Air is visible at 2 places - there are bubbles visible in the tubes to the printhead - that's the easy part - or you need to look to the damper of that color - the little round ink reservoir in the middle of the damper should not be empty. I described above how to get to the dampers. And you should not exclude that there is another reason for the missing nozzles - a normal clog.
There is a latch at the backside - pretty much at the top, you can get there and release it slightly from the side with a small screwdriver, getting there between 2 adjacent dampers. Yours should look similar.
o.k. - fine - you got it out. You disconnected the tube, did you clamp the tub to prevent the ink to run back ? You can pull the ink up when you reconnect the tube in any case.
I had the ET-7750 years ago and finally dumped it since I couldn't get it working again after converting back from pigment inks to dye inks - and some other tests......Anyway - I'll try to find the service manual I got at that time which should let me understand again what I did then, it must be hidden somewhere in older backups , and I probably took apart too many printers so far.
There's a small srew at the bottom of the damper. Unscrewed the damper can be removed. Underneath the damper ther is no valve, but something 'mouth' like, see photo. The nozzle jet check shows empty magenta and photo black sections
You are right, my image of the damper is not the one for the ET-7750, the ink transfer pad is different. We didn't discuss how you replaced the inks yet.
I do it a rather simple way - pulling the old ink through the tube at the side of the damper, the paper clip prevents the new ink once loaded to flow back.
I'm still looking for the service manual, that might give you more information, I got the ET-7750 in Nov 2017 and was trying to find a way to use one of the black channels for a gloss optimizer - which did not work out as intendet but it's all quite some time ago.
I would put the printer back together at this time and see what is still going wrong. It could help to put a piece of kitchen towel paper undertneath the nozzle plat wetted with some cleaner. You may put some cleaner onto the pads in the printhead of those colors which miss some nozzles.