Front brake issues

Have you tried rigging up a bench bleed system? Brake line looped back and into the MC reservoir?
The thumb over the banjo port trick usually works to get things going.
I know that once it's bled you should see the reservoir level drop as you release brake handle; as the piston draws in a fresh breath of brake fluid. So that porting in the piston has to be open and clear. If the piston does not allow make up fluid from the res. into the piston/cylinder and just bounces fluid back and forth, it'll never push fluid down the lines and bleed. Also, you sure the piston is returning all the way against the snap ring with some free play at the lever? I've had that foul me up too. If the piston cant release all the way, it may not draw in fresh fluid from the res.

Usually this goes just fine. Been there tho. Its a bitch.
Be aware that the MityVac tool can push fluid as well as suck it. There are two output nipples on it. Normally you cap the "push" one when using it as a vacuum pump, but you can change it up (move hose) so it pushes fluid. I've done it a few times on a stubborn brake system, pushing fluid back in from the bleed nipple up to the MC. You just have to make sure you have a bleed hose full of fluid so you're not pushing any air into the system.
The rubber o-ring in the caliper won't play well with grease. The grease causes the rubber to swell up. You will probably have to replace the o-ring.
The only cleaning solutions for brake fluid containing systems are: WSW rinsed with clear water, clear water or the correct brake fluid. There are reasons that master cylinders state which type brake fluid to use.
I knew there was a reason I joined this Forum - to get advice on matters concerning the restoration of my XS. I have been flopping around for 4 days trying to solve my front brake issue. Have dismantled and reassembled so many times I can do it in my sleep. I have pumped and bled, filled brake lines and reservoirs, bled the master cylinder, replaced banjos and copper washers, replaced mc pistons and seals, all with no success.
I had my pen poised to fill out an online order for the MityVac when I figured I'd give Gary's reverse bleed technique another shot. Air came through the master when I compressed the piston and when I pulled the lever, I had brake. Unhappily, it faded away. I had put my hand over the calliper piston to avoid it shooting across the room and found brake fluid on my fingers. The seal around the piston was leaking. Just like Leo and John said it would. Sound advice, stupidly ignored.
Waiting on a new seal. Hopefully it works.
When inserting the piston I find it best to rotate it slightly initially until it passes the seal. If the piston is not aligned perfectly an initial heavy push can pinch the seal. How did I find this out......