Sorry I thought I was looking at the finished job
. Land rover disco 300 tdi do a tee with a check valve to do exactly that job , it stops the water thats being pumped back to the expansion tank from the thermostat housing being fired straight back to the top of the rad. It also acts as an ejector tee drawing from the top of the rad, god knows how it does it but they do work.
http://img.photobucket.com/albums/v3...rtankhoses.jpg.
Bob