It's hydraulic. You may have a leak, or the system may need bleeding. As varese says, first thing to check is the fluid level. Check along the bottom of the rack for signs of fluid, below where the two banjo bolts are fitted to the rack body; also where the gaiters join the rack, but the gaiters themselves can hold quite a bit of fluid, so if an end seal has failed, you won't always get a tell-tale sign by way of a visible leak. Grab hold of the outer end of each gaiter (where the tie rod emerges) and give them a good push up and down. You shouldn't be able to feel any free play. One end shim was shot on mine, so I sent the rack off for a rebuild earlier this year.