DO NOT use ATF for power steering fluid, even if the manual in your older car says so. ATF has changed drastically in recent years and has been improved for use in automatic transmissions. It will damage the seals in your power steering system.
Go to the auto supply or somewhere and get a few quarts of "Power Steering Fluid." It is more expensive than ATF, but darn well worth it to prevent seal damage.
It never hurts to flush the system when changing it. To do this use a couple of extra quarts of PS fluid. Put a pan under the pump and remove the return hose from the reservoir and place it in the drain pan. Have someone start the engine and begin pouring the PS Fluid in the reservoir while it is pumping it out the return hose into the pan. After you have poured two quarts through, put the hose back in place, fill the reservoir and again have your assistant start the engine and turn the steering wheel back and forth lock to lock. As the assistant is doing this, keep adding fluid until full.