so officially, everywhere you put the magenta line/damper you get a problem? if so, than you have a supply problem, all being that the line holds ink and doesn't take on air.
start with cleaning out the magenta ink line. don't do this by pulling ink toward the carriage. it works better by forcing fluid backwards towards the cartridges.
get a basic empty plastic cartridge (if you only have factory carts, just be a little gentle, you'll need an empty). mount a syringe on to the ink line without the damper attached. and push about six syringes worth of fluid into the mounted cartridge. you could have a cartridge needle block. this will only come out by reverse pressure. had a major issue with yellow and went nuts trying to find it and finally cleaning the line was what fixed it. also, observe the magenta ink. paint a swatch of it next to another color on some aluminum foil. see if over a few minutes is stays cleanly mixed (no particles, etc). the only other thing that it might be is a cartridge that has the inner bag collapsing incorrectly. and it is starving the magenta under demand.
although it's frustrating, this is the cheapest thing that can go wrong with your printer. next time you will fly through the diagnosis and be back printing in an hour.
YOU CAN DO IT JPRO!!!!! CLEAN THAT INK LINE!!! AND PRINT THOSE ELECTION THINGS!!!