How do you remove the brake fluid reservoir from a 1991 Nissan Pickup truck - 4 cylinder engine?

Carefully. Use something like a turkey baster to remove the brake fluid. Then pull up with a very slight rocking motion away from the master cylinder.

With the brake fluid removed, just check that there is no securing bolt in side otherwise it will be secured with rubber grommet seals and do as the last post advises.

Earlier you asked how to remove the brake master cylinder. I gave you an answer now you asking how to remove the fluid? My guess you'be coming back and asking how to bleed the system.

My question what's with the brake master cylinder