Originally Posted by Twabak
It extends all the way out. Yes I think is hydraulic. But way would it do it when there is no power supplied?
It seems that you don't appreciate how a hydraulic system works. Here's a simplified description.
There's no direct drive to bring the slide in and out. That is, it's not like an electric garage door opener with a chain or a leadscrew (giant screw that rotates in a nut on the door to bring it up and down). These could get muddy and rusty under a trailer and fail to work.
Instead, the motor in a hydraulic system does nothing but pump fluid. The hydraulic fluid (a light oil, of the consistency of cooking oil) is pumped into a cylinder like a giant bicycle pump. This causes the piston to extend (like the handle of a bicycle pump going up) which pushes the slide out.
To bring the slide in, they could suck the fluid back out of the bicycle pump (it's really called a "hydraulic cylinder"), but instead, they just inject the fluid into the other end of the cylinder. Attached is a picture of a typical cylinder, although it probably has less travel than yours. You can see the two ports on the side. Fluid injected on the right port extends the piston (and fluid is allowed to exit the left port). Fluid injected in the left port (and allowed to exit the right port) contracts the piston. A hydraulic valve selects which end is filled by the pump and which empties back into the tank.
Now, back to your question. How could the slide come out by itself? First, suppose that the trailer isn't perfectly level. The slide is on the low side, so if there were no cylinders, it would slide out by itself like a dresser drawer if the dresser isn't level.
To prevent the slide from doing this, the hydraulic valve (see picture of manual valve--yours is probably electric) has THREE positions: Fill, Empty, and Neither. In the Neither position, fluid is not allowed to flow to/from either end of the cylinder.
What the other posters said (in a lot fewer words than this) is that in the Neither position, your valve is leaking a little, like a water faucet that won't completely shut off. There are really two possibilities.
- The valve could be letting fluid bypass, back into the tank/motor, or
- There could be a leak from one of the fluid plumbing connections. In this case there would be drips on the ground.
No power has to be supplied for either of these conditions to occur.
Hope that makes it clear.