booster be kept and replace the single resiviour with a drum/disc master cylinder off a similar size fm car with front disc?
The first year for disc brakes was 1967. Chevrolet engineers took the caliper and disc rotor out of the Corvette parts bin and put them on the front only.
Yes the size of the master cylinder and combo valve are similar for similarly equipped vehicles of the same weight. By similarly equipped I am referring to the brakes being either manual or power assist.
Yes you can bolt on a dual reservoir master cylinder in place of your single and replace the brake lines to match the master cylinder. (you can splice into the brake lines and attempt to bend them and double flair the ends again, but they are probably pretty rusty by now and probably should be replaced)
The problem with your year car is that the steering knuckle had a forged steel spindle pressed into the cast iron steering knuckle. This two piece design makes it hard to upgrade to disc brakes. By buying a replacement one piece steering knuckle (either standard ride height or dropped two inches) with the cast iron spindle induction hardened to accept the wheel bearings you get the added material (tabs) needed to bolt on the calipers. This is required unless you go to a bracket that bolts onto the factory two piece casting and will require a lot of machine work and shimming to get everything aligned.
Just keep in mind that if you have the original unmolested factory springs that you are already riding an inch and a half to two inches below the factory ride height due to spring sag. So new springs and a standard ride height will actually raise you car up noticeably. If someone has heated or cut the springs before you bought the car your ride height could be considerably lower than how it left the factory. You may want to measure what you have and compare that number with what was published in the Chassis Service manual for 1962 (1964 cars required you to buy the 1962 factory manuals and then to also buy the 1964 addendums to the 1962 manuals).
I can also recommend the better kits sold by MPP and Classic Performance Products. Just keep in mind every one
sells cheap kits with cheap parts in them just to keep the Wal Mart shoppers happy. Brakes and suspension parts are an example of "you get what you pay for!"