Poor brakes on 1987 model

Recently bought a 1987 2.9l model. The brake pedal is very soft and she wouldnt stop soon enough in a crisis.
Accumulators cost €350 each so i am reluctant to buy. Would i need more than one and has anyone done such a job successfully?


Before purchasing a new/replacement accumulator have you tried bleeding the brakes?

George …

$381 for a brake booster :dizzy_face:. On a Jag (especially an older one that’s no longer supported in the after market) it’s called “Pay to Play”.

The cost of maintaining the original hydraulic brake system is the number 1 reason that a lot of us have converted to vacuum boosted brakes. The number 2 reason is the simplicity of the vacuum system. The number 2 1/2 reason is no more green fluid leaks.

Probably more information available on this system in the forum archives than any other.

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My car is a mix of 2, the car I have decided to keep is a 1987 XJ6, imported from England back in 1992, but most of the running gear is from a 1988 Australian version sovereign, it had the recall done and the self levelling suspension was removed and a vacuum booster was fitted. Although both were fitted with the 3.6 engines, I opted to use a Ford 5.0l EFI from an 1992 Australian LTD

But as I said it has now got the Ford V8 EFI now.

If posters looked after their hydraulics systems they wouldn’t leak and testing and recharging the accumulator sphere is quick, easy and cheap.

You won’t match the performance of a power hydraulics system by removing the SLS and power brakes.

There’s new accumulator in this post in Classifieds


Dieselman … just a bit of a friendly disagreement

— “If posters looked after their hydraulics systems they wouldn’t leak”

So how exactly do you “look after” your three decade old hydraulic system to prevent it from leaking ? Is there some preventive maintenance that can be performed other than just repairing it every time it leaks. which in my over 30 years of ownership was often.

— “You won’t match the performance of a power hydraulic system …”

I strongly disagree. If DONE CORRECTLY the vacuum boosted brake conversion works every bit as well and is simplicity in itself. No pumps, no reservoir, no fluids. Of course the nightmare rear self-leveling shock absorber system has to have been removed (which of course was Jaguar’s answer to repairing it).

Just out of curiosity how many of you out there that have owned your car for more than a few years still have the original self-leveling system?

Jaguar claim a 450% improvement in braking ability when the power brakes are fitted, compared to a vacuum-servo system.

Hydraulic systems shouldn’t be leaking a lot, they are designed to be reliable given regular fluid changes and checking over.
I currently run three 32 year old cars with complete high pressure hydraulics systems, for steering, brakes and suspension…no metal springs.
They don’t leak and I have the, leak free, MOT inspections to prove it.

Simple physics says a metal spring system cannot compete with a hydraulic/gas sphere system, as the gas sphere provides a constantly variable spring rate, proportional to the load it is subjected to, allied to which it acts as a natural damper to oscillations.

I think it would be good for Jaguar owners to repair and persist with their systems, they are superior, which is why they cost a lot when new.
Accumulators can be recharged for very little cost and alternate suppliers can provide a suitable accumulator significantly cheaper than the Jaguar parts suppliers.

Dieselman …

Wow, now that’s an amazing claim (even for the Jaguar manufacturer to make).

So the braking ability of my car is 450% less than before … I DON’T THINK SO (and please don’t tell my wife that or she’ll never ride with me again :cowboy_hat_face:).

Not to get into too much detail remember the “braking system” remains absolutely the same after the conversion. Nothing changes starting after the input of the pin into the master brake cylinder all the way down to the brake pads applying friction to the wheel rotors. This of course includes the operation of the anti-skid system. All the same, nothing changes.

The only thing that does change is simply the “assist” being provided to depress the original brake pedal which drives that original pin into the original master brake cylinder.

Actually if you think about it the “braking ABILITY” of the system wouldn’t change even without any pedal assist, hydraulic or vacuum (Remember the good ole days of manual brakes) provided you were strong enough to depress the pedal.


Can you quote the source please?

My source is Jaguar XJ40 power hydraulic system - Rens Swart

Second section " Principles of the power hydraulic system"

I’ve no idea where his source is, or how reliable that is, but his claim is that Jaguar say the power system is 450% more effective.

Another poster has reported songy brakes after exchanging the power brakes for servo ones…which does make sense, the firmer pedal feel is one of the features of power brakes.
XJ40 3.6 spongy brakes - XJ40 - Jag-lovers Forums

One item overlooked for the SLS is that it provides a side to side stabilising function, whereby if one rear wheel rises due to a bump, due to fluid flow across the car and both rear spheres always having equal internal pressure, 50% of the suspension upward motion is transferred to the other side as downward motion.
This lifts the body of the car keeping it more level side to side thus reduces rocking.

My preference would be to maintain and keep the power brakes and suspension systems, (to me) they are simple and should be reliable.
I note rens swarts comment:
" See my other pages for a description and fault finding of the self-levelling system. There is a lot of misunderstanding and pure nonsense on the web on its operation (we possibly call it fake news , nowadays)."

Watch the Omega and Xm in this short clip to see the reduction of body movement due to interlinked hydraulics. You only need to watch 20 seconds.

Pressurised hydraulics is a luxury feature and the car won’t be the same with them removed. I would endeavour to keep them working and on the car.

Sorry but this is the copy and paste from that article;

“When the XJ40 was introduced, Jaguar claimed that the power hydraulic system provides a 450% increase in potential boost energy over conventional vacuum systems.”
Increase in potential energy being the operative word.

Ok thanks for the correction, but it still means Jaguar state the power braking system has the potential to deliver 450% the energy to the brakes than a vacuum servo system.
Is the potential energy delivered simply not dependant on the braking force required to stop the car?
The driver presses the pedal whereby the force is multiplied, in the case of the power brakes system, up to 4.5 times greater than a servo system.

Trying to perform an emergency stop from 100+mph isn’t the time to find you have a shortage of braking potential.

Jaguar must have thought it was a worthwhile upgrade, otherwise wouldn’t have spent the money on fitting it.
Other manufacturers have come across the same issue and resolved it in different ways, Mercedes employed a twin servo to increase force multiplication, before installing power brakes (SBC).

I think taking a system designed for hydraulic power boost and installing a vacuum servo could well lead to problems.

From reading further in rens swats pages he does list his reference material and also demonstrates the suspension problems, which appear to me nothing more than worn out accumulators.

His video here, where he demonstrates the “worn out shock absorbers”, is clearly demonstrating flat accumulators, which are the springs, not the dampers.
Shock absorbers are a spring and damper combined system, not one part, or the other.
If not too far gone they just need re-gassing, if left until they rupture new ones are required.
Re-gassing is very cheap.
Failing rear shock absorbers of a 1993 Jaguar XJ40 with ride levelling system (SLS) Jaguar XJ40 on Vimeo

I think there is significant misunderstanding of how the pressurised hydraulics systems function and a lot of fear has been spread.
To me, both the brake and suspension systems appear robust and elegantly executed and as such I think owners should persist with keeping them operational.

Even the web site poster rens swart is confused to the operation of each part of the suspension.
Springs provide movement, dampers control oscillations.
What is seen in his video is there is a lack of suspension movement, to the point the tyre has become the spring.
Lack of movement has increased the resonant frequency of the suspension, hence the additional bounces. stiffer springs always require stiffer dampers. Greater movement would remove this as the damper could then function again.

Without well gassed spheres no hydraulic suspension system can function properly.
Spheres with no gas pressure cause the hydraulics system to rupture due to huge shock loads being imposed.
The first thing to do with one of these systems is to check the gas pressure in the spheres.
A simple bounce test is a reasonable guide, once experienced, but there is no substitute for actually pressure testing the spheres.
Re-gassing when they are below 20Bar can be risky as the membrane may have already been damaged by contact with the filling plug.

I built my own sphere pressure tester and gas refilling rig.
I would need to make a new adapter for Jaguar spheres, but would then be prepared to test Jaguar spheres if running on mineral oil.
I would need to build a completely separate test rig for ones running on brake fluid, to ensure no cross-contamination.

Refilling spheres isn’t hard, I would need to make a new adapter to fit the filling plug.

Sorry gents I’m still trying to get my head around this …

“Jaguar claimed that the power hydraulic system provides a 450% increase in potential boost energy over conventional vacuum systems.”

Ok so we’ve narrowed it down to Jaguar talking about “potential boost energy” and not “braking ability”, a big difference.

So my question is if a vacuum booster is capable of locking up the brakes in a high speed emergency stop (I had to do just that not too long ago at 80 mph to avoid an accident) I would say that it provided 100% of the “required boost energy”. Isn’t that correct?

Once you reach the effort required on the brake pedal that’s required to lock up the brakes they are locked up and even IF the hydraulic system is capable of applying 450% more “boost ability” above that required it doesn’t accomplish a thing.

Again all we’re talking about is the augmented energy required to depress the pin going into the master brake cylinder.

Wow all this technical stuff makes my head hurt … I’m taking a brake :sunglasses:

Screenshot 2024-05-20 at 12.27.52 PM copy


I suspect the reason you locked the wheels (apart from your ABS not working), is because you didn’t modulate the brake pressure to bring the weight of the car to bear on the front wheels before pressing the brake hard enough to lock the wheels.
Due to aerodynamic forces, at speed the front wheels are unloaded more than at standstill, so lose grip readily.

I understand that in a panic situation it may be difficult to achieve, but the correct way to brake is to apply enough braking force to cause the front wheels to be fully loaded, then give them full force, at which point you won’t lock the wheels.
The additional braking “potential” can then be brought into play applying (according to Jaguar) 4.5 times greater multiplication factor to the pressurised brake fluid, thus significantly more pad clamping force, which should stop the car more quickly.

Additionally, a power braking system may also provide a quicker response than a vacuum servo system, as it is pre-primed.

I have cars with both vacuum servo and full power brakes. When I change to drive one of the servo assisted cars, I really notice just how much effort I need to put into braking, especially if at speed.
I can brake to cause the wheels to reach the point of locking, but it takes concentration and effort, the power braking systems give quicker response and achieving full braking effect easier…meaning less leg and mental effort.

What Jaguar are saying is that to achieve the same maximum braking force (enough to lock fully loaded front wheels), the driver needs to provide 4.5* the amount of leg effort.
That probably also translates into shorter stopping distances for a given leg force, at any speed, but especially at high speed as the necessary energy dissipation rises with the square of the speed.

Locking fully loaded front wheels at speed takes a considerable force and is probably only possible if the car has worn out suspension dampers failing to keep the tyre in contact with the road.

In short, I wouldn’t delete the power brakes and SLS system, it will be easier and cheaper to fix what is already there, which in my experience, both systems are desirable features.

Are the disks, calipers and pads the same between the powered and servo systems?
It’s possible, as designed, the powered system has smaller components as it can produce greater pad clamping force.

Hmmm…file this under “Did you ever wonder why?” Along with, why does my daily mutivitamin contain 200% of the recommended daily requirement of certain vitamins/elements? Hmmm…

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XJ40 forum is like a box of chocolates, some soft :slightly_smiling_face:, some hard :face_with_diagonal_mouth: and some nuts :woozy_face:

Bottom line is how do the two shortest stopping distances compare? Locked brakes mean longer stopping distance generally, because the coefficient of friction of tyre to road is inferior to that of disc pad to disc, ergo shortest stopping distance is the point just before tyre lockup. An increase of booster pressure or efficiency of 450% likely means it’s easier to lock the brakes perhaps, but that won’t translate to a shorter stopping distance, unless I am mistaken.
Anyone have actual data?

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Though, as already said, once the point of threshold braking has been reached, it’s quite probable that the servo assisted system can’t readily provide sufficient pressure to reach the point of continuing threshold braking, whereas the power system can.
Not all Jaguar drivers go to the gym and leg press 400Kg.

Stabbing the brakes hard while the tyres are unloaded will lock the wheels, but loading the front wheels sufficiently will stop that happening at which point the shortest stopping distance will be achieved by the brakes that can maintain threshold braking.

When cars had drum brakes and no servo one could lock the front wheels in sudden emergency braking, but servo assisted brakes dramatically reduced stopping distances, due to being able to create enough pressure for the pressure to be near, or even at that required for threshold braking.
Jaguar acknowledge that a heavier car required greater braking pressure per unit of retardation ability. Faster speed compounds this by some factor.
I think posters are confusing braking power to reach threshold braking and brake power required to overcome tyre friction on a lightly loaded wheel.

The formula for calculating brake efficiency includes the weight on the tyre.
This formula is used for the MOT test, which it’s possible the modified systems here would fail.


During the test, the brakes have to be gradually applied so the tyre keeps loading up and increasing the friction between it and the test rollers.
Due to tyre friction being high, the efficiency required is much greater than that needed to lock a wheel on a speeding car.
We don’t know the coefficient of friction of tyres fitted to posters cars, if low then lock-up is inevitable, but better tyres should produce shorter stopping distances if the brakes can achieve threshold braking pressures.

Excerpt from test:

  1. Gradually depress the service brake again, this time until maximum effort is achieved, or until the wheel locks and slips on the rollers. Stop the rollers.
  • Record the reading at which the maximum braking effort is achieved and whether brake “lock-up” occurs. Stop the rollers if they have not stopped automatically.

I suspect Jaguar have all the data needed to explain spending more on the power braking system.
One has to ask, why would a commercial company spend more on a braking system which could only achieve the same result and why do car companies keep installing costly power braking systems?

Jaguar will have designed the brake system to be able to cope with maximum speed, gross vehicle mass and repeated heavy stops.
In the Teves III thread we covered that the power brake system has a greater reserve of applications and quicker recovery time than a vacuum-servo system.
If any modified system can’t match that performance it is an illegal modification, in parts of the world that have regulations.

Here’s a quick question for our XJ40 UK friends …

As I understand it your vehicle MOT is one serious inspection. Can you pass with a vacuum brake conversion ?