[Limey_Dave]

The front hole (nearest the brake line) should be clear with the brake off.
When you pull the lever the cup seal moves forward past the hole and spreads to hold the pressure.The hole may then look to be covered by the back of the cup seal depending how far it has moved.
When the lever is released the cup will move back behind the front hole and fluid can return to the resevoir and pressure removed.
If you had problems moving fluid when bleeding and brake is now holding pressure it points to the spring not pushing the cup far enough back.
A rebuild kit will include a new spring aswell as seals and piston and should sort the problem.
Pressure bleeding can overcome and mask this fault by blowing fluid down the rear hole and past the cup from behind.