Brakes not working?

In my experience (2 times), when an MC fails it acts erratic. Sometimes you will get a firm pedal and sometimes it sinks. The tiniest nick on the rubber piston seal can make it leak. In one case, the MC was fairly new. When I pulled out the piston I found a clean cut on the rubber seal, apparently because the rebuilders had bored the cylinder but didn't bevel the corner where the ports come in, leaving a sharp edge that cut the seal.