Shimano technical training has a process for cleaning the pistons on the disc brake calipers which relies on cutting down a bleed block (such as this design is based upon) to make a piston exposure tool. Having to hack apart a bleed block with a hacksaw seems like a bit of a hack, so I came up with this design that gets the same effect without having to break out a hacksaw.

Basically, it works like a bleed block, but it specifically allows the piston to extend out of the caliper enough that you can then clean the side of the piston before pushing it back in and doing the same with the other piston.

I haven't been able to find this process documented anywhere outside of Shimano's industry-specific technical education, but you should be able to understand the basic idea from the photos.

Cleaning procedure:

1. Push both pads into the caliper
2. Insert this tool
3. Pump the brake and watch that the piston on the side of the tool with the cutout extends.
4. Stop once the piston hits the tool.
5. Remove the tool
6. Clean the sides of the piston - I used rubbing alcohol and cotton swabs. (Q-tips)
7. Lubricate the sides of the piston with Shimano mineral oil (the same stuff that goes in the brakes)
8. Push the piston back into the caliper.
9. Repeat with the other piston.

Park tool has this video available which explains the process without the "special tool".

Note: If you have Saint brakes, (or another Shimano 4-piston design) Mngnt has this version which works for four-piston Shimano brakes