Adding my 2 cents....
I have a 2000 4Runner Limited with 165,000 miles 3.4L V6
I don't think the power steering fluid had ever been changed.
At 160,000 I decided to do the main internal cam belt, tensioners, water pump, hoses and belts. Engine quiet and running great!
But I had belt squeal which I had never dealt with before and new belts?
I thought the new belts would stop the squeal?
No it didn't we took a long 1,600 mile trip everything ran great but the power steering seemed weak and I had to fight the steering a bit and belt squeal when 1 starting the 4Runner and when turning sharply.
1st thought maybe the pump was bad so I purchased a brand new power steering pump from Rockauto $80 when we returned home.
NOTE: Rockauto has brand new pumps WITH A BRAND NEW RESERVOIR for $120. To my surprise the new Power Steering pump did not fix it, puzzling to me was the squealing had changed so I did some more research which lead me to this post.
I decided to clean the Reservoir so I purchased
2 cans of brake cleaner.
4 feet of 3/8 hose
4 3/8 hose clamps
2 quarts Dex/Merc Transmission Fluid
1 Bolt that fits into hose
$44 later
Used this video....
https://www.youtube.com/watch?v=5k61KXRK8bM&t=983s
Or if you are smarter than I am purchase the Power Steering pump WITH the Reservoir $120 because for some reason they don't sell it separately from the pump unless through a Toyota Dealer for $300.
So I could have purchased a brand new power steering and bypass all the cleaning of the original reservoir.
Exception I didn't have to even turn on the 4Runner engine I just jacked the vehicle up onto jack stands and with the hoses hooked up just like in the video I was able to turn the wheel all the way back and forth (engine off) 4 times (checked fluid after 2 full turns, refilled) then 2 more full turns and it passed through about 1 1/2 Quarts of fluid nicely red in color not brown muck anymore. Wow the gunk I cleaned out the Reservoir was ugly hopefully my rack and pinion are still good?
My only regret is that I didn't purchase a fluid extractor (about $75 on Amazon) and suction the entire system clean and refill but the end result was NO MORE SQUEALING and the Power Steering is like a brand new vehicle it steers SO MUCH BETTER I am amazed how I didn't notice over the years how stiff the Power Steering had become.
I hope this helps others struggling with the Squeal issues, wear some rubber gloves and put an old tarp under your truck its a messy oily job even if you are careful.