1985 Volkswagen Transporter fires up but wont idle

Hi my vw t25 1.6 diesel has a bit of an issue it starts ( fires up) but after about 1 second it cuts out I have noticed air in the fuel line ive changed the diesel filter checked the clear fuel tube from the filter to the pump we checked the injectors and there washers and one of washers was rubbish I replaced that one to now I'm abit stuck I'm going to replace the injector to see if that's the problem cause I cant tell if that's defective either and I have no means of testing it