Piston problem, not responsive
Sounds like a sticky piston to me. These can be a real pain to sort.

To get the unstuck you remove the wheel, and push the pistons in and out a handful of times.


Stick the bike in a sturdy stand.
Remove the wheel.
Squeeze the levers a few times to get the pistons sticking out a bit. Don't go too far as dropping a piston out is bad news.
Push them back in. Repeat as necessary.


Another way I tried was removing the pads, and sticking an 8mm allen key in the calliper, clamping it between the pistons and holding the level down with a strap.

Then you can gently move the pistons back and forth together.

You shouldn't need to lube the pistons, just make sure they're clean. The in and out will lube the bits that need to be lubed.

SRAM say 3-5, my Maguras took a lot more than that.