After World War II, the elders in his Brethren assembly agreed to commend him to work full-time in Central Asia.
In March 1947, Bull and George N. Patterson (1920-2012) went to China, travelling deep into the interior up to the border area shared with Tibet.
Bull entered Tibet on 29 July 1950, where he witnessed the last days of Tibetan independence and was soon imprisoned by the invading Red Army on the pretext of being a spy.
[2] At first, he was kept in solitary confinement, but later underwent a re-education and thought reform programme—his captors tried brainwashing, but he claimed that his "faith in Christ kept him from mental breakdown".
The Bulls spent a year in Australia,[4] and subsequently served for fourteen months in North Borneo, now Sabah, from June 1959 to August 1960.