Typically the booster is the cause of a slow return. Many times it is caused by the rear seal of the master cylinder leaking brake fluid into the booster and swelling a certain part. In some cases the brakes will not release until you hook your toe under the pedal and force it to return to the "off" position.
1965 "K" GT fastback Honey Gold exterior, Ivy Green and White Pony interior, many options
1966 Conv., high option, removeable hardtop, thermactor "C" engine, AC, Springtime Yellow exterior, Black Pony interior
1968 California Special, "J" code, ,many options, white with red interior