Sludge in radiator.......

hi, i have a rebuilt slant 6 and now the coolant is turning to a reddish brownish sludge.....i have to add fluid every other day or so if i drive far.....what does this mean/how can i fix it? oh.....and i have one of those flushing adapters put into one of the hoses where i can just screw in a water hose to flush my radiator....my question is what do i need to open up while flushing? is it only the hole at the bottom of the radiator? thanks in advance!