fld120 steering problem ! pls someone help me !

So you had this problem before, and after you changed the p/s gear box? Did you have your gear rebuilt? Very rare to have the same problem after culprit part has been changed! But, I hope to God you have found it! (I'm not religious, by the way... just saying)

 

If the gear is bad again I recommend a complete system flush, all new fluid and filter. You should take the p/s reservoir right off and clean it completely out too. A lot of sediment can build up at the bottom in many types of reservoirs. You may have a bunch of pieces of an oring or seal that broke down and floating around reeking havoc!

 

Unfortunately you would need to disassemble the gearbox to confirm the problem and that would void the warranty. Are there any traces of debris in the filter? The filter being on the return side would allow trash to go through the gear before filtration. Does the truck have a PS cooler? Is there a possibility that one of the PS lines has developed a fissure in the inner lining holding trash and then allowing it to enter the system after it has been flushed?

 

When you replaced the gear box did you set it up correctly? You have to set the plungers in it. You'd have to read the instructions as I can't remember exactly how to do it but I do know we had to do this on my dad's truck. We didn't do it right the first time and the steering was sticky and wouldn't turn all the way to the left.

There should also be an automatic bleeder/breather on the gear box, make sure its there.

Think someone already said this, but make sure there is nothing blocking the passage ways for the fluid. They come with those plastic inserts to keep dirt out.

I'm thinking you have something wrong with the pump itself, not the gear box...

 

You could try ditching the TRW and using a Ross box. Disassemble/flush/clean the whole system before putting it back together.
You could grab one from a junkyard just for sh--- and giggles.

 

Sounds like you got air in the system. Probably from a seal leaking in the pump. Every old straight truck I ever drove did this at one time or another. My old '83 brigadier dump does it now. No more than I use it I just deal with it but it don't go far and usually on gravel.

 

I wonder if maybe the p/s lines or a fitting is too small and restricting flow?