Our power steering has gotten very hard to turn lately (especially at low speeds like in the driveway). There is a small leak of fluid at the end of the rack, and the fluid often gets low, which I have to keep refilling. But, I've had that problem for years without any loss of function. Due to the constant leaking and refilling, the fluid is somewhat foamy with a lot of air bubbles. This also is not necessarily new.

So, what has changed so that now it's hard to turn the wheel? Is it air in the system due to the air bubbles? Are the seals on the piston shot so that the fluid is just pushing by? Is my pump not generating the necessary pressure?

What are the likely sources of the problem, how do I test to see which it is, and how do I correct this?

Thanks for your input.