A mate of mine (pocoyo) had the same problem this weekend with his, it was an o'ring problem but not sure which one. reading what Balf has written it sounds like he's spot on with his solution, do what Balf says and i'm sure it'll workDo what I told you, if LiPS just clean and re-lube you will feel a right twonk, it's part of routine maintenance better to learn sooner than later as you'll have to do it at least once a month
Failing that, pm Pocoyo on here.