on my 72 240,when I push the brake pedal, the rpm's drop to about 450 and car is hard to stop(as in manual braking)and also have to keep foot on gas to keep from stalling. My assumption is that brake booster diaphragm is bad. Any help is greatly appreciated

Could also be a vacuum leak in the hoses leading to the master vac. If you end up needing a master vac, there are rebuilt ones available.

There are only two things that could be wrong in this instance, first as you say it may have a split in the diaphram. Or lastly the atmospheric valve that the push rod operates in the booster is not sealing the area in the front of the booster from the rear of the diaghram before it opens to allow the air pressure in to the rear, letting air go straight through the valve to the booster hose and inlet manifold. Big air leak

Steven