The Delrin I'm familiar with is black, not white. While it machines very nicely, I was leery of using it to make a piston. (K7 caliper, phenolic pistons not available in 42.8 mm) I also have thought of making anodized aluminum pistons. Somesuch, any chance the boiling fluid issue was due to the aluminum pistons expanding from heat, and sticking in the caliper bore, creating drag, which in turn created more heat?