I found this problem in 2.1 experimental and my work around was to set the filter on the pump to match the contents of the pipe -- when the pipe gets emptied by my normal slushi pipe logic (after the train leaves) then the filter being set to nothing will clear the fluid that was stuck in the pumps ...

I would like to be able to choose the color of wire to read each item out on. E.g. Logistics requests on green, logistics contents on red -- or robots in the logistics group on red and requests on green (just as examples)