Steering gear box???

Diagnose it before throwing parts at it. Its cheaper.

If I were to randomly throw parts at it I would not start with the pump. Hydraulic pumps won't start and stop pumping at random unless there's a built in relief that's sticking.

I would start down at the box. A good shop will have a flowmeter they will plumb in and load up the system to see what's working and what isn't.

 

10W40 ? I've always used ATF or straight 10W. But I haven't driven a Frightliner since the cabover days.

 

Atf or power steering fluid.

Atf is cheaper though.

Never heard of using a multi weight oil.

 

IIRC Peterbilt used 15W-40 engine oil as factory fill for the power steering system.

 

I agree with @AModelCat. PS pumps don't fail that often either.

Were it mine, and I was keeping the truck, I'd personally replace both, and PS lines too.

The last pump I bought was 300-325$ ish, gear box 6-900$ roughly.

A good deduction and no worries about failures on the road

 

Like @AModelCat states "Diagnose it before throwing parts at it. Its cheaper."
It would not take a shop long to check out the front end. It could be something like a worn/binding joint in the steering shaft. Have the front end checked for worn/loose parts and grease everything. You mention a clunking noise which would be something other than the steering pump or box. Then move on to checking the hydraulics. If it has a belt driven pump make sure the pump is secure.

 

Yes. But most of the Frieghtliner’s I’ve seen all the way back to the White days called for GM type Dexron ATF. Unless the reservoir has a label saying motor oil I’d bet the fluid may be the problem. I agree.

 

I was always told it didn't really matter which oil is used, just as long as the two aren't mixed. Don't really know how much truth there is to that statement though.