Just as a thought... How old was the brake fluid or better has it ever stood for a while (few years)? Idea: possibly over pressure due to a torn O ring Inside the system from accumulated rust...

 

It needs to be searched but I remember reading a poster said that a ball from a Saab or Mercedes fit perfectly on the XJS...and more reasonable $$ too. I don't have a Jag model that uses the nitrogen sphere but I have had an Audi that used one and my 88 BMW uses one as well. I would highly suspect the o ring that seals the ball to the pump body. As said, the ball has trapped nitrogen of around 1200psi on one side of the internal diaphragm. When the pump is charging the system, the 1200psi of the nitrogen charge plus the pump pressure plus your foot pressure can raise the working pressure close to 2500-3000psi during a panic stop.
Here's something I found on an Antique site: https://forums.aaca.org/topic/282942...e-accumulator/




Says 1200 psi. This not for an XJS specifically but they are all around this pressure.

 

Hi Daim

Brake Fluid 'New' and changed every year just as a matter of course, because as you know even the slightest little bit of microscopic 'Crud' can really mess with the Valves inside the Master Cylinder Actuator

 

Hi Carsandplanes

That was 'Mega Useful Information' so Thank You for that!

Just off out to pick up a New Black Ball from some Kit Car Friends of ours, who when I described what happened reckoned that pumping the Brakes to the Max, may have ruptured the Bladder, causing the Ball to fill with Brake Fluid

Which then with the Brake Pump running as well as the added pressure of the Nitrogen Gas (no longer inside the Bladder) must have forced its way past the 'O' Ring and then turned it into the Brake Fluid Sprayer from Hell!

Adding that I was unlucky as they had never heard of that ever happening before

Except that they have Now!