Brake Booster Repair, Series 1, XJ6

List:

The brake booster master cylinder on the 71 XJ6 is leaking brake fluid into the cabin. I assume I need a new or rebuilt unit. Does anyone have a used or rebuilt for sale in good operating condition? Alternately, I recall discussion of a firm in Virginia USA who has a good reputation for rebuilding. Anyone recall the name of that firm?

Thanks

Lou

The booster for XJ’s are readily available.
I’ve had really good luck with Centric branded parts. That’s pretty much all I can afford and have had excellent results. perfect fitment & everything is included for a successful installation.

Here’s the brake booster for a Series I at Rock Auto. Parts geek is another source I use frequently. or if you prefer a Jag specific company
SNG Barratt - booster

I’ve used brakeboosters.com in the past.

Lou:

Ordinarily, busted boosters do not leak brake fluid.

Indeed, master cylinders do!!

So, shop for the MC.

Or, a kit to rebuild it…

If your booster is leaking, it is admitting air, where a vacuum should exist. big clue, very hard brake pedal.

Carl

1 Like

Carl: Thank you for your response. This may be a a question of my nomenclature. The service manual refers to the device I was writing about as “The Master Cylinder”. What is happening is the brake fluid is somehow going through the firewall, showup where the wall meets the floor.

I some time ago, I ordered whatever repacement parts were available for the Master Cylinder. I think the kit included a valave seal, ring seal, tiping valve cover seal and tiping valave. There may have been others. I installed the parts and reinstalled the cylinder. When I pump the brake pedal (with the engine off, this is the Jaguar with the failed head) the pedal goes to the floor and eventually, brake fluiiod appears inside the car, at the base of the firewall under the brake pedal. I assume it’s from the Master Cylinder, and a result of the cylinder leaking the fluid, but that is a guess.

Lou:

Aye, that was a point of my post. clarity in nomenclature.

Tis quite clear that your MC is leaking. The condition of the bore in the MC is everything when it comes to a rebuild.

As to MC’s/ Nowadays, I see it as a risky DIY process and best to get a rebuild from a reputable source.

Carl

I agree Carl. I understand White Post re-sleeves the device restoring the original interior diameters. As a result, all of the interior devices fit properly. VOILA!

Lou

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It’s a certainty, Lou…:slight_smile:

A failed booster does nothing with the engine not running - a failed master cylinder or other system leaks, both circuits, will always bottom the pedal.

And since brake fluid is leaking down the firewall - a leaking master cylinder is more than a good guess…

Frank
xj6 85 Sov Europe (UK/NZ)
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I have had a couple re-sleeved in Stainless Steel and new seal kits fitted.

This is about the same price as a new one, but will last forever theoretically, although the rubber seals go soft after about 20 or 30 yrs

If you install another brake booster, you may want to recheck the proper length of the adjustment rod which is situated between the master cylinder and the brake booster. Also, when the booster is out you may want to renew the gasket or seal between the pedal box and the sheetmetal. That is likely the path of your brake fluid leaking into the footwell.

Thanks David and Frank:

David:

Sorry, but I was using the wrong name. I don’t plan to install a new brake booster, it’s the master cylinder that I will replace.

Regards

Lou

I should keep my fingers out of this. Clarity has been established.

One could consider the device termed here as a master cylinder as a brake booster!!

It boosts mere human muscle power to much stronger effort via the movement of hydraulic fluid. A squeeze in a way

No, I’ll stay away from mechnical leverage at the pedal design…

Apologies…

Carl

Carl:

No apology is needed. It’s Sunday and if not here, what else would we be doing.

As you know, the Jaguar Series 1 service manual refers to the master cylinder as the “tandem master cylinder” because it consists of two independent and complete hydraulic cylinders.

The same book describes the unit behind the master cylinder as the “mechanical servo unit” or servo unit for short.

Frankly I have trouble remembering two descriptions. Maybe I will just describe it as the brake system. :slight_smile:

Description is the better part of valor.

Lou

List:
Our discussion of brake system nomenclature and the use of engine vacuum to boost the stopping power of the brake system, reminded me that the first thing I need to do this spring is to check for a vacuum leak. At speeds of 20 MPH or more, the brake system works well. However, when slowing down and coming to a stop, the brake pedal requires more and more foot pressure as RPMs decrease, to stop the vehicle.

I think I recall someone suggesting use of a volatile liquid like starter fluid, sprayed around each vacuum hose connection. The idea is that the liquid would be sucked into the intake system at a vacuum leak, increasing engine RPM’s, and identifying the location.

But, the idea of spraying a flammable liquid on a hot, running engine concerns me. I don’t want to make an ash of myself.

Is the above method a reasonably safe way to identify a leaking hose?

Regards
Lou

You wouldn’t be using too much of it, and the engine is hot, but not that hot; so if you keep away from sparks you will be fine. Just in case, keep your head out of harms way. I‘ve used brake cleaner liberally on a few occasions and never was there any fire (which, if you are selective, would probably burn quick and die within a second).
I‘d not worry.

The vacuum connections would all be on the intake or at the master cylinder, and these areas are safe.

List:

Our discussion of brake system nomenclature and the use of engine vacuum to boost the stopping power of the brake system, reminded me that the first thing I need to do this spring is to check for a vacuum leak. At speeds of 20 MPH or more, the brake system works well. However, when slowing down and coming to a stop, the brake pedal requires more and more foot pressure as RPMs decrease, to stop the vehicle.

I think I recall someone suggesting use of a volatile liquid like starter fluid, sprayed around each vacuum hose connection. The idea is that the liquid would be sucked into the intake system at a vacuum leak, increasing engine RPM’s, and identifying the location.

But, the idea of spraying a flammable liquid on a hot, running engine concerns me. I don’t want to make an ash of myself.

Is the above method a reasonably safe way to identify a leaking hose?

Regards

Lou

Our discussion of the brake system nomenclature reminded me that the first thing I need to do this spring is to check for a vacuum leak. At speeds of 20 MPH, the brake system works well. However, when slowing down or coming to a stop, the brake pedal requires excessive pressure to function easily.

I think I recall someone suggestion the use of some volatile liquid sprayed around the volume hols connections. The idea being that the volatile liquid would be sucked into the intake manifold and inncread the engine RPM’s, idneifitying the leaking hose.

However, the idea of spring flammable liquid on a hot, running engine concerns me. I don’t want to make an ash of myself.

Is the above method a erasable safe way to test?

Regards

Lou

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You could use less volatile liquid, Lou - like kerosene. The point of the testing is to notice change in engine as the leak sucks ‘whatever’ into the engine - but it only works if the leak is external…

Actually, while locating the leak point is ultimately required - it’s worth while checking for an internal leak. At the initial pedal application there is a brief ‘hiss’ at the air inlet, coaxial with the pedal rod to the booster. With a steady pedal, the hiss stops as the booster repositions, and closes the internal valve - with an internal leak the hiss is ‘constant’…

There is also another point to check. Crudely; engine draws more air the faster it turns, and the better the ability to maintain booster vacuum. Admittedly, the engine uses heaps of air at any rpms, but as manifold vacuum is constantly ‘fed’ to the booster; the one-way valve is fitted to retain the highest manifold vacuum. With a failed one-way valve brake effect will vary directly with manifold vacuum. This may enhance the effects of a leak…

Working normally, very little vacuum is ‘used’ for every pedal application. So another standard leak test is, as always, to run the engine briefly - then repeatedly apply the pedal. As booster vacuum is depleted, the pedal will rise and gradually stiffen. With no booster vacuum, leaked out/failed non-return valve - the pedal is rock hard throughout…

Frank
xj6 85 Sov Europe (UK/NZ)
**

Lou,

once I encountered the same symptoms, even less violent with a loss of servo power only when rolling to a stop at the last 5 mph and virtually at idle. Nonetheless there was a strong hissing sound making it easy to find the vacuum leak. It will be in an rubber line and fairly obvious - different from vacuum leaks between carbs and intake where brake cleaner, water steam or other indicators may be necessary.

I would keep a fire extinguisher and a bucket of water handy, but wouldn’t be too concerned as on my RHD car the brake servo and the rubber line are opposite the exhaust side, but on a LHD car you should take care of your eyebrows!

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)