Dont grease the pistons. The way a sram brake works is through the friction of a square o-ring seal that deforms as the piston extends, and then pulls the piston back in as it tries to reset itself. One of the common issues for lazy pistons is oil/grease getting on these seals and meaning they no longer bite the piston hard enough to pull it back in.
Best bet is to ‘massage’ the pistons. This should be done by the shop before sending it out, but I guess it doesn’t always happen. I have a piece of old disc to do this, but I think you can use the pad spacer on the tapered end, hence the taper. Basically you need to push the pistons out, and force them back in a few times, depending on how sticky it is, that can be quite a few times. Obviously if the pistons have been running out a while, cleaning them up before resetting is a good idea.