in order to modify the piston for the groove the rubber retaining ring needs, the master needs to be disassembled, and the rear piston chucked up in a late.
you then need to know what push rod you are going to use, so you can determine the distance from the rear edge of the piston the groove needs to be machined.
then using the proper width grooving tool that fits inside the rear piston bore, you can easily make the groove necessary for the rubber retaining ring to fit into.
lacking a late and the necessary tooling to machine a groove, [and most do not have access to this stuff as i do] you can obtain a piston with the necessary groove already there that fits your master, find a master that is the size you need with the necessary retaining groove already in the rear piston, or make a substantial stop that will prevent the brake pedal rod from pulling out of the master cylinder, allowing the rod to fall toward the floor, causing complete brake system failure and possible severe damage to you and your car if this happens at the wrong time.