I can understand your consternation, based on what just happened, but 1. You have already paid for a reliable braking system once, and, 2. You may not have considered that a well-meaning mechanic MAY have installed a wheel cylinder that was (unbeknownst to him,) incorrectly assembled, and may have developed a leaking seal, totally exclusive of what he did.
I think I'd insist that they fix it, since you already paid for it.