What are the possible causes for two break caliper pistons not releasing??

suggested that to the mechanic - he didn't seem to think that would help - he may change his mind if he exhausts any other options

provides short term pressure relief on the piston - once brakes are applied again, the piston extends but does not pull back when the brake pedal is released

That tells me there is residual pressure at the calipers as opposed to just a sticking piston... I agree with answer # 1.. Also 'possible' master cylinder issue... For SOME reason reason the fluid is not returning to the master cylinder!!

Turns out the problem was the master cylinder. When the mechanic initially drove the car, he never drove it long enough for the pressure issue to develop. After he drove it for a 5-6 mile route and then opened the bleed valves, he heard the pressure release. Didn't have to replace the flex brake lines. His description of the low MC fluid essentially matches yours. Even though the fluid level in the MC had been checked previously, it was never so low as to indicate a problem. Thanks.

Glad to hear you're back on the road!!!