I did a search on the topic and didn't come up with any helpful results, so I do apologize if this is a duplicate.

Information:
1999 F150 XLT
V6
5 Speed Manual
Original pump

I've noticed over the past few months that the power steering hisses almost like when the wheel is turned to its max even when I'm just turning normally. The wheel still turns easily and there are no "hard spots." I checked the fluid level and it's full. The last time I added fluid was many months ago. I'll make the assumption that it's on it's way out just like every thing else on the truck

Any ideas?

 

Try changing the fluid. Easiest way is with a turkey baster - suck all the fluid you can out of the reservoir and replace it. Do not use power steering fluid in Fords, use Mercon V ATF, synthetic preferred.

 

I've already got mass stocks of power steering fluid available to me. I'll try what you suggested and replace it with power steering fluid only because I have so much of it, and if that doesn't work I'll try the Mercon.

 

Use Mercon V only!

Don't use anything else... since you said you already filled it up with something else, I'd say you learned the hard way, now you need a new pump.

Unless you want test it, and see if swapping the fluid with Mercon V stops the hissing, be my guest... but I have no faith in that pump anymore.

FYI: A powersteering pump is a simple hydraulic pump, like the hydro pump on a tractor, excavator, etc... when the manufacturer tells you to use only 1 type of fluid, you better use it. Why? It will wreck the pump's internals along with the seals. Sometimes they will seize up due to the fact that when mixing different types of fluids, they have a chemical reaction that is bad for the pump. Same applies to brakes, if it says to use DOT4, you better use DOT4 fluid. Not just any old brake fluid laying around the house or shop.

 

DO NOT USE power steering fluid!!!!!!

If you look in your owner's manual (if you don't have it, download it) it says MERCON ATF. Mercon V has replaced Mercon, Mercon is no longer manufactured.

 

I was observing it a bit closer today and it seems to only happen sometimes. I'll just use the Mercon.

 

what about AMSOIL ATF?

 

Amsoil ATF is an EXCELLENT choice.