Many years ago my husband gave me a Parker fountain pen as a nice gift and I used it often. It had an old fashioned filler which allowed me to fill it from a bottle of ink. For reasons I have forgotten I switched to ballpoints or rollerballs and left the fountain pen in my desk drawer, now I want to use it again.
I had to clean it out well first, but had had little luck reviving fountain pens, they would come back but gradually stop flowing and finally skip so much that they were useless. I read in an article on line where if you bought a Radio Shack desoldering tool that you could attach the pen nib to it and squeeze water through it to open the passages for ink and it would flow again. I soaked the bellows and the nib overnight in water and then attached the nib to the desoldering bulb filled with water. I flushed it very well and now it works a charm. I love old fashioned fountain pens.
In addition to the Parker, I have two old and cheap Shaeffers (one of which is a joy to write with…go figure), and an Osmeroid which is going into the bath soon too. Lucky me.