On another site, the builder of a HONDA racer discovered the
cause of excess play in the brake actuation system.

At the end of the pedal push rod there is a rubber disk the thickness
of four US quarters. The push rod acts against this disk.

In the master cylinder push rod/valve assembly there was another
such disk.

In blow up diagrams of a early Corvette Bendix brake booster there
is a similar "reaction disk".

Has anyone been inside the master cylinder or brake booster on the
Jaguar X30x and seen whether these devices are used?

The HONDA builder replaced the rubber with metal and delrin, but note
that there is a warning that he was converting to non-assisted braking.

Even if we know that these devices exist, it may not be possible to remove
them.

Is the pedal pushrod adjustable as they used to be on other vehicles?

I am also thinking that the way the booster is mounted is part of the problem.

On many cars the booster is mounted directly to the firewall with bolts/studs
that are coaxial with the pushrod forces.

In these sedans they are mounted with the bolts/studs at a right angle
to the pushrod forces.

Big difference in stiffness.

edit
NOT TO BE ATTEMPTED WITH BRAKE BOOSTER INTACT
SEE POST #3

 

I've heard of 'reaction valves' in vacuum boosters....but to be honest I really don't know what they are or how they work.

Cheers
DD

 

Changed the search term to reaction valve instead of disk and all the search
results turn up e-types. It seems that Jaguar had a special way of doing things
and that's what they called them.

Anyways, it seems the reaction disk is responsible for balancing manual force
versus assist.

If it is replaced with a non-compressible material, or goes missing on a boosted
brake system, the brakes are either off or you go through the windshield

It also seems from reading vendor documentation that the reaction disk
characteristics are tunable. Obstensibly, by specification of material and profile.

The problem of course is that the tuning is specified by Jaguar and not the
great unwashed.

The later ATE booster also has a feature called "brake assist". The faster the
driver hits the pedal, the more vacuum assist is applied for a given input force.
Thus, the assist is non-linear. This is done with a set of locking ***** and a collar
which divert the feedback force from the master cylinder 'around' the normal
path and causes greater application of vacuum.

Translation? Live with mush

 

Hello Plums ,

I don't really know the reason or what problems if any you are having to ask this question , but anyway ,

Brake Booster's are basically air tanks that store engine vacuum via the inlet manifold ( petrol engines ) . On diesel engines a separate air pump is used to create the vacuum .

Brake Booster's come in single or twin diaphragm form . The most common is single .

The Reaction Disc / valve that you and " Doug " are talking about is there to give the brake pedal a constant feel at all times . There is also an Atmospheric Valve that works in conjunction with the reaction disc .

When you apply the brakes the Atmospheric Valve opens and allows air to enter the rear chamber of the booster , the Reaction Disc at the same time closes off the vacuum port in the front chamber of the booster , this allows the booster push rod to drive your master cylinder piston forward .

In other words , you have to main EQUAL FORCE either side of the diaphragm .

By increasing or decreasing the thickness of the reaction disc you will change the brake pedal feel and braking performance , ie : more or less vacuum as needed .

In regards to rod adjustments , there is but " DON'T TOUCH IT " especially the booster push rod .

Then there's High Pressure type Hydraulic Boosters , but that's for another time

Cheers ,

Sid

 

The X30x sedans have a reputation for brake pedal feel that
I think is best described as 'vague' in the first part of the
travel.

 

I think it is the pedal box and in particular the mounting
method.

Picture a traffic cone that is screwed down at the base.

Then imagine applying a horizontal force at the top,
which is where the pivot for the brake pedal is located.

The result of course is that the cone wants to pull
up the base on one side and/or bend.

If the traffic cone is your pedal box, then any movement
is movement at the pedal itself. All of the slack, including
movement of the pedal box and other tolerances has to
be taken up before the master cylinder does any work.

From pictures, the pedal ratio looks to be 6:1 at best.
Jaguar wants owners to be comfortable.

This would translate 2mm of slack on the pivot end to
12mm at the pedal face. This is almost a half inch.

4mm would be close to an inch.

Now, take the same traffic cone screwed to the floor
and also brace the top. Displacing it sideways will be
much harder.

Doing that to the pedal box would minimise the bending
motion of the pedal box itself which is part of the slack.
The result would be less slack to be taken up before the
master cylinder does work.

Other makes are affected by this. For example, search:

subaru master cylinder brace

They have at least three vendors addressing the problem.

As usual, nothing ready-to-wear for Jaguar

BTW, the pedal box on the X308 is plastic. The X300
has a metal pedal box in at least early production. According
to ebay listings, some X300 seem to have come with a plastic
pedal box. Perhaps when the TSB for insufficient pedal return
was applied.

 

BTW, in the picture above, item #20 is listed as a reinforcement plate.

There are two tangs. One on each side.

Does the firewall itself have matching holes for the mounting studs, or
is it the case that the pedal box+reinforement plate are dependent on
those two tangs for retention?

 

If as you say " the vague " pedal feel is at the first part of pedal travel , then what you are saying is quite the opposite .

Under heavy braking , the cars firewall or pedal box can flex , that's when you need to
brace the master cylinder , and would be more felt in the second stage of braking .

This was quite common on some GMH models .

It sounds more like your front brake circuit to me ?

Cheers ,

 

I've gotten very used to the X300 brakes now, a good year later, and they really don't fell "mushy" to me anymore, just different. But a lot of manufacturers, makes and models feel different from each other too.

I have to take my booster (and associated brake pedals) off soon though to repaint the booster. It is the most common reman, but they just prime the body and they rust after a couple years. So I need to pull it out and paint it black, which to your point is an odd assembly since it comes out as a unit with the pedals.

.

 

Not quite.

Don't forget that any force sufficient to overcome the strength of any
involved component can contribute to the overall effect.

Consider for example, part #19 which is a gasket.

Not likely to take much effort to compress on one side, and the result
will be movement of the "traffic cone" which translates to motion at
the pedal without effect.

Consider then, part #20 which at most is a sandwich plate assembled
using mass production techniques.

Overall, the "traffic cone" design is probably the worst possible means
of securing the assembly.

At least even the GMH had the conventional coaxial mounting arrangement
as found in most vehicles including all US domestic manufacturers. Still
a problem if either the firewall tin or the assembly is under spec'ed, but
at least a better basic concept.

Just been reading sites about Holden manual conversions where pedal
boxes are ripping out of firewalls btw.

And no, it is not the front circuit.

I am talking about 'contributing factors'.

For example, some people have put on stainless steel lines *and*
reinforced the pedal box mounting with some positive effect on
feel from each. In other words, incremental improvement.

You can have a bubble in the fluid, leave the bubble there and
get some improvement by taking slop out of the system. Now,
take the bubble out, and more improvement.

 

Well it seems that lots of cars are made with 'assemblies' bolted to the firewall,
cheaper to assemble it would seem.

But take a look under the hood of your Buicks. Coaxial mounting instead of the
"traffic cone" method.

And probably better tin than most of the lighter imports.

Drove a 2003 Buick LeSabre a few weeks ago, and the pedal feel is night
and day for modulation at the start of the stroke. No hotrodding. Certainly
not with a 80 year old in the shotgun seat.

I have had access to that vehicle since new and know its history although
I might only drive it once every couple of years.