A double-acting cylinder is operated by a 4/2 pilot operated spring return control valve. When the pilot is energized, the cylinder extends. When the pilot is removed, the cylinder does not retract. A possible cause is __________.

The scenario describes a double-acting cylinder controlled by a 4/2 pilot-operated spring return control valve. The key function of this valve is to control the direction of air flow to the cylinder, allowing it to extend or retract based on the state of the pilot signal.

When the pilot is energized, the valve shifts to allow air to flow into the cylinder, causing extension. If the pilot signal is removed and the cylinder does not retract, it indicates that the control valve is not returning to its default position. A likely reason for this failure is a broken spring within the control valve. The spring in the valve serves to return the spool to its original position when the pilot pressure is removed. If the spring is broken, it cannot exert the necessary force to move the valve back, thus preventing the air from exhausting from the cylinder and stopping it from retracting.

Other options such as a leaky pilot line and loss of pilot air pressure suggest issues with pilot operation but would not specifically cause the spool not to return if the spring is functioning. A removed muffler might affect exhaust flow but typically would not prevent the valve from returning to its neutral position.

Consequently, the most direct cause of the cylinder failing to retract, in this case, is