[xj40] Brake accumulator question?

Having made a recent replacement of this little steel pressurized
sphere raises new questions. Why is it functioning for rear brakes
only? Can hard emergency braking damage it especially if it is
aged? Does it gradually lose its pressure? Will braking performance
whether normal or emergency be adversely affected and noticed? At
what stage of pressure loss will antilock light show if only
temporary? Should noticeably more pedal effort accompany a bad
accumulator failing?

An owner said his brakes took much more pedal effort after an
emergency stop. I suggested checking the accumulator without
knowing any of the above.–
LarryWard '93XJ40 '81MBZ300TD '95 Dodge4x4 CTD
modesto, ca., United States
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In reply to a message from LaJagy sent Sat 22 May 2010:

Larry,

  1. The accumulator sphere does not only work on the rear
    brakes. The rear brakes rely on the accumulator pressure for
    their operation whereas the front brakes are operated
    directly by the brake master cylinder assisted by brake
    boost from the accumulator pressure.

  2. The pressure inside the accumulator sphere is generated
    by the Teves electric pump injecting brake fluid in to the
    sphere thus compressing the nitrogen gas behind the
    diaphragm. The pressure / force inside the sphere is not
    generated by the force applied to the brakes - so an
    ‘emergency stop’ will have no effect on the sphere, other
    than to reduce the internal pressure until the Teves pump
    tops it up again.

  3. If by ‘‘gradually lose its pressure’’ you mean does the
    nitrogen charge static pressure reduce over time, then the
    answer is yes. The flexible (rubber?) diaphragm in the
    sphere suffers degradation and the nitrogen molecules
    gradually escape through the diaphragm by ‘osmosis’ (or
    something similar!) One guy I spoke to who specialises in
    these types of system told me that they assume a degradation
    of about 10% loss per year of the static nitrogen charge
    (which IIRC starts at about 600 psi when new?)

  4. With a failed accumulator, braking performance will be
    adversely affected to some extent for three reasons. Firstly
    the amount of pedal effort required to operate the brakes
    will be greatly increased because you have lost brake boost.
    Secondly, the effectiveness of the rear brakes will be
    reduced / lost - however, since, even in normal brake
    operation the majority of braking force is through the front
    wheels, that may be less noticeable. Thirdly, you will have
    lost the ABS function, so in an emergency stop situation the
    wheels are likely to lock and skid on a slick surface.

  5. I think I gave you information on this previously, but
    here is a generalisation of the sequence of accumulator
    switching pressures (the nominal pressure of the brake
    fluid in the sphere, not the nitrogen pressure of course)

  • Upper switching point (pump off) = 2700 psi
  • Lower switching point (pump on) = 2000 psi
  • Warning switch ‘made’(lights on) = 1500 psi.

On a failing accumulator (as yours was) where there is no
nitrogen charge behind the diaphragm, the instant that the
pump starts, the brake fluid pressure within the sphere will
rocket to the upper switching point because FLUIDS DO NOT
COMPRESS. But equally, as soon as you touch the brake pedal,
that pressure will instantly drop below the 1500 psi level
to actuate the warning switch and the light will illuminate
(as yours did) and the pump will run again to top up the
accumulator.

  1. Providing there is some accumulator pressure in the
    sphere, you will still get brake boost so you would not
    notice any difference. The acid test is to turn the ignition
    off when the pump has stopped as the upper switching point
    is reached and to count the number of brake applications
    then available. 20 plus is good, 30 plus is better.–
    The original message included these comments:

sphere raises new questions. Why is it functioning for rear brakes
only? Can hard emergency braking damage it especially if it is
aged? Does it gradually lose its pressure? Will braking performance
whether normal or emergency be adversely affected and noticed? At
what stage of pressure loss will antilock light show if only
temporary? Should noticeably more pedal effort accompany a bad
accumulator failing?
An owner said his brakes took much more pedal effort after an
emergency stop. I suggested checking the accumulator without
knowing any of the above.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 23 May 2010:

Larry,

Missed out a vital phrase in my previous post - that last
sentence should read ;-

‘‘The acid test is to turn the ignition off when the pump has
stopped as the upper switching point is reached and to count
the number of BOOSTED brake applications then available - 20
plus is good, 30 plus is better, before the pedal ‘goes hard’.’’–
The original message included these comments:

  1. Providing there is some accumulator pressure in the
    sphere, you will still get brake boost so you would not
    notice any difference. The acid test is to turn the ignition
    off when the pump has stopped as the upper switching point
    is reached and to count the number of brake applications
    then available. 20 plus is good, 30 plus is better.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 23 May 2010:

Boosted brake applications? I always did the test to wait
for a hard pedal but sometimes hard pedal is subjective. I
use my hand but still, I am not sure if the pedal has
reached the hard spot.

Is there any other way to tell boosted applications?

I’ve been having the anti lock and brake light illumination.
Sometimes it is just a tap to the pedal, sometimes not.

I recently did the bi-annual brake fluid flush and brake
bleeding. The light seems to illuminate less but still
comes on. Pump does its thing, braking seems to be ok.

I need to attend to a cleaning of that blue connector. Then
think about this but I am not sure about the hard brake.

Is hard when it reached the point that it will no longer
depress at all, like hitting the wall and it won’t move?–
John 90 XJ40
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In reply to a message from John S sent Sun 23 May 2010:

John,

I’ve had no problem detecting when brake boost is lost on my
car. I will try to figure out a way of measuring the force
required to depress the pedal with boost and then when it
‘goes hard’.

Will get back to you.

If your ABS light illuminates sometimes as soon as you
depress the pedal, that suggests that, as in Larry’s case,
your accumulator sphere is not holding charge beyond that
single brake application. Non compressible brake fluid again
after the nitrogen charge has been lost but still giving a
‘one-shot’ boost.–
The original message included these comments:

Boosted brake applications? I always did the test to wait
for a hard pedal but sometimes hard pedal is subjective. I
use my hand but still, I am not sure if the pedal has
reached the hard spot.
Is there any other way to tell boosted applications?


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 23 May 2010:

Should the pump run every time you touch the brake? I’m
guessing that little nitrogen means that there is no reserve
pressure. Therefore the pump goes every time you hit the pedal.

I’m hoping it is not the accumulator ball because that is
one expensive little ball.–
John 90 XJ40
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In reply to a message from John S sent Sun 23 May 2010:

No, the pump should not run every time you depress the brake
pedal. It’s a sure sign of a failing accumulator when it
exhibits that symptom.

Fortunately, they are not that expensive and are quite easy
to change. Mind you, they’re not exactly cheap, but at well
under $150, they are affordable.–
The original message included these comments:

Should the pump run every time you touch the brake? I’m
guessing that little nitrogen means that there is no reserve
pressure. Therefore the pump goes every time you hit the pedal.
I’m hoping it is not the accumulator ball because that is
one expensive little ball.


91 XJ6, 93 Sov, 97 Sov – Driveway’s crowded!
Tampa Fl, United States
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In reply to a message from Win sent Sun 23 May 2010:

Before replacement, with key off the pedal was hard which I
presumed was normal - wrong. Key on without starting gave a softer
initial pedal progressingly firmer with more leg muscle. And now it
is evident the Teves was always kicking in the instant key was
turned on with out starting and maintained normal braking feel Now
with new accumulator it has less frequency and totally eleminated
even a momentary antilock light showing.

Bryan, would the 10% annual degradation of nitrogen pressure have
nothing to do with failing internal bladder but the normal osmosis.
Unlike a tire which has only atmospheric pressure of ? resisting
the internal pressure escape, the accumlator would have some brake
fluid internal maintenance pressure resisting against the neoprene
bladder thus in my theory resisting molecular outflow of gas but
not without some loss through the dropping of fluid pressure.

This begs the question - is there a mfg date that would serve to
warn one to only buy fresh ones? No one seems to know what the
birth date of ‘‘aged ones’’ were on Amazon for $97. Clerk at local
store assured my I was getting a new one but had no way to prove it.
I paid $165 incl tax but I saw online quotes up to $275.

Back in the 60s-80s a shop in San Jose would rebuild Citroen
suspension spheres for $25-30.–
The original message included these comments:

No, the pump should not run every time you depress the brake
pedal. It’s a sure sign of a failing accumulator when it
exhibits that symptom.
Fortunately, they are not that expensive and are quite easy
to change. Mind you, they’re not exactly cheap, but at well
under $150, they are affordable.


LarryWard '93XJ40 '81MBZ300TD '95 Dodge4x4 CTD
modesto, ca., United States
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In reply to a message from Bryan N sent Sun 23 May 2010:

John,

As a follow up to the referenced previous post, I tried to
measure the relative force required on the brake pedal with
and without brake boost, but I couldn’t adapt my wife’s
fancy electronic kitchen scales to do the job! :wink:

However, this may be of some guidance to you in determining
when you do and do not you have brake boost.

I turned the key to position (I) on my car this morning
after it sat for 4 days unused (so the accumulator pressure
would long since have dissipated) and heard the Teves pump
running. When the pump stopped, I switched the key to the
off position and with my hand, operated the brake pedal
easily with boost for 40 applications before the pedal ‘went
hard’.

The horizontal centre line of the top surface of the brake
pedal on my car is approximately 7 inches above the carpet.
With brake boost USING MY THUMB ONLY, I could comfortably
depress the pedal 2� inches (i.e so that the upper surface
of the pedal is now only 4�’’ above the carpet)

After exhausting the accumulator pressure (40 brake
applications later with the key off) so that the pedal ‘goes
hard’, exerting the same amount of force with my thumb, I
could only depress the pedal 1� inches before the resistance
(like hitting a brick wall) was too severe for my thumb
pressure to overcome.

BTW, with the accumulator fully charged (i.e Teves pump off)
but with the key in position (I), the pump will re-start
every time after the second normal application of the
brake and run for a couple of seconds to top up the
accumulator. BUT, if in the same circumstances I hit the
brake pedal hard (i.e stamp on it!), the pump will re-start
after just one such application of the brake.

Neither of those situations is a reliable indication of the
condition of the accumulator sphere - but the number of
applications you get from a fully charged accumulator
after the key has been switched off is. As mentioned, mine
does 40 normal applications before the pedal ‘goes hard’.–
The original message included these comments:

I’ve had no problem detecting when brake boost is lost on my
car. I will try to figure out a way of measuring the force
required to depress the pedal with boost and then when it
‘goes hard’.
Will get back to you.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from LaJagy sent Mon 24 May 2010:

Larry,

I’ve no idea whether the ‘osmosis’ of the nitrogen molecules
is better or worse depending if the accumulator is charged
with brake fluid on the car or sitting ‘dry’ on the shelf.

Since my car is 19 years old TODAY (first registered new in
the UK on 24th May 1991) and it still has its original Teves
accumulator sphere, which is performing perfectly (famous
last words!) I’m a little doubtful about that (Citroen)
guy’s 10% per annum degradation theory. On that basis my 600
psi sphere should have less that 75 psi nitrogen charge left
in it! :slight_smile:

I just wonder - I think the Citroen system used mineral oil
(HSMO / LHM or equivalent) and our Teves spheres are used
with brake fluid so the material used for the diaphragms has
to be different. That may have a bearing on pressure loss /
osmosis. I know the guy who made that statement used to
re-charge accumulators with a sound diaphragm for about �40 GBP.

As I’ve mentioned previously, I bought a $90 AC Delco
25528382 accumulator sphere four years ago in the States
‘just in case’ and it remains in its box. The ‘probe test’
through the oil orifice indicates it still holds nitrogen
pressure, but I guess we will see if ever I get to use it
whether it has degraded much on the shelf.–
The original message included these comments:

Bryan, would the 10% annual degradation of nitrogen pressure have
nothing to do with failing internal bladder but the normal osmosis.
This begs the question - is there a mfg date that would serve to
warn one to only buy fresh ones? No one seems to know what the
birth date of ‘‘aged ones’’ were on Amazon for $97. Clerk at local
store assured my I was getting a new one but had no way to prove it.
I paid $165 incl tax but I saw online quotes up to $275.
Back in the 60s-80s a shop in San Jose would rebuild Citroen
suspension spheres for $25-30.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Mon 24 May 2010:

Bryan,

Do you if the AC Delco 25528382 will fit on a 1990?–
John 90 XJ40
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In reply to a message from John S sent Mon 24 May 2010:

John,

Yes, no problem - it is the OEM Teves accumulator–
The original message included these comments:

Do you if the AC Delco 25528382 will fit on a 1990?


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Mon 24 May 2010:

No the AC Delco parts catalog site that is included below
expands on which cars that AC Delco will fit. So I called a
local parts shop asking for an accumulator ball for a 1990
Pontiac 6000. They didn’t have the AC Delco part but had a
new part for $39.99.

Now if the AC Delco would fit the 1990 6000 the alternative
part would fit the pontiac 6000 and therefore it should fit
the 1990 XJ40.

Hm? What do you think about it?

http://parts-catalog.acdelco.com/catalog/catalog_search.php

ACCUMULATOR-PRESSURE
Part Number: 25528382
Product Notes:
[Brake Master Cylinder Fluid Accumulator]; Front & RR Disc
Antilock brakes (JL9)
Per Vehicle: 1; Years: 1988-1990

Then there is a radio tab titled ‘‘Vehicle List’’ you can
expand to see all vehicles the AC Delco will fit.

BUICK(16)1986-1990 

ELECTRA(5)1986-1990

LESABRE(5)1987-1990

REATTA(3)1988-1990

RIVIERA(3)1988-1990

CADILLAC(16)1986-1990

DEVILLE(5)1986-1990

ELDORADO(3)1988-1990

FLEETWOOD(1)1990-1990

FLEETWOOD BROUGHAM(FWD)(4)1986-1989

SEVILLE(3)1988-1990

OLDSMOBILE(13)1986-1990

DELTA 88(5)1987-1990

NINETY-EIGHT(5)1986-1990

TORONADO(3)1988-1990

PONTIAC(17)1986-1990

6000(12)1986-1990

6000 STE(1)1989-1989

BONNEVILLE(4)1988-1990–
The original message included these comments:

Yes, no problem - it is the OEM Teves accumulator


John 90 XJ40
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In reply to a message from John S sent Mon 24 May 2010:

John,

Your choice - but can you believe the parts guy?

Just shell out for the real thing and you cannot go wrong.–
The original message included these comments:

No the AC Delco parts catalog site that is included below
expands on which cars that AC Delco will fit. So I called a
local parts shop asking for an accumulator ball for a 1990
Pontiac 6000. They didn’t have the AC Delco part but had a
new part for $39.99.
Now if the AC Delco would fit the 1990 6000 the alternative
part would fit the pontiac 6000 and therefore it should fit
the 1990 XJ40.
Hm? What do you think about it?


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Mon 24 May 2010:

I’m trying to find the real AC Delco part but not much luck.
They do have them on Amazon. Maybe I’ll get one from
Amazon and also get the $39 to see if it fits.

I would guess that what you must be aware of is the size
(don’t want to dent hood) and the threads.–
John 90 XJ40
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In reply to a message from John S sent Mon 24 May 2010:

John,

This is the cheapest one I can find on the web - looks to be
the same source where I bought mine with an up-dated site
and now in cahoots with Amazon.

http://ac-direct1.amazonwebstore.com/search.htm?keyword=25528382--
The original message included these comments:

I’m trying to find the real AC Delco part but not much luck.
They do have them on Amazon. Maybe I’ll get one from
Amazon and also get the $39 to see if it fits.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Mon 24 May 2010:

I tried all the parts stores and nobody had it. Called a
Chevy dealer and they had it but wanted $247 for it. Looks
like they made it a dealer only part.

I’m going online to buy at your link.–
John 90 XJ40
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In reply to a message from Bryan N sent Mon 24 May 2010:

Bryan, my first citroen '60 ID and second '63 ID wgn used higrade
brake fluid. I think our '67 DS Pallas also used brake fluid but
recall a later time other fluid was used.–
The original message included these comments:

I just wonder - I think the Citroen system used mineral oil
(HSMO / LHM or equivalent) and our Teves spheres are used
with brake fluid so the material used for the diaphragms has
to be different. That may have a bearing on pressure loss /
osmosis. I know the guy who made that statement used to
re-charge accumulators with a sound diaphragm for about �40 GBP.


LarryWard '93XJ40 '81MBZ300TD '95 Dodge4x4 CTD
modesto, ca., United States
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