In the master cylinder there are two small holes (front and rear halves of the master cylinder). They are there to relieve the pressure and let the fluid escape back into the master cylinder when the brake pedal is released.
Have a helper depress the brake pedal, then remove the caps and watch for the fluid to swirl up into the master cylinder when the pedal is released. You can clean the gunk out of the holes with a fine wire, but be careful not to get any wire stuck in the master cylinder.
If this is not the problem then suspect either the caliper pistons stuck in the bores, or the hoses have swolen on the inside. If the calipers are holding the pads against the disks then something is keeping the pressure on the pads. Either the pistons are sticking (or cocked in the bores), or the pressure is not being released in the lines, or at the master cylinder.
If your rear brakes are not dragging it is unlikely that the fluid return holes are blocked because the master cylinder is split diagonally. That is each half of the master cylinder opperates one front and one rear brake.
Note: the duel safety master cylinder has one cylinder but there are two pistons, one in front and one behind it, that effectively form two seperate chambers. Each chamber applies pressure to two lines, one front, and the opposite rear.
Note 2: Check the dust covers that are on the pistons. If they are split or torn then moisture will cause rust to form on the pistons and they will stick. This is unusual but not rare. I just found this on a 2002 Buick, that is one of the boots had gotten torn and rust caused the piston to stick. The inside pad was worn down to the metal at only 15K miles on the car. Stuff happens
