Power brake assist S1

Hi Folks,

I have this 1972 XJ6.

When driving, when you first hit the brakes, the assist is normal, but rapidly drops to almost nothing.

It looks like someone swapped in an XJS master, and a new aftermarket booster.

I have good manifold vacuum. I can’t pull a vac on the booster, although sucking and blowing show no leak.

I notice the installer used washers between the master and booster. It seems this conversion may not have worked well.

When I push the brake pedal, if feels notchy too.

Also, when the engine’s running nicely at idle, and I push the brake, the rev’s drop a little and the engine chug chug chug’s. I assume a vac leak would make the rev’s rise.

Maybe they put in the wrong brake push rod?

Any ideas?



not sure whether I get your symptoms correctly: what exactly is a “notchy” brake pedal? :slight_smile: Also, when you say you lose power assist during braking, does it change with revs or with duration. May sound stupid, but on an empty road you may lift your foot, switch to N and keep revs at, say 2000 RPM, to find out whether you still have the same effect. If not, the problem is vacuum related. Normally, vacuum leaks hiss, but then, what is normal in an engine bay anyhow …

Another thing: I never heard of brake operation affecting engine operation, except for once when brake fluid from a leaking MC was drawn through a torn diaphragm in the booster into the intake. Must have been a frightening experience for the driver as it was a Diesel engine and irrespective of the gas pedal the engine would continue to speed up running on brake fluid … or so I was told …

Good luck

75 XJ6L 4.2 auto (UK spec)

Yep just google run away Diesel engines and ‘enjoy’ ?

Hi Rob. I doubt if I can help with anything you can’t figure out, but I’ll try anyway.

What do you mean by the above? If no leak, then you should be able to pull a vacuum? I can’t see the needle on the vacuum gauge in the photo, BTW.

Just to confirm, I assume that the in-line valve near the intake manifold is there and functional?

Not sure there’s any difference among the masters, although the XJS should have had an integral reservoir at some time? Space between booster and master is not right IMO, but may have been to compensate for the wrong booster. I know the S3 booster differs from the S1, for example, at least in mounting. Good luck; be sure to report back.

I replaced both master cylinder and booster in my S3 rebuild and the instructions called for a 1/4" of travel at the brake pedal after the linkage slop was taken up. My assumption is to not create a vacuum leak or partially applied brake when the pedal is at rest.


Thank you all.
I’ve been doing some Googling and You Tubing ( I guess I should have before asking here),
and now I’m pretty sure the new booster is no good.

Pressing the brake pedal while running cause engine to faulter, like a vac leak.

When shutting off the engine, there’s no residual vac.

Yes Dave, I saw that rod-end adjuster that points into the master, I didn’t know there was one. This one was loose; the lock nut not locked.

The funny thing is, sometimes I can pull and hold vac on the servo, and sometimes not at all, depending where the rod is positioned. That threw me off.

I’m going to try swapping in a used servo, but I’m not sure which to use, S1 or S2. This S1 car has a new S2 master on it now, so maybe a S2 servo would be correct. I’m dreading that rod with split pin that connects the servo rod to the pedal rod, inaccessable and blind down by the firewall. Is it easier to remove the whole servo and pedal box to get that pin back in?

Thanks again, and happy Easter.


Idle the engine and stop it, Robert…

Apply brakes repeatedly; you should ‘feel’ power assist - gradually diminishing as booster vacuum is depleted, and the pedal becoming rigid. Also, idling the engine, stop and wait - booster vacuum should be retained…

If booster vacuum is not retained; there is a leak; to be sorted…

With booster ‘empty’; the pedal is mechanically connected through the booster to the master cylinder. With firm pressure on the pedal; sinking pedal means a fluid leak…

There should be some play in the pedal as it is applied and released - the return spring of the pedal releases the rods. This ensures that with the pedal released, the valves inside the booster to operate correctly. As pedal is applied; the valve separating the two booster chamber closes, and the vent allowing air into the booster should open. This allows air pressure to boost braking.

As the pedal is released; the separation vent opens and the air vent closes - rebuilding booster vacuum. Incorrect pedal rods/adjustment, or failed pedal return spring, may cause incorrect booster valve functions.

Washers between the booster and m/s will indeed simulate rod length differences - but may be deliberate, and necessary? There were some differences in rod length related to different set-ups…?

Idling, the engine uses plenty of air to maintain booster vacuum - sufficient for ample booster assist. However, at speed, taking the foot of the accelerator to apply brakes; the throttle closes - raising manifold and booster vacuum, increasing braking effect compared to reaction coming to a stop.

However; the one-way valve is there to ensure that the highest manifold vacuum during driving is retained in the booster. Very little vacuum is ‘used up’ during braking - and vacuum is constantly ‘produced’ with a running engine…

A vacuum leak should indeed raise idle as air is drawn through the booster into the manifold

xj6 85 Sov Europe (UK/NZ)

Hi Robert. It’s possible that the gap between the booster and the master cylinder is the source of one of your problems. There should be a seal between the vacuum side of the booster chamber and the master cylinder. If that seal is missing or displaced you would not be able to hold a vacuum. It might be that the seal is disrupted when you step in the brake.
It is more likely an internal problem in the booster, though.
I would also check the integrity of the non return valve that you are connecting to on the booster to draw a vacuum. Make sure that has an adequate seal and is working properly.
Refitting the correct parts would be advisable, if you can find them. I’ve seen a few pop up on eBay.
I keep my 72 XJ6 at work so I can’t just run out and look at it now to compare it to what you have. That’s the time we live in.

My next project is to separate the master cylinder and booster to measure the rod length. After recently replacing the rubber seals in the master cylinder the brake appear to function Normally at first but after a little pedal travel the brakes grab to aggressively. Was working fine before my recent r&r.


Thanks. I was just at the point where I discovered that the master and servo needed to be sealed as well.

I was busting some parts open from a parts car and wondered why they seemed glued together.

Of course, the push rod only goes through a seal, and could not be air tight. No wonder I couldn’t pull a vac when the servo and master were not connected. It’s funny, I’ve swapped master a few times before, but must have gotten luck, because I never had a leak.

So, I remove the washers that were between the master and servo, smeared sealant on the O ring, and bolted it tight. The next day…still a problem. But, I’m on my way to understanding and diagnosing.
The 2 one-way valves work, so it looks like the new aftermarket servo’s no good.

David, the push rods were the same length on these 2 I had open; a '72 and '74. Maybe the tech who told me they were different (S1 and 2) was wrong, or there was a change later.

I’ll have another go at it tomorrow,


It seems like the pushrod shown is adjustable, Robert?

xj6 85 Sov Europe (UK/NZ)

Hi Frank,

Yes, the push rod is adjustable.

So, I’m confused at a higher level.

Last night I noticed that the area on the servo where the master mates, has manufacturing creases. These would seem to make it very hard to seal.

So, I really gooped the whole area, and into the servo, and bolted up the master. I was able to pull 10 in Hg vac, and needle didn’t move all night. But, this morning, when the brake is pressed, assist diminishes, and the engine slowly dies.

What gives?


Any leak through the booster should increase engine air, Robert - raising rpms…

However; this leaked air is ‘unmeasured’ and leans out the mixture, which, if on the lean side, could slow rpms or cause the engine to slowly die…

A second theory; a leak somewhere may draw brake fluid into the engine - and the engine doesn’t like it…?

Since you applied measured vacuum to the booster (through the non-return valve) do the same test - but now apply brakes. Light and constant pedal force and and position - while watching the vacuum reading.

This will close the commuting valve and open the air valve into the booster. The divider inside will move, closing the air valve, but with no further pedal movement, the commuting valve will stay closed. Which means that the vacuum reading should not drop…

If the vacuum does drops, and/or the pedal continues to move; there is an internal leak in the booster - probably(?) a failed commuting valve.

And if the air valve does not close completely, and the commuting valve stays open/leaks when pedal is released - vacuum is gradually lost in the booster reducing brake booster effect.

The booster effect comes from ambient air pressure in one booster ‘half’ while manifold vacuum is constant in the other ‘half’. With equal pressure/vacuum in the two halves of the booster there is no booster effect…

It doesn’t fully explain the engine behavior, but a leaking booster is a fault anyway…

xj6 85 Sov Europe (UK/NZ)

Hi Frank,
Thanks agaian, you are always helpful.

The explanation of unmeasured air helps me understand why the engine dies.

So today, I can’t pull any vac on the booster at all. The commuting valve position?

Next is a replacement booster. I’ll report.



We are running out of options, Robert…

I’ve been musing a bit on the purpose of the adjustable rod. It may just be an adjustment of pedal travel, but it may also relate to valve operation?

All vacuum boosters use the same principle; letting in air to work against (manifold) vacuum - the pressure difference exerting the booster force.

I have always assumed that vacuum is always present in one ‘half’ of the booster, while the inlet of air into the other half is regulated by pedal action. And that pedal action does not interfere with vacuum supply. However, this may not be entirely correct; the valving may block vacuum inputs - and rod length may play a part?

Generally, rods may internally ‘push’ on valves, but may not ‘pull’ them. Obviously, the pedal return spring retracts the pedal rod with the pedal released. The ‘push’ on the master cylinder rod comes from the return springs in the m/s - but what if the rod is too short to work the valve(s)…?

Just musings - but your symptoms is like the repeated operation on the pedal with the engine turned of. Vacuum is gradually ‘bled off’ - and without vacuum ‘resupply’, boost is gradually lost as pressure between the two ‘halves’ of the booster equalise…

I’m not delivering gospel…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

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. If you cannot establish a vacuum in the front chamber of the booster, it means that the rear chamber is open to the atmosphere, perhaps due to dirt or defect in the valve. But if that were the case you would have difficulty modulating brake pressure.
If there were a fault with the valve that isolates the two chambers it would cause an increase in pressure or decrease of vacuum in the front chamber when you step on the brake. The booster might still work but not effectively. It would also be seen by the engine as a vacuum leak.
What it doesn’t fully explain is the drop in RPM you experience when you step on the brake. As everyone agrees, assuming that your engine is in proper tune with correct mixture and ignition timing, there should be an increase.
One last thing. What if we assume that your engine has been tuned correctly, but it has been tuned to run properly with a defective booster. A booster that is leaking front to rear and to the atmosphere due to a defective valve. Then you step on the brake and the front and rear chambers of the booster are isolated and normal vacuum can be established. Now the engine is running richer and will slow down. Interesting idea.
Either way, it looks like you will need a new booster.
I hope this all made some sense and helped a bit.