Xj6 series one brake booster leaks?

Hi Gents, I am new here. Still looking for buying my first classic xj.
I went to inspect and test drive a series one in the weekend. When the car was on the road, it got attention from almost everyone. I had a lot of fun in the 10 mins run, but also found it a bit scary.
It has a very soft and weak brake. I had to stand on it to have it stop. When reverse gear was engaged, the brake did not feel sufficient to hold the car still. And there was a continuous air blowing sound when my foot was pressing the brake pedal. Does it sound like a brake booster leak? Would it be an expensive fixing job?


There is a vacuum connection from the brake booster to the inlet manifold. If you disconnect this at the booster while the engine is running ( it just plugs in, so should remove easily) you should hear it hissing and the engine speed rise. If the speed does not increase it suggests that there is a vacuum leak within the booster and perhaps the idle speed has been adjusted to compensate.

Sounds like the booster is shot (although it may be a hose).

These can usually be repaired for anything between $US200-400

They are known as Girling Supervac 100, but are the same inside as a Bendix

They can only be repaired by an experienced brake specialist that knows what they are doing

If you Youtuube “overhaul Bendix brake booster” and look at the type that has the crimped shell, you can see what is involved

I would ring and ask locally whether someone can overhaul the said booster

They often fail because the brake master cylinder rear seal is leaking

imo, you should 100% get the brake master cyl overhauled, unless you have documents that show it has been done within the last 10yrs

Sure does, Fan Yi…

If there is a continuous hiss when the brake pedal is applied, but not otherwise - the booster is leaking internally and must be replaced…

xj6 85 Sov Europe (UK/NZ)

Thanks Kevin. That was the first series one I inspected. So I dont know much about where to look and where to check yet.

Thanks Tony for the info. There are a few Jag specialists near my area (Brisbane). Will definitely get their opinion.

Yes Frank, when the brake pedal is applied, hard or not, the hissing sound is always there.

If otherwise the car is in good shape this is a minor fault and easily repaired.
Why don’t you post some pictures?

Brand new Series 1 booster for $269

This is from an earlier post I did on brake boosters and might help explain what you experienced.

I hope that this will give a better idea as to what might be going on with your brake booster.
As you all know, the booster is divided into two chambers that are separated by a diaphragm. The chamber that is closest to the master cylinder is connected to the vacuum supplied by the intake manifold or a storage reservoir if fitted. Under normal operating conditions this chamber should always have a vacuum. The back chamber, under normal operating conditions, ie, when there is no input from the brake pedal, will have the same negative pressure or vacuum as the forward chamber, resulting in no force being exerted on the brake master cylinder rod. In this state, the front and rear chambers are connected by one or more passages.The system is in a stasis or equilibrium.
When the brake pedal is depressed, there is a system of valves and springs that move and do two things at once. The passage between the front and rear chambers is closed, isolating the two chambers from each other and another valve (they are usually two parts of the same valve) opens allowing atmospheric pressure to enter the rear chamber by way of an inlet that is located concentrically around the input rod. There is usually a rubber bellows with holes in it closing off the input rod end of booster. Inside there is a filter to keep dirt from entering the valve system.
As atmospheric pressure enters the rear chamber, it exerts a force against the diaphragm due to the lower pressure on the front side. This force is what assists the braking effort.
Between the the input rod, which is directly connected to the brake pedal and the diaphragm plate and the output rod going to the master cylinder, there is a reaction disk of rubber or some other deformable material. This reaction disk is critical to the modulation of power assist supplied based on pedal pressure input.
When the brake pedal is released, the valves return to the resting state and pressure is again equalized between the two chambers. A heavy spring in the forward vacuum chamber and springs around the input rod force the diaphragm and rods back to the rest position. There are also springs in the mater cylinder itself that return the pistons to the home position.
Based on this description of operation, we have to surmise that the problems you are describing indicate a faulty valve system within the booster or a defective diaphragm that is allowing vacuum to be drawn through the atmospheric port. This will reduce the power assist and it will require more of an effort to stop the car. You say it had a soft pedal which could also indicate air in the brake system and or a faulty master cylinder. When a booster isn’t working properly it usually as a very firm or hard pedal. Whatever it is, replace the booster and check the master cylinder.
A leaking master cylinder does not necessarily cause booster failure. What it will do is draw brake fluid into the forward vacuum chamber and then into the intake manifold.

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One across the bay in Oakland on International Blvd.


Hi Aristides, the car has a straight body, just like most cars in Australia, no rust issue. That is why I drove 250km to inspect it. There are more minus problems with it. I took some pics, but didn’t asked the owner for permission to post images. Since it is still listed for sale on local classifieds, I would hold from posting the images for now. I inspected a few more S1s and an SIII in the last few weeks, and finally put deposit on a SWB S1. Will become an owner very soon.

Thanks Greg, I have saved this website.

Thanks a lot for the details! I kept thinking about the car in the last 2 weeks. It is one of the rarest model. However, after a period of cooldown, I decided to give up due to multiple issues and modifications on the car.