I like the unit made by Towbrake the best of all the ones I've seen. It's the unit I plan to use on my rig after the rebuild.
This unit is not a pump, but rather a large linear actuator pushing the piston of a normal automotive master cylinder. I like this design.
Check out their website (towbrake.com). It's very informative.
As well, here is a link to the "Trailer Life" article that I've mentioned on the forums here in the past.