lock release

[1herb1]

Hi , I am new so have mercy on my lack of knowledge of the 1898 marlin. now my problem, I have a chicago arms pump shotgun exposed hammer, it it obviously a marlin 1898 make . it is in very good shape except that the release buttom on the right side beside the hammer will not release the lock up. the pump works fine if the hammer is down ,or you fire the gun, there seems to be a spring that pushes the button back out about 1/8 inch when I push it. is this a hard repair any suggestions would be appericated. I can dissamble and reassemble if need be as I repair other guns . there are no older gunsmiths avaible that know anything about it locally and I would rather do it myself.. Thanks all