Button Instance request
You could also just make a button out of prop_dynamics instead of using a prop_floor_button. I've done it before when I experimented with Portal 1 mapping. You could also try using the prop_button that you press with the USE key (E is default) to make things a little more simpler.
I realize what you're trying to create, but if other people were playing your map, would they understand the concept you're trying to create with floor buttons? It just doesn't seem "correct" for a floor button to be used that way unless a Cube was placed on it instead of the floor button locking after being pressed.
My Work
[spoiler]Maps:
Revenge of the Angry Turrets
Capture the Cube [Co-op]
Capture the Cube 2 [Co-op]
TPWEGTH Sample Map
Aperture Aquatic Testing Center
Aperture Aquatic Testing Center 2
Aperture Time Testing Center
ML's Halloween Trick - 1000 downloads!
ML's Halloween Treat
ML's Combination - 1000 downloads!
ML's Jailbreak Labyrinth
ML's Tricky Teamwork [Co-op]
WIP:
"Capture the Cube 3"
Workshop Maps Link: http://steamcommunity.com/profiles/76561198008890579/myworkshopfiles/[/spoiler]
This is my last attempt, then if this doesn't work. Then sorry, I can't help you.
i haven't tested this yet.
if you can still push the button, then make the brush taller.