I took a blowtorch to the disc (in an attempt to burn off any contamination).
They’re stainless steel, so washing/scrubbing with hot soapy water is fine. Just make sure to rinse with clean hot water. Using flames on a rotor is just not necessary.
I removed the pads and did the figure of 8 on some wet and dry. Also gave them a blast of the blow torch.
Sanding is good as it levels off the compound surface – I’ll come back to this. If there’s oil in the pads, heating is only going to make it soak further in.
Removed reservoir cover and made sure there was no air (there wasn’t).
You’ve looked into the open part of the system. Air bubbles between the lever piston seals and the caliper piston seals is what’ll cause you problems. This is invisible without bleeding.
I took it out for a ride yesterday and it’s working pretty well now. I’ll give the back one similar treatment.
They also started working better once they’d got a bit of muddy water on them.
My own hunch is that the sanding of the pads is what’s made the difference. Although it means that the pads/rotor have to bed in again, it does get rid of any glazing (from dragging) or impressions on the compound from being left against the rotor when very hot.
I appreciate that it feels as though you’re doing something constructive, but setting about your brake parts with a blow torch is a waste of time. Sand the pads; clean the rotors; start again.