A sliding valve assembly (30, 130) for introducing and/or exhausting a fluid medium relative to a cylinder containing a reciprocating piston for an energy conversion engine which includes pressure within the system, said sliding valve assembly (30, 130) comprising: a valve housing (45, 47) having first fluid conducting passage and second fluid conducting passage separated by a valve cavity; a valve body housed in the valve cavity of the valve housing (45, 47) and having a fluid conducting port adjacent to a fluid impervious surface; an actuator for translating movements of the valve body within the valve cavity of the valve housing (45, 47) into a fluid conducting position wherein the fluid conducting port of the valve body allows a fluid conducting relation between the first fluid conducting passage and the second fluid conducting passage of the valve housing (45, 47) and into a fluid blocking position where the fluid impervious surface of the valve body obstructs a fluid conducting relation between the first fluid conducting passage and the second fluid conducting passage of the valve housing (45, 47); wherein the valve housing (45, 47) of the valve body have cooperating surfaces for constraining the valve body during the translating movements of the valve body within the valve cavity of the valve housing (45, 47) between the fluid conducting position and the fluid blocking position of the valve body; and at least a first sealing assembly located within the fluid impervious surface of the valve body for forming a first sealing relationship between the valve body and the valve housing (45, 47) characterized in that the cooperating surfaces (63a, 63b, 65a, 65b) includes cams (112, 114) and cam followers (67, 69, 71, 73, 101, 103) responsive to the actuator for releasing the sealing pressure of at least the first sealing assembly from the valve housing (45, 47) when the sliding valve assembly (30, 130) is in its blocking position. a valve housing (45, 47) having first fluid conducting passage and second fluid conducting passage separated by a valve cavity; a valve body housed in the valve cavity of the valve housing (45, 47) and having a fluid conducting port adjacent to a fluid impervious surface; an actuator for translating movements of the valve body within the valve cavity of the valve housing (45, 47) into a fluid conducting position wherein the fluid conducting port of the valve body allows a fluid conducting relation between the first fluid conducting passage and the second fluid conducting passage of the valve housing (45, 47) and into a fluid blocking position where the fluid impervious surface of the valve body obstructs a fluid conducting relation between the first fluid conducting passage and the second fluid conducting passage of the valve housing (45, 47); wherein the valve housing (45, 47) of the valve body have cooperating surfaces for constraining the valve body during the translating movements of the valve body within the valve cavity of the valve housing (45, 47) between the fluid conducting position and the fluid blocking position of the valve body; and at least a first sealing assembly located within the fluid impervious surface of the valve body for forming a first sealing relationship between the valve body and the valve housing (45, 47) characterized in that the cooperating surfaces (63a, 63b, 65a, 65b) includes cams (112, 114) and cam followers (67, 69, 71, 73, 101, 103) responsive to the actuator for releasing the sealing pressure of at least the first sealing assembly from the valve housing (45, 47) when the sliding valve assembly (30, 130) is in its blocking position.