Read Melev's article on CA Reactors here:
http://www.melevsreef.com/calcium_reactor.html
I found it very helpful when I set up my Reactor. Just ignore the part about the cup if your Ph probe is mounted into the reactor.
Also you should use a bubble counter as you dont want to add too much Co2 too quickly when the solenoid opens.... the PH Probes are slow to react to the change in Ph and you will get wild swings and the reactor won't work properly.
Don't worry about the fluid in the BC going into the Co2 Bottle... it would have to get by about 1200 psi of pressure first!
Also get yourself some good quality test kits! It is very hard to adjust the reactor properly if you can't accurately measure the changes in calcium levels in your tank.
Good Luck!