Not being familiar with your slide system, but knowing a bit about hydraulics, the hyd. pressure of the system has to be powerful enough to overcome the weight and friction of the slide room to operate it. The hyd. fluid that is piped to the actuator/cylinder normally would be controlled via a solenoid valve of some type. This solenoid valve also functions as a device to change the flow of hyd. fluid so that the actuator/cylinder has the ability to change direction.....thus pushing the slide room out or pulling the slide room in. Inside these directional solenoid valves are "spools" that slide back and forth, thus changing the flow direction of the hyd. fluid which of course changes the direction of the actuator movement; bringing the room in our out. These directional valves in a coach or trailer would be controlled electrically by 12VDC operating coils on the directional valve itself. Many times in an Industrial hyd. system there are proximity switches or limit switches of some type that detect when a device has reached its end of travel, and it then opens a set of contacts and de-energizes the operating coil and the actuator stops travelling.....think slide in or slide out position. Now a couple of possibilities......
1. The switch that should control the travel distance of the room has failed and is not stopping it at the proper place.
2. The spool in the solenoid is sticking and not returning to full cutoff position, thus allowing some hyd. pressure to try and continue to bring the slide in further.
There are most likely other possibilities, but since I have zero knowledge of a coach hyd. slide system, I probably don't need to go any further than my very basic hyd. system explanation. Some systems "dump" back to tank when travel distance has been satisfied, but those normally will have a hyd. check valve in line to prevent creep when system pressure is relieved via the dump sequence.
So hopefully, someone with a fair amount of experience troubleshooting coach hydraulics will chime in.