V50 Monza rear brake lost it's power

[jjwithers]

I have a 81 V50 Monza and the rear brake pedal now goes all the way down, and barely actuates the calipers. 

Is it safe to assume that a Master Cylinder rebuild is in order? 

I seem to remember looking into this years ago and there was a different solution.  The reservoir has brake fluid.  The lines are OK and not leaking.

TIA - Josh

[Tkelly]

Try bleeding it first,sounds like air in the line.

[turboguzzi]

does it still have linked brakes?

[guzzisteve]

Try bleeding it first,sounds like air in the line.

This, a normal maintenance practice, should do every year plus a flush w/new fluid

[paulbricey]

Adding together the two posts above - If you bleed it AND it has linked brakes, you would be better to remove & raise both linked calipers as high as possible to the link pipe level on top frame tube & leave air bubbles to settle up overnight before bleeding BOTH calipers.