Removing the pistons and cleaning everything is probably a good idea at this point. The $20 seal kit may or may not be necessary, pending the condition of your existing seals, but I would probably buy it. Also if it were me, I would remove the existing seals to ensure that their seal groove is scrupulously clean. Crude can build up in that groove and force the seal radially against the piston, which may prevent the piston from retracting fully. Be careful when removing the seals so as to not scratch the groove or bore.
After cleaning the pistons and bores, install the seals and smear some brake fluid in the bores, seals, and pistons before assembling them.
After bleeding the air out of the brake system as much as possible using the bleeder valve, put a weight on the brake pedal to fully depress the master cylinder and maintain this configuration over night. This last step magically removes any remaining air in the hydraulic system. Remove the weight the next morning and pump the brake pedal several times and you will be good to go.
P.S. There is a rear brake bleeding video in the "How-to..." section of this forum.
Last edited by Norton 850; 07-31-2019 at 10:30 AM.