I suspect the seal has gone but am in no hurry to try to dismantle having read the Houdaille manual on the subject. Amyone had a go and is it possible??
you should see signs of a leak if there is one.The type fitted to the aprilia is very long lived and reliable and in practical terms cannot be (and doesn't need to be) serviced outside of topping up oil.I had mine tested on a test bench because i assumed they would be worn but they perfomed very well.
In addition the mot here involves a very demanding suspension test which vibrates the wheels at varying frequencies with an electronic graph of the damping performance and balance of the two sides. The houdailles comfortable exceed the requirements whereas several nieuwe models actually don't reach them eg the nieuwe fiat 500 which is causing legal problems and warranty disputes with dealers in belgium.
I understand there is only one reliable company that can take them apart (in the US). attempting this yourself or in a local willing workshop will simply end up as destructive testing .