My 78 Oil Leaking Pig was losing PS fluid and losing steering force in a curve.
Looking underneath, it looked as if the PS return hose was leaking. It wasnt...

The old PS reservoir cap seal was no good, it didnt take any force to screw it in place.

The PS reservoir fills up and blows fluid out when turning the steering wheel without the engine running. I found the reservior filled to the top.
I suctioned it out to COLD level, started the engine and the fluid level was so low the pump was noisy, so I shut the engine off and filled to COLD.

The excess fluid backing up in the reservoir could not be kept in the reservoir with a bad cap seal.

I made a new seal from a new tire inner-tube for a garden tractor (ATV, tractor). Find a fairly flat portion and cut:

a.) a 1-5/8" circle
b.) inside the circle cut a 1" square
c,) on two sides of the square, cut a 1/4" arc (as if two sides of a 1-1/2" diameter circle, but only two sides

This is for the PS pump on a 78 Blazer, your vehicles cap might be different, but the process is the same.

This fits on top of the original seal (with the PS cap upside-down) without having to remove the old seal. Now the cap takes noticeable torque to install.