a fluid bladder including a check valve inturned within the bladder . the bladder is formed of two thin sheets of plastic material sealed together around the periphery of the bladder . the check valve is formed of an extended mating pair of strips of the same two sheets of plastic overlying one another and heat bonded together by a high frequency electric field along two substantially parallel lines to form a central flow channel with a closed gap - free configuration . in forming the check valve , at least one pair of electrodes are employed , and insulating material is used to enclose the electrodes so that the two sheets of plastic material are maintained parallel and in contact with one another during welding and the subsequent cool - down interval , thereby assuring reliable check valve operation .