hel with expansion bottle cap?

[dridgy]

i have a 1.6 d 1981 model
my expansion tank has the old type rad cap, the metal one.
now when the cap is on i disconnected the pipe to the top up tank then i very lightly press one of the coolant hoses and water comes out the pipe that goes to top up tank.
i dont think the rad cap is sealing at the bottom.
i have two rad caps i tried them both and they both let water through?
surely this isnt right, dosent the pressure need to be quite high to push past the rad cap?
if this is the case it would expalin why my old engine had exactly the same problem.
what should i do? maybe the exapnsion tank seal is worn which is why its not sealing?

any advice?
cheers
John