how can you use a disk drum m/c with 4 wheel disks?
You don't. In addition to the different balance, drum master cylinders retain a small amount of pressure in the lines so that your brakes function in a short amount of time. Drums brakes are removed by springs, disks just by pressure. There is a difference in the master cylinder.
It sounds like your brakes aren't working at all though. Even just the stock disk/drums do a good job of stopping these cars. Maybe not by new car standards, but certainly by old car standards. I'd make sure that the brake system was bled very well before going any further, and be sure to do it in the correct order so the proportioning valve is set up right. The disks in the rear should stop the car better than the old drums did, so with a stock proportioning valve you should be locking the rears first. The adjustable prop valve should be needed to dial the rears off a little.
First thing I would rule out is a leak or air in the lines. Bleed them with a vacuum bleeder too, sometimes gravity bleeding or the "pump the pedal" method doesn't get all the air out. While you're bleeding the brakes, take a look at the brake fluid coming out. It should be relatively clear, if its got a lot of dirt or rust in it, or is really cloudy looking you should flush it through with new fluid until its clean at all the bleeders.Take a look at the pads too, if they're contaminated (oil, brake fluid etc) they won't stop you either. Short of a leak or air in the lines, I would look at the master cylinder next.
