90 fuel tank breather problem, got me baffled

For some reason i cannot fill the (petrol) fuel tank on my 90 over half way, when the level gets to the top of the inlet pipe which is just over half way up the tank it stops and wont take in any more fuel. At first i thought it must be the breather pipe blocked, so i disconnected it and poked a rod through it, all clear. Then the next time i disconnected it, it had fuel in the breather pipe, trapped in the sag so i cut 100mm of the pipe to keep it straight with no sag but i still cannot fill the tank over half way. WHY???
Anyone got any suggestions?

mine did this but it started leaking soon after and had to be replaced. no help at all i'm afraid! have you tried filling it slower and giving the fuel chance to flow in? there is not rally much to go wrong with a fuel tank!! maybe your sensor is dodgey and its actually full, but reads half. easy to remove and test. just remove drivers seat and there it is! have you had the sensor out recently?