Brake master cylinder seals leaking

[Disco]

Hello all 

I am trying to bleed the brake system and find I can get a hard pedal which when released air rushes back into the reservoir. The seals between the reservoir and cylinder are leaking so this where I think the air leak is . I have fitted a seal shaped like cylinder and pushed the plastic reservoir pipes into the holes but is doesn’t seem very sealed . Do I need a different seal which has a thick top to it to seal between the cylinder and the bottom of the reservoir?
or do I fit a remote reservoir as supplied by kitspares ( the seals don’t seem to be part of the kit 

thanks in anticipation 

Chris D