brakes just quit - no brake pedal pressure

Look again for external leaks. If none found again, Pull off the rear drums and inspect the rear brakes. Are the wheel cylinder pistons engaging the shoes? Has any linkage broken? Are the wheel cylinders leaking? If everything looks OK, then adjust the rear brakes and put them back together. Now look at the top of the brake pedal. Is the pushrod that goes into the booster attached to the top of the pedal? Look in the master cylinder reservoir. Is it empty? If it is, you've missed a leak somewhere. If it isn't, the master cylinder is most likely your problem.