I've done it many different ways, but have found the following to work the best:
First, you need to hold the master cylinder on something stationary.
I use an old microphone stand with a 7/8" tube. It's got a heavy base and stays put on the bench. You could also use an old chunk of handlebar or 7/8" pipe in a vise.
I also use a lighted magnifying glass to help see down in the recess. It REALLY helps! My vision is still good, but this makes it SUPERMAN good.
The next hard part is holding down the piston while you try and get your snap ring pliers down in there. You don't need a fancy clamp or tool. Just put the lever screw back in and use a flat blade screwdriver as a lever to push the piston down.
Now you can hold down the piston with your left hand and insert your snap ring pliers with your right hand (or reverse it if you are left handed).
It's usually best to soak the snap ring in some Kroil or PB Blaster for an hour or so. That helps loosen up the 30+ years of corrosion and rust.
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It works for me. Hope it helps you
