hey pigdog,
i had the same problem with the booster i brought off you!
the pedal was realy sort and the brakes would lockup without any force,
coming off the pedal and the bosster would get stuck and make a loud click.
gave me the ####s until i got another 1 from the wreckers, which doesn't work either now

and yes, the reaction disc is at the end of the booster pushrod.

might be a faulty return spring
good luck sorting it out