Neub's answer sounds right and he should know based on similar experiences.
I would also/first check to see if the 5/8" cap on top of your Manifold "A" is not leaking?
And I would check the fluid level in in your reservoir to verify it's not low, which would indicate a leak elsewhere in your system.
The HWH rule of thumb, when it comes to "creep," suggests the solenoid valve is not holding the pressure.
Below is a diagram I added to the explanation on "creep" from HWH.
OP: Which slide is popping out 1" after it's closed?
...And I just pulled this video out of my HWH Library about the check valve being this owner's problem and he says in the video his bedroom slide would extend 1-1/2" after he stored it. So apparently it's not always the extend solenoid... if your motor is "screaming" or winding down after you release the button, which should like another symptom worth asking about.
I.e., maybe a screaming motor is indicative of a check valve problem... which is located under the 5/8" caps.
Check this out: