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the "M" caliper for solid rotors seems to be a good choice
They bolt right on. Good choice? Same size pad with a 48mm piston vs. a 42mm piston. You're not gaining anything in friction. Just screwing up the bias with the rears.
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less force on the brakes than with the 17mm
Well... it actually takes more force to operate the pedal but, that's negligible at this point. I would replace any 17mm with a 19mm at this point (if it needed to be replaced).
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IMO - if you want better braking, just get a better class of pad. Porterfield R4S are the hot setup. A solid-rotor 911/912 caliper will be OLD, and cast iron. You're better off with what you've got - just use a decent pad. I had an early ('65) 912 and I don't think its brakes were even a little better than a 914's...
What she said.
914 brakes in tip-top order with killer pads should rock any narrow body world. Are they gummed up? Do they have old soft lines? Is the venting clearance set on the rears (most are inoperable)? Did you bed the pads? Even old pads can be re-bedded and you'd be surprised at the difference.
Those Brembo's in the classifieds are the hot set-up but, you'll need vented rotors (or, there's a guy selling his that have the spacers removed... bolt on for your car. PM me for his details). The problem you'll find is, there's a "lot" of pad there for a solid rotor. Time to get some cooling in there.
Most people would benefit from a thorough system rebuild (top to bottom), new pads and new tires.
Best... E.