Hi how do I mate the the black piece onto the 2 pistons like in the bottom picture?
There can be several ways. Like create points in the black part and then using them to mate with those pins OR using distance mates to position the black part.
But just looking at the pictures is not giving exact requirement or intended design intent. So please share more details and files if possible.
If the brown & black parts were both created symmetrically about their respective origins, you should be able to use a central reference plane from each to create a coincident mate.
Also if both grey parts are at the same height, selecting one of the top faces and the underside of the black part can be used for another coincident mate.
Without knowing the geometry better it's hard to know what can be mated next to complete the fixed part.
If you know exactly where your two brake pistons are going to make contact with the back of the disc brake pad you could simply sketch two circles on the rear of the brake pad and then mate each of the pistons to the sketches with a "concentric mate".
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