Breaking a spoke while truing a wheel sounds like you are overtensioning some spokes while others are
just torquing. When spoke tension rises above an ill defined point the torque to turn the nipple on the
threads becomes so high that the spoke twists instead of the nipple tightening any further on the
thread. It appears that something is happening when in actuality the spoke tension becomes constant
and the spoke just twists. The cure is to take the wheel off the stand and stand it vertically on the floor and lean your weight on the top of the rim and bounce your weight up and down on the rim, rotate the
rim 4-5 spokes and repeat til you stop hearing pinging sounds. The ping is the spoke unwinding its torque.
Then retrue the wheel and repeat. That is the reason some people use lube on the spoke threads, to reduce the tendency of the nipple to stop going down the threads and the spoke to wind up instead.
Once you are close to true and desired spoke tension the spokes, when plucked, should all have a similar
tone, rear wheels will typically be higher tension on the drive side than the L side. Current era rims
should be very easy to true as they are so much more rigid than the light weight rims of the '60s-70s.
Truing a 260gr Fiamme yellow label was much harder. Steve