If you pull it out without depressing the 3rd valve, does it make a popping noise? If so, the seal is still tight and everything is fine. As a rule, Getzen slides and valves move very easily, and if the horn plays like you want it to you have another reason not to worry. My only gripe with the plastic stoppers is that:
a) they don't look nearly as cool as the rest of the horn
b) I couldn't quite get low c# low enough without using the first slide too
c) low f is impossible, so no good for Carmen or Heldenleben.
Those little dents on the end of the third slide are a pain to get out -- sometimes a thicker grease will hold the slide better, but will not be as lightning quick.
Have fun with your horn!