After having my 1988 Jaguar XJS sit for a week, I returned to find poor braking performance with the pedal sinking all the way to the floor. I also found that the reservoir was dry, but there was no sign of brake fluid leakage under the car.

I've done some searching, and it sounds like the master cylinder is the obvious culprit, but that's been rebuilt within the last year. Bleeding the brakes hasn't yielded any positive results, either.

I'm at a loss, and my shop is just as confused as I am. Any tips on what to try next or where to look next would be appreciated.

The car is not equipped with ABS brakes, and brake performance was acceptable before the rest with no fluid loss.

 

Fairly typical, rebuild your rear calipers and try another master. A slow leak on the rear caliper will be very hard to distinguish from a differential leak.

These iron masters rust quickly and once the piston travels to an unused area of the bore it rips up the seal.

 

Top up the reservoir, ask the garage to put the car on a ramp. Someone presses the brakes hard and someone else looks at the calipers and brake lines. If there is a leak, and it sounds a big one, they will see it. They might have to bleed the brakes first though if the lines are full of air.


I agree with ICS America that the master cylinder sounds to have failed, unless the system is totally empty of fluid, so the above test, with topped up reservoir should settle the matter.


A long shot: I have a vague idea that the fluid can sometimes/somehow get into the vac servo (booster) from a master cylinder leak and from there get sucked into the engine and disappear, or just pool up in the booster. No idea if this is true or not. But you could remove the master cylinder and see if fluid seems to be escaping from the booster/cylinder join area.

 

Can't find fluid, its totally failed and happened while the car sat?

Is it leaking into the booster?

 

I've see this too... but in my experience the some of the fluid seeps out the back of the master and drips along the booster's face and on to the body,along the frame rail and never actually makes it to the ground as the fluids gets absorbed and softens paint and rust proofing. The tell tail sign of this are a rusty booster and softened paint visible when the MC is removed.

 

Thank you for your replies. I didn't want to accept that the master could have gone again, but it looks like the only plausible explanation for the behavior we're seeing.

By the time I'm driving again, I'll be needing snow tires, hah.