master cylinder

I just put dual disc brakes on my 79 750 twin, the question is do I have to change the master cylinder? the brakes work but there is a lot of travel in the lever

Not if you have the stock 79 750 twin master cylinder you won't. The 79 750 twin has a 5/8" master cylinder stock (so does the 78)
Getting all the air out of the system can be real time consuming unless you have a pressure or vacume bleeder.

thats what I figured, it seems to get better after working it,I let it set for a day and will bleed it again