I had the same experience with my 2003 S-Type 4.2 ...

I noticed an annoying squeak from the power steering and thought it was low on fluid. Finally, I decided to do something and pulled into a convenience store and bought a bottle of power steering fluid.

It was dark and I decided to top off the fluid right in the parking lot. I just wanted to get rid of the squeak ASAP..

About 3 weeks later, the squeak was still there and I decided to do a little more research on this forum. That's when I discovered the PAS additive (Jaguar p/n C2S48887) that fixes the problem.

That is also when I realized I had poured the power steering fluid into wrong place -- I had added it to the brake fluid!

At this point, the brakes were still working fine, but I was alert to any potential problems. A few days later the brakes started getting mushy and then failed completely.

When my mechanic opened the brake fluid reservoir, he noticed that the rubber seal under the cap had been melted by the power steering fluid. He tried to suck out the power steering fluid (which was mostly on top), but by this time the damage had already been done and the brakes no longer worked.

Fortunately, I found a rebuilt master cylinder for around $100 (Cardone 11-3294) that was exactly the same as the original. My mechanic sucked out all the old fluid before doing a flush (as recommended above) and replaced the master cylinder. I also found a new Jaguar brake fluid reservoir cap (Jaguar p/n XR824768) for $28 on Ebay to replace the old one with the melted seal.

My mechanic inspected everything and could not find any damage besides the master cylinder.

BTW, I finally got rid of the squeak. This time I made sure to add the PAS fluid to the "real" power steering reservoir.

It looks like everything is back to normal and I will be paying much closer attention to where I pour any new fluids.