Method and system for inhibiting cement deposition in a jack and pull (JAP) expansion assembly

A hydraulically actuated Jack And Pull (JAP) tubular expansion tool string comprises: - an annular non-return valve arranged in the wall of the JAP string (7), which allows mud to flow from a surrounding annulus into the bore of the expansion tool string when the pressure in the bore of the tool string is lower than that in the annulus; - a second non-return valve (8) arranged in the JAP string below the annular valve(7) which allows fluid to move down only; - wherein during the cycling of the jack the annular non-return valve (7) is continuously open and the second non-return valve(8) cycles with the jack movement to inhibit curing of cement in the expansion tool string.