Update: I replaced the rack, power steering pump, PS pressure hose.

I bled the system and the power steering worked GREAT! I let the truck sit overnight and started it in the morning. The PS pump whines and after a while of driving around, I have no power steering. I'm at a complete loss. It's doing the exact same thing as it did before I replaced everything! I'm now on my second pump and cannot figure out what to do next.

I don't see any signs of leaks. The only thing I haven't changed was the fluid. I've been using cheap supertech ATF. Maybe I should flush it with some synthetic ATF. I wouldn't think the fluid would make a difference.

Also, after driving it without power steering yesterday for a few miles I parked it in the garage for a couple of hours. I came back out to look at the steering system again and started the truck up. To my surprise, I had power steering again and no noise from the pump. I didn't do anything other than let it sit. Could this be temperature related? This morning it was 20 degrees in the garage.

Last night I once again vacuumed the filter screen but instead of leaving it in place, I punched holes through it. Problem solved. Pump is quiet and power steering works great. I'm guessing the screen wasn't allowing the fluid to flow until the fluid was warmed up. I refilled with the same cheap fluid. I may install an inline filter but after dealing with this for almost 4 weeks, I'm going to take a break.

This was the most frustrating issue I've had with my 4Runner. At least I now have a brand new power steering system and I replaced the LBJs while I was in there.

Interesting! How did you punch holes without creating mesh particles/pieces in the fluid?

Bump for info. I'm not punching hole in my screen but I'm wondering if I need to replace my rack. I have replaced the fluid in my system, rebuilt the pump and finally replaced the pump with a new one and still having whining sounds

happen to take any photos you can post and share?

Some say on Yotatech that they bought multiple PS pumps before they got a non-whining one. But the common problem lies in the filter screen in the pump.

Try flushing the fluid like these prior or even after you have cleaned your inline filter: DIY: Power Steering Flush

now that I have this exact same whine and have read this entire thread I have a stupid question...

why wouldn't you just replace the filter rather than poking holes in it?

Noticed this thread got bumped. The screen is part of the reservoir assembly and it cannot simply be replaced. I eventually ended up pulling the screen out and placing an inline filter on the low pressure hose. I haven't had any power steering issues since.

I get it.

Here's the part at Camelback (my goto):
1999 Toyota 4Runner Parts - Camelback Toyota Parts - Genuine OEM Parts - Free Shipping

At $150.00 it makes sense to try the flush first.

My local, trusted mechanic quoted me $90 to do the flush and can fit me in today. It would take me an hour or two to do myself and I'd have to wait 'til the weekend and burn my time off to do it, so, well worth the money. I'll update with results. Thanks very much.

Aaaaand... it's fixed, driving completely normally, no whine/whirr.

Thanks very much to the OP, and T4R.org, this place is so great!

Aaaaand... it's back. The whirr returned yesterday. When they did the flush job it only took 20 mins. I think they likely didn't do the procedure outlined here, turning the wheel back and forth... thoughts on how to proceed?

Just call them up and see what they say?

mechanic says next step is the reservoir, if that doesn't fix it, the pump/hose, and lastly, the rack.

and, apparently, sticky LBJs (lower ball joints) could be causing this problem, also.

just to put all my research results in one place

So you have or have not pulled the screen from the reservoir?

Sticky lower ball joints is not the cause of your issues. Though, if your lower ball joints have any issues you should replace them immediately.

You have already ruled out leaks by checking the fluid in the reservoir while running the truck, correct?

You would then want to rule out leaks in the steering rack. Are you having to refill fluid frequently?

If you dont have any leaks, I would start with the reservoir/pump. But I wouldnt buy new parts. Deathcougar is parting out a fairly low mileage 99 in the classifieds right now. I would get those parts from him.