depends on your definition of "smallest" depends on your definition of "smallest"
Which piston? what's wrong. If it's a piston in the engine, you'll need a major rebuild. If it's a master cylinder or wheel cylinder piston, just replace the cylinder, it's not worth the trouble.
Buy a small universal piston retractor from an auto supply shop (square in shape with strange different shapes coming out of it) find the correct side of the tool, the one that best fits the grooves on the piston. Attach the piston retractor tool to your ratchet Rotate the piston back into the caliper as you would with a bolt or screw.
In a hydraulic system, the force on the applying piston is multiplied by the reciprocal of the area of the piston, to find pressure which is multiplied by the piston area.
The piston rings may be worn out .
it would be better to buy a piston kit. It wouldn't be a good idea to try to fix a cracked piston.
the piston would push air down and fluid up.
the piston would push air down and fluid up.
Worn piston rings, bad valve seals,
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That is not a knock, that's a piston skirt slapping against the cyslinder wall. It is either #2 or #4 piston, and if you remove the piston you will find that all the moly coat has been rubbed off the skirt. A new piston is what you need.
you either have bad piston rings or your valves are staying open. Other Possibilities: Blown head gasket; cracked cylinder head; cracked piston.