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My usual method of pads out, pump the pedal to allow the still connected hydraulics to push the pistons out as far as possible, got lost in the unrelated disassembly process.

I.e., the pistons are mostly fully retracted with the calipers apart. I read the manual, to see the use of various BMW special tools, holding one piston while using compressed air, etc.

Step 1., I know I have to make a custom nozzle, cover the Osha safety side vents with some semi-sealing rubber, tape end to keep the air from escaping from the nozzle into the caliper brake line hole. Yes ?

While shooting my face with brake fluid, from the caliper half fluid transfer holes on the ends, it occurred to me, perhaps the alternative to the BMW method is having the calipers assembled, with O rings, the bleeder screws closed. Doing that, is there a better chance the air applied to the hard line input holes will push the pistons out ?

When I did this I had the calipers completly off the car. I guess it never occurred to me that I needed to pull the pistons out haha. Anyways, I tried the compressed air and my small compressor with 125psi wouldn't pop them out so I grabbed them with a pair of vide grips and a rag to keep from damaging the piston lip and slowly tapped and twisted the piston out. Took a while but nothing else would work.