I finally figured out why the fluid would hiss out of the reservoir when I applied 15 psi to it with a hand-pump brake bleeder. The fluid wasn't coming from the threaded adapter... it was leaking out of the both of the black buttons that you press to test the circuitry of the brake fluid level indicator. I can't believe I didn't see that before. It was bubbling/hissing out from under the pushbutton which means that the seal below that is gone. It wasn't easy to see either.