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If you use the turkey baster method, it takes about 3-4 cycles of sucking out, refilling, driving and repeat to get the old stuff out. I use Redline Power Steering fluid in my 87 and I think I used 2 quarts to do a flush to where most all of the old fluid and the new/old mix was out.
You could remove the return hose from the resevoir and plug that port, start the motor and let it idle while catching the old stuff from the hose and filling the reservoir with new. This really takes two people to do but it works. As the engine runs, have somebody turn the steering wheel lock to lock to cycle the fluid from the rack piston.