Recently I had a vacuum leak and while inspecting all vacuum tubes I actually broke the nipple of the Fuel Pressure Sensor part AJ87977 where the vacuum tube connects to it. It was extremely brittle and snapped right off as I tried to take the rubber end of vacuum tube from it. I started searching for the Jaguar part number AJ87977 and stumbled upon a Ford cross reference number for same exact part labeled to fit Ford and Lincoln engines in same year range as my 2004 Jaguar 3.0. I found the following Ford part F8CF-9F972-BH on an eBay listing for under $25 with free shipping. It was listed as used part but when I received it it's new old stock and never been installed. It looked identical to the one I removed from engine and worked perfectly.

Here's how to replace the sensor. First go to fuse panel inside car and remove fuse 17 which is fuel pump then crank engine and allow it to go dead which will take pressure out of fuel rail. Then disconnect battery. You will need to disconnect all electrical connectors, remove the air breather bellows, and loosen bolts from the upper intake manifold. No need to disconnect throttle body from manifold. Lift the manifold up and tilt it towards coolant tank just enough to gain access to the fuel pressure sensor area. I propped my intake up with a short piece of wood while doing this replacement. Don't forget to wad cloth or shop towels into each intake opening to make sure nothing can fall into open engine.

You need to then place shop towel below the fuel pressure sensor to catch what little fuel will leak out once removed. Disconnect the electrical plug on the sensor. Also remove the vacuum tube off nipple. Then using a 7mm gear wrench you will take off two bolts that connect sensor to fuel rail. Gear wrench makes it easier but regular wrench would work. Very tight quarters for access. With firm pressure pull the sensor out of end of fuel rail -it's very tight with two orings. There will be a little fuel that leaks but you really need to make sure to run engine with fuse for fuel pump removed to relieve pressure first.

Insert new part in reverse steps, connect electrical plug and also vacuum tube and refit the upper intake manifold. You just replaced your fuel pressure sensor. Reconnect battery , replace fuse, turn key to run only and allow fuel pump to charge system. Crank the engine and check to make sure no fuel leaks.

 

Here is photo of the end of vacuum induction tube where it connects to the barb on fuel pressure sensor and snapped off. The rubber connector is fused to that plastic barb and with age and heat essentially welded itself

 

I've had a different failure with my fuel pressure sensor. After 150K miles on the car plastic parts start to fail and become brittle as you mentioned.