Why do you think you have a bad shuttle valve? You do NOT, but I want to know why you think it is bad. The reason the jacks are not fully retracting is simple. BOTH jack inner upper seals are bad and are not allowing the jacks to fully retract. You should NEVER see any part of the silver inner piston when they are fully retracted. 625 system will not cause the jacks to extend in transit from road heat. The springs are OK if they retract until you get to the 3 to 4 inch mark. The repair is to either rebuild both rear jacks or replace. HWH will rebuild yours, but you must send them in for rebuild which is about 1/2 the cost of new cylinders. Since the jacks are not fully retracted, road heat and engine heat can expand the fluid in the lines and cause the jack cylinders to slightly extend and THAT will cause enough movement to trigger the down switch in each jack. 625 system down switches have MORE play in them than 300/325 systems. You have just been lucky the past year. Doug