On the new ones you aren’t meant to
shimano method of squeeze, open nipple, close nipple, release
until they towards the end of the bleeding procedure. fluid wont go through as it gets air locked and the air pushes the fluid back up when you release the lever.
They can be fiddly so I just pump fluid up from the bottom with a syringe, then lock the nipple, back pressure the syringe and jam the syringe open so its holding a large vacuum. then open and close the nipple rapidly to suck the air and fluid back down. For me it works better than their official method which I couldn’t get to work to save my life.