It is not uncommon for a MC to "give up the ghost" during a routine maintenance procedure such as bleeding the system.

As described above, there is a normal operating area for the Master Cylinder (MC). If the seals are pushed beyond that area into a part of the cylinder that may have corrosion or deposits, the seals can easily be damaged.

Another premise here is that if the MC is going to potentially lose its ability to make pressure, it might as well do it during maintenance operations. I wouldnt lose any sleep.
Install a rebuilt unit and move on.

Another thing to consider is the one man bleeding system. Use a bottle, run hose from bleeder into the cap into the bottle. Be sure to loosen the cap during use to allow air to escape. Air will be displace as fluid enters the bottle.

With the car on jack stands, the operator can look under the vehicle at the (clear) hose going into the bottle. This allows the operator to not only see air bubbles if they exist, but also see the colour change and this know when the flush is complete. Close the bleeder and move to the next closest component to the MC.

Note:
It is helpful to use compatible fluids of different colours from one flush service to the next. (ATE blue and gold for example is what I use)
Use a turkey baster to remove all the old fluid from the reservoir before starting.
Do not push the pedal beyond the normal operating range while pumping.
No air will enter via the bleeder. Fluid flows OUT the bleeder simply by gravity.


Best of luck.

__________________
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.