Right, I stripped the forks down this morning (lowers and air spring, left the damper alone as it seemed to be working fine). To be honest I couldn’t see anything wrong, but gave them a good clean, put them back together and a quick spin down the track suggests that they feel a bit better. Wont know for sure until I get a chance for a proper outing later in the week though.
By the way, I discovered a way of checking whether the little groove that balances the positive and negative chambers is blocked. I’m sure I’m not the first person to think of this, but I’ll pass it on in case it’s of interest.
1. Pump the fork up to pressure (e.g. 70psi)
2. Check that the fork is fully extended. If the slot is clear then both chambers are now at 70psi
3. Sit or lean on the bike enough to compress the forks a couple of cm. The plunger is now clear of the slot and the air is trapped in the negative chamber.
4. Press the valve to release all the pressure in the positive chamber. The forks will compress and resist extending as there is now a higher pressure in the negative chamber.
5. Pull on the wheel/bridge and stem to fully extend the fork. If the slot is clear you’ll hear a short hiss as the fork reaches full extension and the air rushes out of the negative side through the slot.
If you don’t hear the hiss then either the slot is blocked or one of the seals has failed. Either way you are going to need to take it apart. Mine hissed just fine by the way, so I don’t think the problem was a blocked slot or faulty seals. In fact I’m no closer to finding out what it was really.