This is my home made tool for winding back the pistons of rear brake callipers on typically Volkswagen vehicles.
As you can see its simply an M10 x 100mm bolt, large washer, 2 M10 nuts, piece of 1/8th plate and 2 M6 set screws.
For smaller/larger pistons the washer with nut welded onto it can be unscrewed from the bolt and changed.
This cost me nothing to make really and saved a lot of stress, proper units are typically £10 upwards.
Not pretty but it works!