An automatic bleed valve is disclosed, suitable for attachment to the inlet manifold of an internal combustion engine in order to provide an effective embodiment of the 'air bleed' principle. Small amounts of inlet air are allowed into the inlet manifold at periods of high vacuum (low pressure) present during deceleration. The valve closure (38) is biassed by a biassing means (36), e.g. a compressing spring, which acts on the face of the closure disposed away from the seat (22). Other aspects provide for a valve seat which is adjustable in position relative to the closure, a closure and seat of different plastics materials, and a closure with a conical aspect, preferably paired with a seat with a complementary contact portion.