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This one is an odd situation.

My front brake has always pulled in further than the rear, which I put down to having to fill 4 pistons. So, with dual brakes, more lever movement.

But this lever has been getting closer and closer to the handle bars when pulled, to the situation now that I can't really get good braking, the lever won't pull far enough.

The calipers seem to actually be 'too free'. What I mean; I serviced them and cleaned off all rust, etc, lubricated. Now, you can push the pistons back easily with just finger pinch.

But, this means the pistons go TOO far back after release!

The pistons now pull so far back, so easily, that most of the brake lever travel is doing nothing useful but just moving the pistons in air to get to the disc.

Is there something I'm not understanding on these brakes?

Have you bled the brakes properly and regularly changed the brake fluid—or topped it up when necessary?
The brake fluid reservoir must be filled to the top (no air bubbles!).

Yes, bled perfectly.

There is a teeny tiny bubble, but I can't believe it is because of that, surely? I wouldn't know how to get a bubble that small out of it.

The pressurisation is on the piston side. They just seem to come back a long way after release, because I got them so free-running.

I could replace the fluid, I don't have an issue with doing that, but I don't think it is that. I guess it could be the brake lines are bad and walls are weakened and expanding? Could that be it? I should replace the brake lines for better quality?

What happens when you pull the brake for 20seconds and then relase it and that repeat it for three times?
Does your brake get better?

If that happens then you need a new brake cylinder, as it lets air in the brake lines.

Hi, no, if anything it gets worse.

I have routinely attached a lever-lock to the front, this is like a 'hand brake' seeing as there is no gear-lock like on a regular bike. I put it on mainly to stop it getting budged off the centre stand accidentally.

So, it does sit there sometimes when I am out with the brake lever pulled tight, and now it barely puts the brakes on.

I think it is the brake seals are not holding the pistons against the disc.

I cleaned up the pistons 'too good'. Now, they just slide back in too easily, so the pistons/pads sit away from the disc.

I mean, that is great for avoiding brake drag (a bad thing for electric bike range!), but I can't get maximum braking from the front that it could give.