I just got a 1978 KZ650 and the front brakes are very soft. I am pretty sure the problem is the master cylinder. It looks like the guy who had it before me put a Honda master cylinder on it from a bike that had just one front caliper. Since this bike has dual calipers, the master cylinder is having some difficulty maintaining enough pressure.

I have found several master cylinders around for other bikes (mostly Harleys) and was wondering if they are interchangeable.

Thanks in advance for your help

Personally For Myself, I would use a Different Caliper than stock, provided it Worked well.
And Tested good, away from traffic.
If brakes are spongy, might need to be blead, to get rid of air.
If Brakes creep down when applying pressure, then Master cylinder.