More likely you have bad seals in the master cylinder, fluid is being pushed past the piston in the clyinder. ABS will not likely cause no peddle. Bad Booster will cause hard peddle. Some might say not to do the following but if you need to isolate the master cylinder to see if it is an internal leak. Crimp off the calipers at the rubber brake lines with hose crimps or vise grips with rubber tubes on them to protect the hose (if the hoses are old it may damage them so use discretion) don't have to crimp the hell out of it. Then if the peddle still sinks to the floor with light pressure assuming the system has fluid in it you have bad seals in the master. Abs has valves that move when the wheels slip basically the same as pumping the brakes, it allows fluid to pass freely if it's not actuated this is why you have base brakes with a bad ABS module. If the peddle is hard with the crimps in place you have a leak elsewhere your not finding. Or just rebuild the master for good measure and cross your fingers;) peddle sink with no leaks more times then not the master.