Not sure if anyone else has run into this problem, but in my level in have a group of NPCs (turrets) resting on top of a thin, invisible func_brush, which in turn is resting on top of a solid platform. At a certain point, I want the func_brush to disappear, allowing the turrets to fall the short distance and rest directly on the platform. I am currently using the "Disable" input to make the func_brush disappear. However, the turrets don't move at all when the func_brush is disabled.

To test this, I retextured the func_brush with a generic brick texture. The func_brush disappears visually, but the turrets remain resting upon it. I also created a thicker func_brush and stood on it before it disappeared, and when it did, the character fell down to the ground, but a turret placed on top did not until I lifted it off the func_brush and then moved it back to where the bursh used to be. This has lead me to believe the problem is specific to NPCs. Any idea why they do not move in the absence of the func_brush and how I could fix this?

I don't know if it will work, but try using a func_breakable. Just a thought.

I had a problem with this when I was trying to make an "energy field" holding a cube. There's a bug with the Source engine that causes it. Any physics objects (props, NPCs, etc.) won't respond when a func_brush they are resting on is given a disable output. However, the turrets will fall if the brush is removed via a kill output. The only problem with this is that there's no way to "recreate" the func_brush if it's sent a kill output; it's gone for good in that session. You are probably best going with a func_breakable; if you can eliminate the gibs and the noise it should work perfectly.
Hope that helps.

By the way, here's a little more info on the Source bug. When the brush the physics entity is resting on is sent a disable output, the physics entity itself remains floating in the air until it's acted on by another physics force--for example, being shot, bumped by player/other object, or affected by a physexplosion. When it is affected, the physics entity falls right through the (now disabled) func_brush as though it wasn't even there. No clue why this occurs with the disable output but not the kill output, but at least there are workarounds.

I actually solved it by making the func_brush a func_movelinear and moving out of the way, but I may try the func_breakable out too. Thanks for your help.