I recently spent some time getting my '58 Mk VIII back on
the road and have been having an issue with locking
brakes. NOTE that the car was converted to the MK IX
caliper system some time back, though retaining the Mk VIII
master cylinder.
The brakes operate fine initially, but as you drive the
car, they begin to progressively lock until the wheels
won’t turn. If I shut off the car and pump the pedal, the
brakes free up and will again operate normally for a
period. If I pinch the vacuum line to the booster, I
obviously lose all power assist, but the brakes do not lock
up.
This seems to be an issue with the booster, though it was
new when installed many years ago when the car was first
garaged. All the calipers were also professionally rebuilt
and all flexible brakes lines to the calipers replaced.
Any suggestions or thoughts as to what might be causing
this would be greatly appreciated!
Regards,
Tom Golodik–
Tom Golodik
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
Something that could be causing this is a malfunction in the
vacuum section of the booster since you eliminate the issue
if the vacuum line is pinched off. By pumping the brakes
with the engine off you are bleeding down any remaining
vacuum in the system.
Or it maybe that the slave cylinder in the booster is
sticking and not releasing immediately after you release the
brakes. A after a period the piston retracts and releases
the brakes. As a test get the brakes to stick on and with
the car on jack stands, open a bleeder screw and see if
fluid comes out under some pressure and the brakes then
release.–
The original message included these comments:
The brakes operate fine initially, but as you drive the
car, they begin to progressively lock until the wheels
won’t turn. If I shut off the car and pump the pedal, the
brakes free up and will again operate normally for a
period. If I pinch the vacuum line to the booster, I
obviously lose all power assist, but the brakes do not lock
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
could be a collapsing rubber brake line (internal)
imo all ruuber brake hoses become subject to this risk at +20yrs
or, if your booster is equipped with a reaction valve, there
is a tiny piston inside that valve which commonly becomes
encrusted and immobile–
The original message included these comments:
The brakes operate fine initially, but as you drive the
car, they begin to progressively lock until the wheels
won’t turn. If I shut off the car and pump the pedal, the
brakes free up and will again operate normally for a
period. If I pinch the vacuum line to the booster, I
obviously lose all power assist, but the brakes do not lock
up.
In reply to a message from awg sent Mon 31 Oct 2016:
Tony: All the flexible brake lines are new, so it’s
unlikely that, plus all wheels lock up, not simply one.
I’m interested in your comment regarding the reaction valve.
Although the booster unit is new, it has sat for many years
unused after being installed. I would be interested in
checking its operation. Can you provide any further details
as to where this reaction valve is on the booster unit?
External? Internal?–
The original message included these comments:
or, if your booster is equipped with a reaction valve, there
is a tiny piston inside that valve which commonly becomes
encrusted and immobile
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
I am unsure on the MKVIII, but it is part of the servo assembly.
I dismantled some servos, including replacement PBR, and
they ALL had the tiny pistons frozen, (which is why I
assemble with rubber grease, not brake fluid)
it was so long ago, iirc, the PBR is external, but the
MKVII, you may or may not need to remove the hydraulic
cylinder from the vacuum shell to get at the needed parts.
There is an adjustment somewhere as well?
a pushrod needs clearance.
The MKVIII is one of the few Jag manuals I do not have.
Hopefully others can give you firmer direction–
The original message included these comments:
checking its operation. Can you provide any further details
as to where this reaction valve is on the booster unit?
External? Internal?
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
Hi Tom,
I saw your post and related it to e-type known issues… I don’t know the
MkVIII brake servo, but if it’s a ‘remote’ type with an integrated air-valve
(reaction valve) then the issue is that this valve is leaking when hot.
The valve should allow air into one side of the servo on brake application
(the input fluid pressure rises, the air valve opens, the boosted fluid is
pushed downstream)…if it leaks air at other times you get the symptoms you
describe.
On e-types this reaction valve is on the master cyl…but no matter,
functionality is same. In that setup it’s one of three things: The rubber
sealing surface has degraded, allowing an air leak. The light return spring on
the air valve has lost tension, so the valve doesn’t close… or the little
hydraulic piston that actuates the air valve is partly stuck due to corrosion.–
Dave S3E,RR SS, BMW’02,Tr4a,Elise, http://www.xketype.com
Georgetown, Penang, Malaysia
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
Tom,
It certainly sounds like a reaction valve/sticking piston problem,
but I don’t know what type of booster you have. Could you post a
picture?
If it has sat for 20 years it may just be hardened grease, but it
might also be corrosion or a hardened seal.
Andrew–
The original message included these comments:
Although the booster unit is new, it has sat for many years
unused after being installed. I would be interested in
checking its operation. Can you provide any further details
as to where this reaction valve is on the booster unit?
Your Mk VIII probably has the same brake booster that the MK-IX has. If so,
it will be located between the frame rails directly under the passenger side
of the front seat. In fact the air intake vent for the booster will get it
air from an intake under the seat on that side. You may be able to get to
the reaction valve without taking anything else apart. Depends on how it
sits between the frame rails. I’ve had the same problem with both my MK-IX
and my Mk-2.
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
could be a collapsing rubber brake line (internal)
imo all ruuber brake hoses become subject to this risk at +20yrs
or, if your booster is equipped with a reaction valve, there is a tiny
piston inside that valve which commonly becomes encrusted and immobile
The original message included these comments:
The brakes operate fine initially, but as you drive the car, they
begin to progressively lock until the wheels won’t turn. If I shut
off the car and pump the pedal, the brakes free up and will again
operate normally for a period. If I pinch the vacuum line to the
booster, I obviously lose all power assist, but the brakes do not lock
up.
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from David McKay sent Tue 1 Nov 2016:
Dave right you are and sometimes it is as simple as cleaning
60 years of crud off of the breather–or correcting a ‘‘new’’
carpet install!–
The original message included these comments:
of the front seat. In fact the air intake vent for the booster will get it
air from an intake under the seat on that side. You may be able to get to
In reply to a message from George Camp sent Tue 1 Nov 2016:
I had the exact same problem, got the booster rebuilt and
have not had the problem since.
On my car the brakes would not release after braking, unless
I tapped the brake pedal again, lightly, so I drove like
that for awhile. And I pinched off the vacuum line and
drove like that for a while. Joecurto.com rebuilt my booster and did a fine job.
P.–
Peter J. Smith, 1966 3.8S MOD
Carson City Nevada, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from Tom Golodik sent Mon 31 Oct 2016:
Thanks for all the suggestions and advice. I will follow up
on these.
In the barn, I found the old booster that was removed years
ago. It looks exactly like the one pictured in the Mark IX
parts manual, but nothing like the one that is pictured in
the Mark VIII parts manual. This may have been a simple
upgrade that occurred during the model’s manufacture.
However, when I crawled under the car to look at the booster
that is now installed on the Mark VIII, I see it is somewhat
similar to the Mark IX booster, but not identical. A large
vacuum line comes out of the end of the large bellows, which
is different from the original unit. I will try to get the
car up on a lift tomorrow and see if I can get a picture of
the installed unit or a part number.–
Tom Golodik
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from Tom Golodik sent Tue 1 Nov 2016:
Tom : I don’t know if it is any help, but the original Mark
VIII booster was made by Clayton Dewandre so if it has a
Clayton plate it is probably original . The Mark IX had ( I
think ) an uprated booster to cope with the higher pedal
pressures associated with discs, but whether it was still made
by Clayton I am not sure–
christopher storey
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from christopher storey sent Fri 4 Nov 2016:
My mk9 has a lockheed servo similar to those fitted to the
later Stype and Mk2 range. The old Rover 90 range also
used Claytons. Don’t think you can get the parts for
these anymore.–
MK9.
Yorkshire, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
[Commercial use of subscribers e-mail addresses prohibited.]
In reply to a message from christopher storey sent Fri 4 Nov 2016:
XKs is offering a brake booster for the Mark Saloons at half
the price I see for the rebuilt original units. I just
fitted one. I think this will be correct for cars with disc
brakes.–
The original message included these comments:
Tom : I don’t know if it is any help, but the original Mark
VIII booster was made by Clayton Dewandre so if it has a
Clayton plate it is probably original . The Mark IX had ( I
think ) an uprated booster to cope with the higher pedal
pressures associated with discs, but whether it was still made
by Clayton I am not sure
Well, it took longer than I thought it would thanks to grand-parenting duties and replacement of reluctant glow plugs on the Mercedes diesel, but I finally got the Mk VIII on a lift for pictures to post of the brake booster for those of you wiser than I on these matters. The bench shots are of the original sticking Lockheed booster that had been removed; the installed pictures are of its replacement, a new Rockwell booster, installed some time ago. For some reason they were uploaded in an order different than the groupings I had attempted. There’s also an exploded view of a booster illustrated in a non-factory Mk VIII service manual that isn’t like the one I removed. Very confusing!
I checked out the breather line to the fitting under the front seat and that appears to be clear. Can anyone tell me if either of these two boosters would be of a type fitted with a reaction valve and where that valve would be before I start any attempt at disassembly. What am I looking for? Is the installed booster the same one fitted to the S-type and Mk 2 models?
In the meantime, I’ve pinched off the booster vacuum line and have been driving the car to limber it up a bit. It needs more work, but it does bring a smile to the face!
