I just pinch pole tubing betwee my fingers to stop water if not near trolley.
using taps is better but the microbore ones tend to drag along floor and bust, not cheap!
some guys on here have fitted remote controls to their freedom trolleys, this is probably the best way, i remember someone saying there sold on ebay the remotes.
The only trouble i find with freedom trolley is it's heavy and when it comes to relacing pumps batteries it's a pig.
Keep meaning to make my own trolley, or if i had a spare freedom trolley, i'd put pump from a backpack and link 12v battery, these would be small enough to fit in the freedom battery box.
oh and try to use electric light dimmer switch as cheap flow controller. (pipe dream?)