im cam, a licenced diagnostic tech.
it sounds like you have brake hose (flex hose) problems.
you will have to determine if the master is holding pressure or the flex hose.
when the brakes hold up again, loosen the brake fitting free just before the brake hose. if the caliper is still stuck untill you loosen the bleeder, than its the brake hoses that will need to be replaced.
if no brake fluid pressure comes out and the caliper is still held up, than you will have to go further up the line.
crack the lines at the master, if pressure releases there, than its the master cylinder that is holding the system up.
if it has abs, you will have to also loosen fittings there untill you see where the pressure is being help up.
im probably leaning towards the flex hoses as this is pretty common issue