Weird pad contamination

Looking for some help with this. I’ve been helping a friend with her howling Shimano brakes. Did the usual clean discs with meths, wet and dry sandpaper, the dremel sand the surface off the pads. Disc cleaner to the cslioer with a bit of pumping back and forth of pistons.

All fine at first, but after about an hour, the howling is back. Taking the pads out, they have what appears to almost be a plastic like material on the surface of the pad st the leading and trailing edge of the pads. About 0.5cm wide at each end and the full height of the pads.

I’d normally put this type of howling down to a leak in the caliper seals but no sign of oil at all and I’ve never seen the plastic type contamination before.

Any ideas? In the meantime, I’m going to swap the caliper over, but I’m proper puzzled by this one.

My bet is a micro leak even if no visible oil.

I’m with you on that, buy I’m puzzled as to the type of contamination. Never seen that, and I’ve quite a few shimano caliper leaks over the years.

Had it been stored/not ridden for a while? It’ll be a leak. It’s always a leak with Shimano. I’m a big fan of shimano stuff in general, but I’ve had it with their brakes.

I can’t explain the ‘film’ as I can never be arsed to delve that far into it before binning then.