Was driving to work tonight and when I approach traffic lights, I pressed the brakes and... nothing. Foot straight into the board

Tried pumping the brakes to no avail - they would not catch.
Managed to crawl into work and took a look underneath. P/S rear has lost the pads and fluid is pissing out of the piston from where I tried pumping them to get them to bind.
Just wondering what could be the cause of this? I'm guessing that the pad carrier is shot. This is the second time this happened recently - first time the mechanic replaced the pads, slides and the metal tag and said all was good. I'm not doubting that he installed the pads correctly.
The pad carrier is original - is it possible that it has corroded/worn to the extend that it would throw the pads?