Well I've got to the bottom of the problem why brake fluid wouldn't flow freely through the bleed nipple, after stripping the brakes down the top slave cylinder was seized and completely solid with crud not allowing any fluid to pass to the secondary cylinder (with the nipple).so I'm now working my way round the complete system. Thanks to Paul hunt at powertrack brakes for supplying all the seal kits so I could retain as much of the original equipment as was safe to do so.