Check the shock-absorber for existence of damages and leak of oil.
Check whether there are no traces of damages or wear on the rubber plug.
Put forward and drown a shock-absorber rod not less than three times. Make sure that force of resistance does not change and there is no unusual noise.
If it not so, replace the shock-absorber.
Drown a rod of the shock-absorber and release it.
Make sure that the rod moves forward completely with a normal speed.