1999 Honda Odyssey Rear brake cylinders

A chain-store brake repair facility mechanic told me that the rubber covers at each end of the rear brake cylinders were to keep dust out, and not to prevent leaking. He then preceded to pull back the rubber covers, at which point brake fluid poured out. If the rubber covers were not leak-proof, why was the fluid not evident before he pulled the cover back? Also, was there supposed to be no fluid loss when the rubber covers were pulled back? Were the cylinders definitely leaking?

Thanks much!