Certainly on a new bike to me I press the pistons back into the caliper. After that, only if I remember.

If I can't push the pistons back with my hands then I take that as a sign to service the caliper. Removing the pistons and cleaning the seal groove and seal and reassemble using a smear of rubber grease usually sorts them out.

 

When I'm changing dirty fluid I generally take the caliper off and rotate it so the bleed nipple is lowest point. Then leave it open to drain overnight and flush a fair amount of fluid through it. I always thought I had a lazy approach, as the real way is popping out the pistons and servicing the whole thing. I am gonna try @Scuff trick next time of pushing the pistons right in.

 

When I'm changing dirty fluid I generally take the caliper off and rotate it so the bleed nipple is lowest point. Then leave it open to drain overnight and flush a fair amount of fluid through it.

I'm not sure that would help as it's still just the brake line that is emptying as the caliper is effectively air locked, unless you have removed the banjo bolt and allowed the caliper to drain out?

 

Nope, I just hoped the open system would drain out overnight, bleed nipple the lowest point.

 

Have to say I never considered this. I assume you are saying there's a difference between radial and conventional calipers such that the "squeeze lever, crack bleeder, close bleeder" method doesn't touch what's in the caliper itself?

 

I assume you are saying there's a difference between radial and conventional calipers such that the "squeeze lever, crack bleeder, close bleeder" method doesn't touch what's in the caliper itself?

That depends on who made the caliper but in essence.. yes.
Some conventional (floating calipers) have a separate bleeder so there is some separation between the banjo (the feed) and the bleed nipple, so some fresh fluid gets in it.

On the KTM/Brembo floating calipers the bleed nipple is still on the banjo bolt, so not ideal.
But the manual states you should push the caliper inwards before bleeding as this will empty the caliper, which is good but I think I'd be careful about doing that without first cleaning the pistons as it might push dirt in and damage a seal.

With the radial types it's not possible to push the caliper over as they are fixed (not on sliders) but also there are pistons on both sides, so you have four fluid cavities per caliper that are not in the bleed flow path.

So basically the best way to flush them is to use the pistons to pump them out first.
The procedure above is what I'm going to do the next time I have my calipers off of the forks to give them a good clean.