So, you’re rebuilding your brakes and it’s become apparent that you’re going to have to replace those old pistons in the calipers. If you’re like me, your bike sat for so long the pistons are stuck fast, and won’t budge. Here’s the solution I found.
Warning! If you follow my steps here, you will get the pistons out, but will likely ruin them in the process.
Step One
Disassemble the caliper completely and remove the brake pad so the piston is accessible. (In my picture I still need to remove the brake pad)
Step Two
Weld a large bolt to the inside of the piston
*here come the tricky steps*
Step 3
While the piston and caliper are still hot from the welding, put ice or cold water on the inside of the piston. (The point here is to cool the piston, but NOT the caliper. By keeping the caliper hot, it will expand and give you valuable clearance to get the piston out)
Step 4
With the piston cool, and the caliper body hot, clamp the bolt into a vice and slowly turn, pull, and work the caliper free from the piston.
Step 5
Allow the caliper to cool slowly to avoid warpage from the heat.
This process worked like a charm on my ’74 caliper. It was nice and clean inside and should be ready for a new pistons and seals.
Here's some more pics of the same process for the other caliper:
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