i belive that since during normal pumping bleeding there aren't massive pressures involved that the viscosity of the fluid keeps it from coming out the threads, but the same leak is present. personally i like knowing that i drew fresh new fluid straight through from the source and seeing the old nasty fluid in the plastic container. that's kinda my guage for when i'm done...it's all fresh fluid in the cup. this obviously wouldn't be the case it you'e going new everything...no old fluid. i'd probably draw a few full cups off of each wheel and twice off the first rear i did to make sure no air was lost at the Y. what is it $4 for the big can of fluid? it's pretty much trash after you open it since it'll be a year before you need it again.