Install a second return spring. With the engine running depress the brake petal and release. If petal returns fully try a few more times. If that fixes the problem that's good. If not and the petal dose'nt return without touching the petal shut the engine down and have someone pull the check valve from the booster. If the petal returns to the proper position you've got a bad booster, if not you've got a possible restriction in the brake system. It might be the fluid is not returning compleatly to the master cvlinder. Something to try.