Cannot remember the size but you do need the green nitrile ones, if you have an original one try a hydraulics company, when I had a twinmaster top I got some extra seals from a local hydraulic company for a few pence each, thankfully they had some in stock, if they didn't I would have had to order them, the minimum order is/was £5, that would have got me a few hundred seals (note this was 6 years ago)