The possibility also exists that your brake fluid has been contaminated by a petroleum-based fluid such as power steering fluid or ATF. The only way this could happen is if the fluid was deliberately poured in to the brake Master Cylinder (it is not possible to cross-contaminate any other way). If, and only if, that has occurred, denatured alcohol run through the brake hydraulic system is the only way to remove the contaminant. You will also need to replace anything rubber ( petroleum eats rubber), which means:
Master Cylinder
Calipers
Wheel Cylinders (if rear drum brakes)
All Flexible Brake Hoses
Mechanically, brakes are very simple: rotor (or drum) spins with the wheel, pads (or shoes) are applied to slow the rotation by use of friction. How? By hydraulic force. How then do the brakes "unapply"?
This is where it gets complicated.
In drum brakes, it is by the use of springs that return the shoes to their original placement once the hydraulic pressure is not given by the wheel cylinders.
In disc brakes, it is by the tendency for rubber to return to its original shape.
"What?"
I will try to be brief. In a brake caliper, there is a Square Cut Seal which allows the caliper piston (the thing that makes the pads contact the rotor) to extend under hydraulic pressure. This Seal does not want to be distorted, and wants to go back to being square, which pulls the piston away from the contact surface (the "brakes") once the hydraulic pressure is no longer applied.
If the rubber is contaminated, it does not snap back.
Result? Locked brakes.
Without having looked at your vehicle, this is the best I can come up with.
Have your vehicle inspected by a qualified professional.
