If this is the same car you replaced the turn signal switch in, here is a thought.

I recently replaced the switch in my D150, pretty sure it is the same switch.

After a short while it developed an awful squeak or screech when the wheel is steered, left or right, coming from the column.

Thinking that I had somehow screwed up the column jacket where the wheel might have been rubbing, I drove it that way for a while, finally pulled the wheel last weekend.

The contact post for the horn, spring loaded, is made from a different, more abrasive looking material than the original, and LOOKS to protrude more toward the wheel, with what feels like a stiffer spring. Slightly.

It had worn a groove into the horn contact ring of the wheel, not deep but definitely "catch a finger nail" deep.

I used a little "grease" on the contact ring, noise gone, horn works.

Is it possible this is dragging on yours?

Try jacking the car up, support it on jack stands under the lower control arms, and perform the left/right tests.

I am thinking some one has "adjusted" the steering box, and has it too tight. Small adjustments can make a big difference. If this is the case, it will feel like you have to move the steering wheel over the nose of a cam lobe when turning away from straight, left or right, and will make the car wander down the road, trying to stay on the cam nose.

Figuratively speaking.

My experience with pumps is, either they work or don't. Allowing for the leakers.

There IS a pressure control valve in the fitting behind the big nut on the pressure side, and it CAN be shimmed to cut pressure, perhaps the PO tried for "firm feel" and got carried away ...

All just food for thought ...