It should work fine by default - I just attached a func_brush to a traintrack test I had in an old testmapp and it's working fine (it's the small reversed square penis parented to the big brush - that hanging sack is func_physbox):
I suppose the problem lies either with your game or the way you set it all up?
EDIT: Did you try setting it up in an empty testmap? I usually do this to see whether the basics are working and if my map is interfering with something. During all the mapping it is sometimes hard to keep track of everything that's happening if your working out a new idea.