I exerienced identical problem. Solution was pressure bleeding the system to evacuate air pockets. One caveat, when M/C was pressurized to 10 psi, (pressure relief?) valves at rear of expansion tank bled profusely. perhaps 5 psi is enough to accomplish the job. Please note, this solution was only employed after needlessly replacing the master cylinder due to misdiagnosis. IF UNABLE TO DRAW FLUID FROM ANY CALIPER, TRY PRESSURE BLEEDING PRIOR TO REPLACING EXPENSIVE HYDRAULIC COMPONENTS.