I assume you mean the wheel cylinder. It has two pistons that push outward. If I understand you correctly, one of the pistons comes out of the cylinder.
I assume the drum is on when this happens.
So, if this is the case. It sounds like the little rod that the piston pushes into the brake shoe must not be hitting the shoe. Causing the piston to just pop out.
Are your shoes secured correctly to the backing plate? Correct shoes?
Even if the bottom adjuster if way off, it shouldn't make the piston pop out.
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Frank
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