Not sure exactly the name of the "control valve" used in ATE/Lucas brake booster but briefly the concept is as explained in the "how stuff works" excerpt:

"On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates a partial vacuum inside the vacuum booster on both sides of the diaphragm. When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder. As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position."

So, what appears to have happened in your case is the spring that operates the vacuum "control valve" that is located inside the booster chamber has failed which not only reduces the effort required to apply the brakes (probably fully opened valve when you apply the brakes) but also allows the brakes to stay applied because the control valve is not returned to the closed position as the brake pedal is released. Mark