When I recently took mine for test the front brakes were 12% out of balance.(within limits ) but I wasn’t happy with that. I rechecked all the pads ,replaced the front discs and re bled the system.took it back and they were still the same.rellaced front hoses ,re bled and tried again. Still the same.garage said they were fine and nothing I could do would make them 100% perfect. No cars are apparently.no signs of any pull either.
I still wanted them better so I rebuilt both calipers noticing that the lower side DID have some slight rusting on the piston ( the others were finger push,this was tighter but not in any way seized)and a SLIGHT wetness on piston.
Rebuilt,all finger push to retract now,rebled and back again.
Brakes now less than 1% out of balance.(0.7%) 229/230
So mine was definately a caliper issue. To be honest I don’t think it was the slightly stiff piston,more like the slight leak drawing a very small amount of air in on release.