The only thing you can do with it is to lubricate the larger of the 2 'o' rings inside with a little silicone grease or petroleum jelly.
I haven't been able to source a replacement 'o' ring. Steve has had the forethought to make the 'o' ring a none standard size.
. I'm sure it can be made to order in quantity.
I have only taken mine apart twice, once to lubricate the 'o' ring and the other time was to reseat the tiny 'o' ring on the end of the plunger. TBH, if the 'o' ring has worn to such an extent that its leaking then my guess is that its better to replace the whole unit.