It seems very unlikely, but then so is winning the lottery and someone does nearly every weekend.Hmm, this being the third master cylinder - the original one, then a bearmach one, and then an OEM one - they can't all be duff surely?
If you check your brake fluid level carefully, noting it's height. Then spend a while repeatedly pushing the pedal to the floor, if the level doesn't drop then the fluid isn't leaking out anywhere. So, it's probably leaking past the seal.