The junction block can cause a loss of fluid or make the brake warning light come on but not will not cause any particular brake to "stick". It may be bad and causing the loss of pedal--thus requiring some pumping to get a reaction. Look at the junction block for signs of leakage.
The sticking brake can be caused by a caliper itself or one or more of the flex lines deteriorating internally forming a "one way valve". Check your calipers -- push each piston in after pumping the pedal to get them extended. If one (or more) is reluctant to retract--try again--pump the pedal and then loosen the bleeder screw--if the offending piston retracts easily your flex lines need replacing. It's possible that a sticking brake could cause the fluid to boil (loss of pedal) but I don't think it would just pump back up in that case.
See the other brake related post (why do they do it that way?) regarding loosening the brake lines. -- Dave