Toyota Previa 2WD - power steering fails to function most of the time. Fluid reservoir is full. Although belt makes squeaky noise (probably slipping), it isn't clear to me whether it is the primary cause or side result of major failure. How to find out?

Is it full of fluid ?

spray down pump and hoses with degreaser and wash off. After its dry, use recomended fluid to fill. Start engine and turn steering wheel all the way left, and all the way right a couple of times. This will blead the air from the system. Now shut off and check fluid. Make sure it is full. Now drive the car down the road and make turns, and drive it back and check fluid level. If its still foamy and not leaking fluid, your pump ma be bad.

Assume the noise is coming from under the hood. If the noise only occurs at the far extreme turning point in both directions, this is normal as it is straining the power steering system. If the noise occurs all the time, check the level of fluid in the power steering reservoir. If low, probably a leaking hose or reservoir. Add power steering fluid to get level between min and max lines. Should make sound go away. If the noise does not go away and a flush of the system does not correct it, you may need a new power steering system, although this is pretty rare unless a leak has gone on for a long time with little or no fluid.
If noise is inside the car, one of the parts of the steering column or steering wheel may be bad.

Hi there, check to see if your power steer reservoir is full, if so then your power steer pump has failed, they usually make a noise when low on fluid, that would be the first place I would check, then check for and fluid leak from the reservoir , hope this helps

hi Eva.
As you turn your steering wheel it is this fluid pressure that aids in turning the wheels of your car. ... If the pump impeller is trying to move power steering fluid and starts catching air bubbles it can cause whining noises and vibrations to occur.
Maybe you've heard it at a stop light. Maybe you've heard it as you pass a car on the freeway. Or maybe you've heard it in the parking lot at work when someone is leaving. The unmistakable whine of a power steering pump can make even the most confident mechanic wonder when that car will breakdown.
Power steering pump noise is a unique noise that your car may make and once you've heard it you always know what it is. The noise is a mix of a whirring and a whining and will always vary with the speed of your engine whether your car is driving or in park. This whining noise can be subtle or extremely loud and may be worse in colder weather, or right when you first start your car.
Power steering pumps are most often vain style centrifugal pumps that are tasked with pressurizing the power steering fluid and pushing it down to your power steering gear or rack. As you turn your steering wheel it is this fluid pressure that aids in turning the wheels of your car.
There are a few reasons why you are hearing power steering pump noise from your car. First, it may simply be due to the design of the pump. Sometimes, noisy power steering pumps can continue to work flawlessly for years after they start making noise. This may simply be due to the manufacture's design or a tight tolerance in that particular pump. In this case, you would hear the noise but not see any other adverse effect like a loss in power steering, leaking fluid or a wobbly pulley. In this case, you may be able to help reduce your power steering pump noise by choosing a different type of power steering fluid, like a synthetic fluid.
Your power steering pump may also be making noises due to air trapped in the system. If the pump impeller is trying to move power steering fluid and starts catching air bubbles it can cause whining noises and vibrations to occur. Even tiny air bubbles you cannot see that are trapping in the power steering fluid may be causing the whining you are hearing. In this case, the best thing to do is flush your power steering system to try and remove the air from the system. Having air trapped in your power steering system can be difficult to diagnose so you may need to simply try a power steering flush to see if it solves your problem.
Power Steering pump noise may also be due to a failing power steering pump. As the bearings on the impeller go bad it will start to make noise as the pump is turning. If you have a bad power steering pump you usually will also experience leaks from around the pulley, a wobbly pulley, or difficulty turning the steering wheel when you are stopped. If you have multiple of these symptoms together, it is probably time to replace your power steering pump.
Lastly, and most usually, a whining power steering pump is an indication of a low fluid level. Your power steering fluid is probably something you have rarely if ever checked, so it often comes as a surprise if the level is low. Your power steering fluid reservoir is also relatively small so even a slow leak will cause a low fluid level relatively quickly. To confirm this is your problem it is as simple as checking your power steering fluid level. Some power steering reservoirs are clear so you can see through them to check how full they are. You may have to wipe away some dirt and grime to find the "full" line, but it should be easy to check. If you have an opaque power steering fluid reservoir there should be a small dipstick under the cap that will help you measure the fluid level. If it is low, then you have a leak somewhere and topping off the fluid should quite down your power steering pump.
If you discover a power steering leak, rather than simply continuing to add fluid every time it gets low, you should fix the problem permanently so you don't have to continue to check your power steering fluid level. Replacing lines or seals can be a surprisingly difficult task due to the location of your power steering pump and steering gear or rack. The easiest way to seal your leak is to do it from the inside out!

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Power steering whining sqreeching Google Search

It sounds as if you have a volume of air in your power steering system. This is causing the bubbling and also some of the noise you might be generating when you turn the wheels hard right or hard left. This air might have been introduced if your old belt was slipping so much that the pump was essentially not operating and fluid wasn't regularly circulating throughout the system. Another, more expensive possibility is that your old belt was slipping because the power steering pump itself was starting to fail, and the new, grippier belt is turning the damaged pump and showing you its lack of functionality.
The first and easiest thing to do would be to bleed the system. Open the hood and take the cover of the power steering fluid reservoir. With the engine off but the steering wheel unlocked, slowly turn the steering wheel all the way to one side and then to the other. Repeat this several times while watching the power steering reservoir. Is the fluid bubbling? If so, good, you're purging air out of your system. In general, this is easier with the front wheels off the ground, but that is not absolutely necessary (if you have strong arms).
Once you have purged the system, put the cap back on the power steering reservoir, topping it off if the fluid level has dropped to the fill mark. Now, restart your car and observe the power steering pump. Does it seem to be operating quietly and smoothly? If you touch the pump 1-2 minutes after the engine has been running, is it hot to the touch? It will probably be warm, but _hot_ is an indication that something is overheating inside the pump. Is the power steering fluid very dark colored or does it smell burned? This is also a sign that the fluid should be changed ASAP and that some internal damage may have occurred. Burned fluid isn't necessarily an indication that your power steering pump is bad. However, if after changing your PS fluid the new fluid quickly becomes dark and starts smelling burned, then it's probably time to start saving money for a new or rebuilt power steering pump.

if you have taken off the belts and there is no sound check the bearings on the pulleys wiggle the bearings and spin them any sounds come from them they need replacing. replacing oil help with the problem but a bearing could be damaged .

No, this is a power steering issue. Are you certain the power steering fluid level is correct? The squeeling noise is caused by air in the power steering pump. The leak must be repaiored, usually by replacing the pump, unless it is just a leak in the reservoir.

raise the truck up and turn the wheels left to right, you most likely have air in the lines.

Could be one of three things: Low on power steering fluid loose belt do to a weak belt tensioner or the power steering pump itself.