This is about my '85 300TD.

My power steering pump had been leaking for a year or so. I'd turn at a stop and hear the groaning at each far range of steering. I saw the frothing in the reservoir in unison with the groaning.

This year, as I started driving the car again, the power steering had been offering no help, hard to turn at low speeds. Last week, I resealed the front and back of the pump, following Stretch's DIY thread.

Yet, the pump still seems unresponsive. Still hard to turn the steering wheel.

Any thoughts? I've never let the pump run dry, always kept a bottle of fluid in the car.

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"Blessed are the cracked, for they shall let in the light." - Groucho Marx

Did you replace the filter & bleed the system?

I imagine you checked the belt tension. Yes make sure there is no air in system, the problem could be in the box binding up you could play with the adjusting screw to see if that helps. On mine mine steered a lot easier after I resealed the box and all new fluid in and most recently put proper tension on belt with a new belt.

There are specs on how much effort is required to turn the steering wheel. Effort increases and peaks at the center of the range of motion. The effort curve looks like --^-- but more like an inverted U. I think the max effort is 8 inch pounds.

Some vehicle manufactuers say to take the measurement with the steering box on the bench. My Dodge Ram is an example. Mercedes says to connect a torq wrench to the center of the steering wheel and lays out a method in the FSM.

To trouble shoot, you want to disconnect the the steering box from the linkage and determine whether all the parts from the box up to and including the steering wheel are within specs. Disconnect the pitman arm from the likage at whatever joint is easiest. On my Dodge this happens by removing the pitman arm from the steering box.

If the upper required force is too high, disconnect the steering shaft and measure the box alone and the steering wheel by itself. You may need to jack the front end up and make sure that the wheels and linkage turn freely also.

I don't have access to the FSM until I get XP set up in a virtual box. It won't run on win 10 or I'd post the pages.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.