I saw an interesting modification idea in one of the trade mags a while back. The user took two heavy duty breaks and bolted one to each end of a length of 2x4, countersinking the bolts to allow only the face of the board to touch the floor. They then welded a rod to the handles of the breaks to allow them to be set down at the same time. This gives you much bigger surface area for stability and strength. I tried to find a photo but couldn't..... and I don't have a chance to draw it out.... So I apologize if my description isn't clear...
I can't wait to give it a try myself though!
This is the method I use - has sure helped reduce the amount of bent wagon brakes. I found the hint here: http://www.communitytheater.org/forum/forum_posts.asp?TID=4463
I cut-n-pasted David McCall's method and made a PDF for my shop....