I recently picked up a set of wheels from a friend who has no need of them any longer. They are Mavic 891s on XT hubs and are in excellent condition.

They’ve not been used for a year and when pumped up the front goes flat in a hour and the back goes soft (too soft to ride but not 100% flat) in about 4 hours.

Do I add more sealant or do I remove the tyres clean out all the old gunk and start the fitting/sealant process from scratch?

Tyres are Hutchinson Piranhas if that’s relevant.

Any advice from tubeless owners would be appreciated.