It really is essential that the brake rod be securely locked into the master cylinder. As Robert said, make sure that the master cylinder has a receiving groove in the hole where the pushrod goes. FWIW I've used master cylinders from both trucks/vans and from 5th.Ave's [ "M" bodies ] and most of them had power brakes and also had the necessary receiving groove. I use grease inside the master cylinder hole and a little on the rubber lockring. Put the lockring on the pushrod and shove it into the master cylinder. For an alternative removal method, remove the brake light switch from under the dash and pull up on the brake pedal. Watch your hands when it breaks loose as it will usually let go all at once. The factory lockring is best since it has a groove that allows the ring to compress and also lets air escape when the pushrod is installed, but an "O" ring or a short piece of vacuum hose can also be used.