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With no disk fitted, both pistons move a bit when level squeezed, and retract as expected. Holding each piston in alternatively, the other advances fine and gradually inches out. But in practice, with the disk fitted, the lever moves pretty much to the bar (road levers), the brake only just fully engages with the disk, and when lever is released both pistons retract to start positions.
There is quite a "large" gap between one pad and the disk, that I feel should have been taken up with piston advancement.
SRAM force HRD, about 5 years old, and done a fair few miles, but been well looked after and piston cleaned each time pads replaced.
Not really sure what to try next.
I'd take the caliper off the disc/bike
Carefully pull the brake lever to extend the pistons
Drip some brake fluid (whichever fluid is used inside the brake) onto the piston/s cylindrical surface
Push the pistons back in
Repeat two or three times to ensure smooth operation.
I do clean the pistons, and have done recently with last change of pads. Both seem to move fine. I've never lubed them with anything though. Perhaps I'll try that next.
Sounds like there might not be enough fluid in the system so I'd give em a bleed which would sort that out as well.
in practice, with the disk fitted, the lever moves pretty much to the bar
This does sound like they are underfilled. Bleeding would be a good place to start.