I have bought a one way valve system which goes onto the bleed nipple, i found the problem. For some reason the bleed nipples on the rear two callipers when you unscrew them half a turn, the fluid comes out of the nipple, but its also leaking out of the base of the nipple onto the calliper, so thats where the air is getting in, even though i have a tube on the nipple. Think i will have it sorted now with this one way valve, i just need to get my mate to put positive pressure on the rear nipples and i can tighten em so no air can suck back in. Bit of a pain but should be good