The brakes on the Sovereign had become significantly down on efficiency over the winter months after a lack of use. Even with the pedal pressed to the floor the brakes wouldn't stop the car sharply, so it had become quite dangerous to drive. The diagnostic display on the digi-dash had started displaying a "low brake pressure" warning.
I think after a lack of use the calipers were initially somewhat sticky, but the braking problem persisted even after giving the brakes a good workout.
My number one suspect component in the brake system was the accumulator sphere used in the hydraulic brake system used on the car. This is a sphere shaped metal part that stores pressure built up by an mechanical engine driven pump, this pressure is then released when the brake pedal is pressed, operating the servo. This is an unusual and complex design of braking system, considered innovative in the '80's a employed by a number of car manufacturers including Jaguar and Rolls Royce. The added complexity lead to reliability issues and this system was dropped by Jaguar in favour of a much more conventional vacuum servo system on the successor to the XJ40, the X300.
Having done some research on the system, I found that they can be serviced and the accumulator sphere re-gassed so thought perhaps this was the way to go. As this was beyond my experience and DIY ability I booked the car in at a local luxury classic car specialist near to me. They replaced the brake fluid and bled the calipers, in addition to topping up the hydraulic system with the special brake fluid required. The chap at the garage showed me the brake fluid that had been drained off, it was a dirty brown colour and obviously hadn't been changed in years.
The result of the work has completely restored the braking, I'm very pleased. They work better now than at any time since I've had the car, the pedal feel is vastly improved. Sometimes it's hard to know what is condition is optimal if it never has been during your ownership. Since having the work done I'm really enjoying driving the '40 again and am feeling inspired to go through some of the other issues with the car.
Now that the Sov was driving well again I let my wife have a go at driving it! At the last insurance renewal I put her on the policy as the cost was minimal. Being used to small hatchbacks she was a little daunted initially but enjoyed the experience.