I changed handle bars on a 2002 harley 1200C. When I reconnected the brake line there is absolutely nothing happening when I squeeze the lever

First and foremost, has the wrong brake fluid being used when the master cylinder was replaced? DOT 4 is the correct fluid for these bikes or DOT 5.1. Do not use under any circumstance DOT 5.0 as this is a silicone based fluid and will turn to a jelly like substance after a while. It can also ruin your ABS module. DOT 5 is coloured purple (at least HD fluid is). Open the top cover of the master cylinder and inspect this. I have got seized ABS modules in the past due to poor servicing (brake fluid must be changed every two years as it absorbs water (DOT 5.0 does not)) causing corrosion in the module.

Dual Disk Brake.
The problem is critical. I would not ride with this condition present. There are a couple of test I would do to determine where the problem lies. What I would be trying to determine with this test is to see if the problem is in the lever or the brake itself.... On the caliper down at the disc is a brass or stainless fitting there for bleeding the brakes. Fitting a wrench here tightly as to not damage the fitting, open the fitting. Now squeeze the brake lever. The handle should pull with almost no resistance. Fluid will squirt out. If the pull is very easy you have determined that the problem is not the handle itself and is in the caliper. A new caliper is the easiest solution however the original can be rebuilt. If the pull is still hard you need to determine if the problem is in the pivot of the lever or the internals of the master cylinder connected to the lever. I would now loosen the similar brass fitting at the Master cylinder(retighten the bleed screw at the caliper first). Lossen enough to allow fluid to leak by when you squeeze the lever. If the pull is difficult replace or rebuild the Master cylinder/lever assembly.

the master cylinder on the handle bar is not adjusted properly, sounds like it is too tight and not letting the pressure bleed off. the handle connects to a rod that should have an adjustment on it. the new handle may not be made exactly the same, so the adjustment needs to be made to allow some slack in the stroke.

The front brake switch is located inside the right handlebar control. It is a plunger type switch. The correct way to install a new one is to solder the wires on the new switch to existing wires from the old switch. Cut the wires about 2 inches or so from the old switch. Be careful when reinstalling the front lever or you may damage the plunger on the new switch.

Yes as long as the brake line is designed to be disconnected at the splitter.

try using a line locker close to master cyclinder ,lever should go rock hard if it has no air in system,work line locker down lines to see if lever stays hard.
also check the lever travel to make sure that lever is pushing on master cyclinder plunger as a lot of after market levers dont have correct casting and have to travel in a long way before brakes work even when system is bled. also check to make sure calliper carriers are not bent and pads not sitting square on rotors.
hope this helps

If the screw is all the way tight and still slipping ... try to find a fatter part of the handle bar to mount the grip to. You will probably will not find any such that is convenient for you to continue to use the brake ... remove the grip, wrap some electrical tape around the handle bar, reattach the grip. Tighten it and give it a try.

I hope this works for you

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fuse their under the seat and their marked good luck