You can try to return it but the way many warranties work is that once the unit is disassembled the warranty is voided. A sharp counter man would see that the m/c has not had brake fluid in it and know that it was not used together. If the manager is a good hearted guy and you tell him what happened and why (changing the ball and switch kept you from opening up the brake system, left your original pump and cylinder on the car, etc) he may honor the warranty anyway since those two parts just screw on to the m/c and would have failed either way. If he will not then I think the ball and the switch are now available aftermarket. They are not cheap, but they can be found.
P.S. You said that the switch was seeping. If the brake warning light is not coming on then the problem may just be the switch itself. If you changed the original just because you had a new switch then swap it back and see if the problem goes away.