Disc Brake Conversion
Using these instructions you should be able to convert any AMC with front drum brakes to front disc brakes. Anyone who has done some brake work and has a reasonable amount of mechanical knowledge should be able to perform this swap. When you read in the instructions "bleed the brakes" if you don't know how to perform that function stop now and get help from someone with more mechanical experience. These instruction assume that you know enough about brakes to properly bleed them, check for leaks and test their stopping power in the driveway before going out on the highway. If the previous statement in not true for you stop now and get assistance. David Crooks said it best in a response about this message:
"If you're not well versed in brake systems get help from someone who is. If you modify your brake system you could be liable if it fails and you hurt someone or destroy property. Brakes aren't difficult to work on, once you've been taught how, but they're one of the most important parts of a car, and deserve a lot more respect than they usually get."
Parts you will need from a donor car:
Rotors Including Bearings
Spindles Including All Bolts Nuts Etc.
Backing Plates
Caliper Mounting Brackets and Mounting Bolts
Proportioning Valve
All Front Metal Brake Lines
Power Booster Including Check Valve (For power brake systems)
Parts you should buy new or rebuilt (You can take these from the donor car but unless you know it to be a good system I would not):
Master Cylinder
Calipers (These are cheap for rebuilt ones approx $20.00 each)
Front Flexible Brake Lines (Kennedy American had these on special for under $20.00) Replace these -- do not reuse old ones.
Brake Pads
Find a donor car, almost any 71 and up AMC with front disc brakes will make a good donor car. I highly recommend that you not use the four piston caliper setup from 70 and down disc brake cars, (instruction are included to swap this older type to the newer type) they are far more expensive and harder to maintain and from what I am told they just plain don't work as well. There were 2 types of single piston caliper systems used in different years, not sure which was used when, but there were Kelsey Hayes and Bendix systems, some people prefer one system some the other, both work well and the instructions provided here will work for either brand. Be sure that you know what kind of car that you used for a donor so that you can order brake parts later. Remove all parts listed above from the donor car (take the calipers for cores if you are buying rebuilt) this should be fairly simple and self explanatory. Be sure to make a quick drawing of the proportioning valve and where each line runs from each connectionin the valve, when you go to install it you will be glad that you did. I have been told that there were 2 different proportioning valves used that had different pressure ratios front to rear, to assure that you have a correct one for your car be sure that the donor car is of similar weight, (Ambassadors and Rebels will use one while Javelins, Concords, Gremlins, etc. will use the one for lighter cars) If your donor car is of the wrong variety to use the proportioning valve of the donor car you may look for the correct valve from another car (don't know if they are available new or not) or you may purchase an adjustable valve. If you choose the adjustable valve you should have a brake expert adjust it for you I would not attempt this myself, misadjustment is asking for trouble. Also there is a short metal line going to a double female sleeve and then to the rear brakes, be sure to get this line and the sleeve, if you do your old rear brake lines will connect right up.
Have the rotors turned and checked to see if they are in specs. Remove the drums and all brake hardware from the car. Disconnect the metal line from the flexible line that runs to the wheel cylinder, this can be difficult flare wrenchs may help or in my case I had to resort to vise grips. Remove the nuts holding on the backing plate and spindle. Remove all the bolts , there is a gotcha here one of the lower bolts may hit a 90 degree turned spacer between the lower control arm and the strut rod, save yourself the hour I spent removing the strut rod and stripping a bolt and take a hacksaw and cut off the bolt, you can use one from the donor car. Since the bolts come in from the front on the disc system this will not be a problem in reassembly. Remove the master cylinder, power booster and all front metal brake lines as well as the brake pressure warning unit.

Install the spindles, backing plates and mounting brackets, if you watched how everything came off this should be easy. The calipers should mount toward the top front of the rotor. Mount the rotors onto the spindles and secure the bearings with the washer, nut and cotter pin and put on dust cap. Connect the new flexible brake lines to the calipers, another gotcha here, if you car is equipped with the loveable trunnions, like mine, then the metal part of this brake line may want to run right through the middle of your trunnion. I overcame this problem with a tubing bender, if you have to do this connect the line to the caliper first then bend the brake line down and toward the middle of the car to run behind the trunnion. Bend it a little at a time and keep setting the caliper in place until the line is no longer touching the trunnion. Others have told me this was not a problem when they did the install, I have to assume that the brakes lines come out of the calipers at different angles depending on who anufactured the calipers you are using, if you don't have this problem it will save you about an hour on the install. Install caliper with brake pads. Connect the free end of the flexible line to the mounting plate inside the wheel well. Install the power booster from donor car being sure to connect the vacuum line, this is an excellent time to clean it up and paint it. Install the master cylinder and be sure to bench bleed it first. Run all the metal brake lines that you removed to their appropriate places and connect securely, do not mount the proportioning valve yet, let it hang by the brake lines until you have finished. The brake lines may require some custom bending to mount to the firewall but this isn't too difficult, just be careful not to crimp or break them. Once you have the brake lines securely fastened then secure the proportioning valve inside the engine compartment.

Lastly, bleed the brakes on all four wheels, check for leaks, and test for stopping power.

Please also note that for true balance on the brakes the braking system with disc brakes were designed to be used with 10" rear brakes which I believe came on all disc brake equipped cars and all V8 equipped cars, the I6s, with front drums, came with 9" rears. There are some people running disc front with the 9" rear brakes and working fine, but to accomplish optimum balance between front and rear if the rear brakes are not already 10" drums these should be swapped as well (this swap is not covered here).

Many people also mentioned the swap of early disc (70 and back four piston caliper non vented rotors) to late disc (71 and up single piston calipers with vented rotors). This is accomplished the same as above except the master cylinder and power booster are identical to the later model cars and the proportioning valve and metal brakes lines should work just fine. All parts at the wheel including spindles, backing plates, mounting brackets, calipers, rotors, and flexible brakes lines should all be replaced as above.

i got this from www.amxfiles.com