Hi, i just made a custom ui and run with some trouble with dissapearing buttons, made some research and now, i know some menus just cant be customized at this point. So finally, managed to make a ui that satisfied my needs, but, found a very annoying problem, the ctrl z function is just gone. I made some tests and found a solution and a possible reason of why this is happening so ill leave you guys an image showing my custom ui, the process i followed and my machine specs:
This is my custom ui:
After the ctrl z problem, i switched to default ui and used the store config button, closed z brush, opened it again, loaded the same tool (that is almost 8 million polys) and ctrl z was back, then, i loaded my custom ui, and problem solved.
I closed z brush again, opened it, but now i loaded my custom ui first, loaded my tool using the customized load tool button, and ctl z was gone.
I tought it was the custom load tool function or something in my custom ui that created the problem, so i made 1 more test:
I closed z brush, opened it, loaded my tool, ctrl z was ok, loaded my custom ui, then, loaded another tool using the custom load tool button, and ctrl z was ok, loaded the same fist tool again and ctrl z was ok.
There seem to be a problem with store config and load ui before loading any tool.
I`m using zbrush 4, my machine have 6 gigas of RAM, intel i7 processor and windows 7 64 bits.
Hope this is usefull to you guys, and can fix those problems with custom interfaces in future releases. I`ll keep using my custom ui and let you know if i find more problems, cheers. 
