Ok, so starting work on 5 lug parts

Flair fittings and bleeders are both free

pulled hardware, cleaned on wire wheel, pads look new but for price will replace

I tried air gun in flair fitting hole, pistons did not move, what is my next step? Pull the seals and get pliers on the lip?

Do I need to split the halves to rebuild these?

Will

NO - NO - NO !!!

NO PLIERS!
Do not split them!

Guys who do this all the time have a bench-mounted MC and a hand lever to use hydraulic pressure.

Interestingly - the threads on a grease gun are (or mate happily with) the 10mm x 1,0 thread pitch for the brake fitting. So - get a tube of whatever grease is on sale at the FLAPS today and GENTLY use the grease gun pressure to squeeze the pistons out.

Yeah - you need to clean all the grease out. You were going to clean them up squeeky-clean anyway, yes ?

Never use pliers!!
I only use air after rebuilding, and even then with a lot of care; pistons come out like cannonballs when you use air. For popping pistons I always use a hydraulic system; a pedal cluster with master cylinder and reservoir. You get much more pressure, and only a tiny fluid leak when the pistons are free. I don't know of any easy way to do a proper job without splitting. I always hone the bores. It is a lot of work to do properly.
Since you are in the USA, I'd suggest having Eric Shea rebuild them for you. It is not worth the time you spend doing it yourself, and his are basically new when delivered.

I was just trying to make the point that splitting the halves is not the way to get the pistons out...

grease gun it is

yes, Eric does great work, (my rears are from him) trying to keep costs down and I like doing my own work.

Trust me if they don't move easy, off to Eric they go.