I swaped my 68 Dart over the the 4 piston KH caliper disc brakes, it was a used kit I bought at the Nationals.
Everything went together great except I cant seem to get the brakes to bleed. If I pump them alot I can get the peddle to come up but it spongey and as soon as I let up on the peddle for a couple seconds it will go to the floor. I was trying to use my master cylinder that was on the car for drum brakes so I thought that was the problem. I put the master cylinder on that came with the brakes, it was a bendix master but larger that the one on my car but my old car repair manual says it for drum brakes also. do I need a specific master cylinder for these brake? Is there a trick to bleeding these calipers? 4 piston and on one bleeder seems like it would be tough to get all the air out.

take off the M/C lines and stick a drill bit deep into the port next to the firewall (the front disc half) and if you do not hit/feel a spring loaded RPV then it is fine for discs (the bore size might not be perfect but that's no worry at this moment). "bench" bleed the M/C on the car then hookup the line for the front discs and start bleeding them RF then LF.

Did you rebuild the calipers? Did you replace the brake hoses? If not, take it back apart and do those things.

Make sure you installed the calipers with bleeder screws facing up.

Just an easy way to remember that the bleeder screws always are on top. Think that air will go to the highest point. That is where you want your bleeder screw so the air will easily escape.

I got the calipers switched last night and checked the master cylinder, both have the RPV in them, need to order a new master cylinder, will get on for a 71 Dart with Disc brakes, should be good to go at that point, thanks for the help guys.

Got the new master cylinder for $30.00, now I have a master for my kids 66 coronet. Thanks for the info though.