You have a failed seal somewhere sucking in air, the pump will always get the blame as it's a common place to fail but get some attention on your steering gearbox and the reservoir too.
I'm beginning my 6th tour of the power steering system and have replaced and rebuilt everything but the paperclip cooler and the steering gearbox. I was ignorant of how many components the system had and just fixed the issues as they appeared but if I ever buy another 80, I'm doint the whole dang thing at once! Just when I thought I was finished and nothing leaked or groaned, the gearbox input seal (where the steering wheel shaft goes in) now has a leak, again introducing air, and leaking copious amounts of fluid and groaning like mad.
I came from a Chevy background where a single belt driven unit is both the pump and reservoir replacing that single unit replacement is very simple typically fixed all leaks and it was not a common issue. The 80 series trucks are a much more complex system of 7-8 separate components all working together, not much unlike a cooling system. There's a pump, a cooler, a gearbox, a reservoir and a handful of high and low pressure hoses and clamps which ALL need to be addressed at the same time unless you want a multi-tour journey as I have been enjoying for the past 5 years.
Dig into the whole thing and fix ALL the seals and hoses at once. Don't ignore that gearbox either, it's going to show as the weak link eventually, the entire system is the same age and 20+ years is about it's life span. If you are in a rust prone area, pay attention to the paperclip shaped cooler bolted to the frame under the fan.
There's nothing horrible in there, just a bunch of rubber hoses and seals but it does take patience and homework. Be prepared to take the battery tray, alternator and possibly distributor out if you want to make your life much easier. They are hard to work around. Don't overlook the reservoir, it will leak around the crimped metal lip, not just the screw on cap. Toyota just reduced pricing on the reservoir. Bite the bullet and replace it with the hoses and seals in everything and you be much happier later when nothing is leaking!
Good luck!
