Well I just jacked my up and the last owner had the same problem,he changed all the front calipers,mount bolts,and turned rotors.Still does it and my other Bee from the 70's did it also.I'm in the process of getting the m/c and booster done,but I've heard this may not help...I'll keep you posted.I've not removed the calipers and checked the slide area of the mounts.
Also I've been wanting to replace the hoses as they will "flap over"and hold pressure and give you the same result.So jack it up,hit the brakes and get it to act up,then crack the bleeders loose and see if that helps,it could be the hoses.. Just my.