If it's the line, it'll just be in the rubber part. You can replace just the rubber line on some bikes, but some are all made together so that you have to replace the whole line.
Are your return springs on the pedal working properly? If it just started acting up after you worked on the rear, I would check all the linkages and mounts first and make sure they all look good.
Did you remove the piston during the cleaning process? If not, you may need to push it out past where it was running and clean it really well. If you have already done that, I would check the pedal and linkages, then go from there.