Is it possible to use mix and max hydraulic braking system parts?
Deore XT levers with Formula calipers or something like that?
yes but the results might not be very good
It'll be technically possible, given enough tinkering, perhaps a lathe and ensuring the seals and fluids are compatible. Whether you'll stumble upon some kind of piston ratio that isn't worse than both brakes or useless altogether isn't particularly likely though.
If your bored enough though it'd kill an afternoon.
You can mix up freely within the Shimano XC ranges, I've got an XTR rear caliper attached to a XT lever frinstance, the stroke and bore are identical. But that's not always going to be the case. Deore and Formula as you mention, may not be a good idea since one is made for mineral oil and one DOT, so even if they're otherwise perfect matches that's not so good.
Still, it's not an automatically stupid idea, just that you'll need either impeccable research or luck to make it work, and also possibly custom hoses. Whereas none of this is true of just using a Formula lever with a Formula caliper 😉
I presumed that on the single piston systems they caliper pistons must be close in size.
Sounds like I should just buy the right things to start with.
