I just had the same problem. I rebuilt my mc on a 2001 Heritage. Couldn’t get fluid out of the reservoir. I bled by putting my finger over the hole reattached brake line and nothing. You can pump for days and get no pressure. Here’s the fix and it took me 10 minutes. Pump the brake lever and hold it down release the bleeder on the caliper. Do it over and over. Took about 10 minutes and I have front brakes. When you take off the brake line at the mc and pull lever nothing will come out. Like it’s not working. It’s a pressurized system and it will never come out if the lines are off. Just put it all back together and bleed as normal it will work.

 

Short bus, the passage between the reservoir to the plunger channel, is not open until the plunger is sprung all the way back. This should happen when the lever is all the way out, but somethings the plunger assembly will stick if installed dry.

So when rebuilding the master, use acetone to clean the plunger channel to get the walls shiny clean to start with, then lightly coat the plunger, the plunger channel, and the spring with silicone grease and makes sure that the plunger is being sprung all the way back, so the front of the plunger seal does recess back behind the channel with the lever all the way out to open the passage.

If you shine a light down the reservoir channel before you add brake fluid to start with, pulling the lever should see the black plunger passing the passage channel towards the middle of the bike, and when you release the lever, should see the black plunger coming back and clearing the channel so fluid can pass through to start with.

As for bleeding, with the plunger all the way back./lever all the way open, best to just start down at the bleeder valve with vacuum bleeder tool, to draw the fluid from the tank to the calipers to start with. Hence empty lines means a lot of fluid that the plunger will need to push to start to fill the lines to start with, as well as get the initial vacuum amount to prime the plunger channel as well. Once you do have the most of the air out of the system, put the cap back on the reserve tank so your not spraying fluid everywhere, do a few hard lever pulls and releases, to dance the last of the air out of the plunger channel and off the piston down at the caliper, do a final lever pull with bleeder open to get any last of air out of the system, then pull the cover to do a final fill of the reservoir tank to the full indicator level.

 

And now you know why you are told to put a spacer between the brake lever and mount when removing and/or reinstalling the mc.
​​ Additional text states that damage to the brake light switch will occur if a spacer isn't used...

Figure 3. Put a piece of cardboard between brake lever and assembly before removing master cylinder

 

Huh...Never had one Newer than 1996!!! Something I never would have thought of!!

 

Yeah that is a necessary step. The manual actually says how many thousandths of an inch it has to be. I have one around here built up with layers of tape to make the proper thickness. I must have done that on my 2006 Softail.