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I ordered the Laser 3940 MINI specific brake tool from the U.K. a while back and I really like it. Unlike the Harbor Freight universal set the end that goes into the piston won't fall out. Not sure if this is the seller I bought from or not but it's the tool:
the harbor freight set i have the disks are magnetic and stick to the end of the tool. falling out of the piston is not an issue. One of the better deals i've got from h-f (some of their stuff is junk.., the brake set is good. )
on mine the 4 prong adapter does not quite fit into r53 calipers, the pins are spaced out a tiny bit too much
I rented a set that looked like the Harbor Freight set from the auto parts store one time and found that as well. Can't remember how I found this tool but I didn't hesitate to order it as it was MINI specific. With two R50's to take care of this was money well spent.