very little pressure from hot/cold to tub

[pwl]

hi, i just reinstalled my tub after doing the floors and painting the tub -- put in new flex steel lines from valve to tub for both hot and cold and they worked fine yesterday but very little pressure today with a hissing sound... from other posts i have read it seems like a debris problem in the water line to the shutoff valve?? how do i clean this out ?? do i remove the lines to the tub and turn valve on? while i'm at all this should i just replace the old shut off valves as well? ugh.

any help is appreciated... thanks!