Injection of steam into the ascension pipe of each coking chamber in a coking plant is continued for a period of time after charging of that chamber so as to create a slightly sub-atmospheric pressure in the chamber during the initial stage of the coking process, in order to reduce leakage of gas and fumes to the atmosphere past the doors and filling openings. A time clock operated by the coke pusher machine or by the coal charging machine stops this injection of steam after a desired period of time.