They are quite dependably garbage. There are two issues with the OEM XJ-S pre-ABS master cylinder: FIrst, the fluid level warning switch looks like it was designed by a child with his Lego set. It makes other Lucas products look reliable. Even if the stupid cork float doesn’t cause problems, the contacts themselves might, getting corroded and failing to make connections. And this is a serious safety concern; it’s important to know when your brake fluid has left the building, preferably BEFORE you really, really need to stop.
The other problem is that brake fluid should not be exposed to the atmosphere because it absorbs moisture from the air, which lowers its boiling point and causes corrosion to brake components. Better brake reservoirs have a rubber diaphragm that sits on top of the fluid and allows the level to rise and fall while still sealing it off from the air above. Of course, the reservoir in the XJ-S is simply vented to atmosphere, so your fluid is absorbing moisture from the day you changed it.
The fix is simple: A reservoir from a Mitsubishi. There are apparently even a couple different Mitsubishi reservoirs that will work, a round one and a rectangular one. The round Mitsubishi reservoir has the rubber diaphragm, and it has a reliable fluid level sensor. Cost is nominal. You’ll have to fab a bracket to mount it. Perhaps the biggest issue is the brake line connections are seriously larger than on the Jag, 10mm I believe, so you’ll need to adapt the hose sizes down to connect to the XJ-S master cylinder.