... and wipe off any brake fluid that gets on the grommets. I didn't properly seat the supply lines when I installed a new M/C and at some point, they leaked.
Sounds weird, but I think the leaking fluid caused the grommets to deteriorate. The grommets just disintegrated when I attempted to pull the lines back out to re-seat them
Some people remove the fuel tank, disconnect the supply lines from the M/C reservoir and press the nipples into the grommets before bolting the M/C to the floorboard, then re-route and reconnect the supply lines to the reservoir. This way they're sure the nipples seated completely.
For the sake of safety, if you haven't already done so, read up on the differences between DOT-3, DOT-4 and DOT-5 (and DOT-5.1) fluids. Know what's in your system and don't change it without knowing all the rules.
Someone at a dealership 'accidentally' added some incompatible fluid in one of my cycle's front M/Cs when a new Russell braided brake hose was installed. I noticed the front caliper had started to leak. We had to purge the system, flush with alcohol, replace the seals in the M/C and the caliper, and refill with the proper type fluid.