Not to burst anybody's bubble (heh ), but I have tied back the front brake lever on a failed bleed that I could not get any pressure whatsoever and the lever feel (pressure) came back overnight. At the time I thought the rationale was that the master cylinder, in the "activated" position, allowed air to rise into the reservoir, Do doubt, this has only worked for me with the master cylinder fluid supply line angled above the cylinder with the reservoir higher.