Has the car been standing long? Dragging brakes suggest to me that something is gunged up - the mc is unlikely to act as a non-return valve to that extent. So, I' m not sure I'd be looking at the MC yet - I would persuade the front pad(s) out, clean, check for corrosion, possibly chock/block/reinsert all bar one piston and carefully check that it moves (making sure you don't stomp on the pedal and pop the whole thing out), put them all back in... ...move to the back, remove the yokes, clean/check etc., try a little handbrake to check for piston movement (I'm assuming standard single-piston Sierra calipers here), use a wind-back tool to retract the piston... And then mebbe flush through some new fluid. Then, once you are fairly sure that all the moving bits are in good condition - then I might start to wonder if the mc might be at fault. Its been a long time since I documented brake fettling - but these pages on my site might be of some use (especially bearing in mind that I documented this work as a http://alcester-racing-sevens.com/brake_maintenance.htm