not flushing brake fluid is literally playing russian roullette - brake fluid is an alcohol and readily absorbs moisture right from the atmosphere, watered down brake fluid lowers the boiling point of it by about 80C - quite dangerous. This moisture also rusts the brake calipers and master cylinder inside out (expensive repair - and can even have deadly consequences)
its not too hard to flush the fluid - all you need is a litre of DOT 3 or 4 brake fluid and a bottle like this.

a spanner to unscrew the bleeder valves and a jack to raise the car - the fronts can be done just by turning the wheels, the rears need the wheels removed.
the bottle should have its pipe dipped in brake fluid, hang the bottle so that the pipe always go upwards from the bleeder valve and the bottle should be above too, top up the master cylinder open bleeder - pump the pedal (only half stroke), keep on pumping and refilling the master cylinder till the fluid exiting the bleeder is the same colour as the new fluid. Repeat on each wheel. Once all is clean - pack the job.