Advice for Suntour XCR Forks

I have a problem with my front forks. I have a fourth hand 2006 Carrera Banshee with Suntour XCR 100mm forks on it but the previous owner of the bike had little knowledge on suspension so he never serviced or cleaned them. The problem is that when I preload the forks, it will physically stop at 50mm no matter what i do to it. Also most of the time the rebound takes about 2 seconds to go back to 100mm :x . I think the seals have been damaged or clogged up as a bit of the fork has been scratched up by some sand or muck. If anyone know how to solve this problem, I would greatly appreciate it.
Thanks
Thanks

My god road cycling is scary! I'm going to keep my relaxing rides to the trails where everything is green, fast and less crazy.
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PS, do they have hydraulic damping?
Remove lowers by loosening the 2 allen bolts on the base, tap with a hammer (gently) to free them, then undo and pull apart, 2 minute job.
Simon
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Having ridden both the XCR's are a much more confidence inspiring (comparing hyrdaulic damped versions, both 100mm).
Simon