Slide out sliding out in tow

[Eng208]

I have a 1995 Four Winds fifth wheel with a super slide on it. For the last few trips, the slide has slid out about 5" while traveling. I had to pull over and slide it back in, only to notice it would again be out just a bit down the road. I can't figure out why the hydraulics are leaking and not holding the slide in. I can let it in all the way and as I release the button, I can hear the pressure relieving and see the slide easing out just a little.
Any suggestions? The manual valve is closed tight.

[mbrower]

On my last two fifth wheels, I had lock bars that I had to install between the slide flange and the wall to prevent the slide from creeping out. RV manufacturer supplied lock rods

[RoyB]

My 2004 model FLAGSTAFF 5th wheel has a couple of square boards that fits between the slideout and the wall when they are pulled-in for travel... These were in the top shelf of the bed room closet... They wall above the slide has a marker where to place these boards...

Roy Ken

[B_O__Plenty]

No idea why but for now you could hold it in place with a board between the trim on the slide and the inside wall next to it.

Bo.O.