I will never ever add new fluid to old fluid.
A number of times in my life I have seen adding fluid cause problems, each time the callipers have pushed the pads against the discs when things got hot, this caused things to get even hotter and caused more pressure.
Here is a couple of EG's:
I had a 1976 R100RS BMW with the MC under the tank, before a big trip I was looking over the bike and the fluid level was at the low point so I put in enough to get it to the high level.
While on the trip the pressure kept building, about every 10ks I had to stop and release the presser at the bleed nipples.
Both front discs cracked from the heat, a very expensive fix in those days.
A friend purchased a second-hand car in Sydney and was to drive it up to Queensland, the brakes kept coming on when things got hot, I rang the original owner who we knew and he told me he had no problems with the brakes but he did top up the fluid just before we picked up the car, the brakes would come on enough to stop forward motion but the discs never cracked.
In both of those cases a complete fluid change fixed the problems.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.