the simple answer is a cooler make sure the drain hole is closed
where u have it mounted u can bring a cooler in remove the lid and then place it directly under it this will capture the water......let the water in the line drain out into there ...........do your work rehook the system then run water adding to the cooler to get all the air out and reset
then u have 2 choices
1 a water pump in the cooler sucking it all out (if u are ground lvl or own land u can just use a 10foot hose and pipe it outside tho a window)
2 u drag the cooler to where the drain is and open the drain hole on cooler
they sell huge coolers in walmart in the hunting section ............those will do 20 40 gallons .........just for the love of god make sure u turned the shut off valve to the filter off otherwise u are fucked with a half flooded grow room
if u do not know where it is
1 home on a well means u have a stwich in the fuse box ..find it flick it now your pump is off go to lowest point in house open up sink so flows go to the highest point in house do the same thing ........this takes a few mins now your system has bleed and no pressure no worries about flooding just rem when u hook it back up leave both sinks open and turn the pump on u will hear the air getting out of the system and the water come out of both and u are back to normal
city water ...............they should have valves all over the places look at your system and turn the one closed after the water heater and the cold water feed........do the sink thing and u are good to go just rem leave it open turn the valves back air out good to go