Allen wrench for rear pistons?

[Alexander van Gerbig]

    I totally forgot about having to turn the pistons in vs. pressing them
in on rear calipers.  I am doing a rotor/pad job all around on my sister's
5ks while she is off from work.  I hoped it would be a 10mm allen but it
ain't.  That is the largest I have so I tried a 13mm headed bolt, no go.  So
I assume a 12mm allen is the ticket, turn clockwise while pushing in, right?

TIA,

Alexander van Gerbig -- '88 80

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