When the port (valve) of the Becker adjustable breast implant is removed, does it loose volume, does it leak?

No it should not leak or cause any loss of volume if done correctly if you are concerned please follow up with your surgeon

It should not.  If you are concerned, see your PS.  If you don't like what you hear, get a second opinion in person.

Removing the port (valve) of a Becker implant, provided it is done correctly, should not lose any significant volume, either at the time of removal, or subsequently in the long term. It is not normal to lose as much as 25 to 50 cc.