My driver's seat keeps sinking down

When i drive my truck and then turn the truck off, within 2 hours the driver's seat sinks from the height i keep the seat at when driving to all the way down to the floor, as low as it can go. Doesn't that mean there is a defect?

 

IIRC, on all the Freightliner Cascadias i drove before this one, the driver's seat would hold its position when i turned the truck off. I could go on hometime for a week, and when i came back to my truck, the driver's seat would still be at the same position as when i was driving.

 

Update: the mechanic replaced the seat valve. The seat does not seem to sink now.

 

Next update: the driver's seat started sinking again on the drive from the repair shop to the truck stop. I have given up on the mechanics fixing this for now.

I am going to try to fix this myself. At my next hometime I will put over a hundred pounds of barbell playes on the driver's seat, and i will see if i can find the leak myself.

 

What type of soap do you use for this?

Will simple hand soap mixed with water work?