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Thread: Brake Servo/Booster information/specifications

  1. #1
    Member Shug's Avatar
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    Post Brake Servo/Booster information/specifications

    Hello,

    I've been enjoying getting out and enjoying the XJ6 Coupe in the sunny weather but I'm growing a little disappointed with the stopping power, a recent incident lead to stamping on the brakes which lead to the rears locking but not the fronts. So I'm now looking to understand the braking system a little better especially the brake servo. I have a reasonable understanding of how it functions but trawling the internet doesn't give a lot of specific details about the servo.

    My brakes are 4 piston Girling's to the front with vented discs standard solids at the rear and are in good condition eg new/refurbished, master cylinder refurbished, EBC Green Stuff pads and Goodridge over-braided hoses. Caliper pistons are all moving freely.

    Some info found so far about the brake servo/booster, but I need to get some confirmation:-

    1. Girling Type 100?
    2. Dual Diaphragm?
    3. the servo may have been used on subsequent Jaguars including the XJ series 3 and XJS models.

    Has anyone got any details? or serviced a servo/booster and has some photo's they would like to share? Or can anyone recommend somewhere in the UK to get the servo refurbished.

    Is there a way to test the servo is functioning to full potential especially if it is a dual-diaphragm? (could one diaphragm be functioning correctly and the other not?)

    Are the later model servos better than the 1977?

    Any information is welcome.

    Thanks
    Jaguar XJ6C 1977 4.2 (HIF44 carbs) UK RHD ZF4HP22

  2. #2
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    Had a wooden brake pedal.
    Servo wouldn't hold a vacuum.
    Replaced with CAC1226/7 from David Manners - £155.00 last year.
    My bulk head needed tidying under pedal box mount and a new gasket to sit on - used some old lino, as I wanted to get it finished.
    Also bought vacuum hose as fitting were different between servos.
    This is what I replaced.

    Click image for larger version. 

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    Regards
    John

  3. #3
    Senior Member rchiv's Avatar
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    Did all that sawdust come out of the Servo no wonder you had a wooden pedal haha.

    Roger
    HEY I THOUGHT I HAD RETIRED, Ex Auto Electrician
    2006 X Type 2.2d SE Manual Estate. Zurcon Blue Champagne/Mocca, F/R Parking sensors, ,Auto Lights, Auto Wipers, Headlight Washers, Sat Nav 2012 update, 17 in Wheels, 116K Mods:- Electric Autofold Mirrors on Locking
    Simple Autofold Mirrors on Locking,http://www.jaguarforum.com/showthrea...irrors+locking
    UPDATE YOUR SIGNATURE http://www.jaguarforum.com/showthread.php?t=91945

  4. #4
    Senior Member rchiv's Avatar
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    The Servo does not operate front and rear brakes. It assists only the pedal pressure. The master cylinder is a split system applying pressure to the brakes and normally operates O/S/R, N/S/F and N/S/R and O/S/F so for the rears to lock up together under emergency braking could be the weight being transferred to the front making the rear lift. Front suspension could be weak.

    Roger
    HEY I THOUGHT I HAD RETIRED, Ex Auto Electrician
    2006 X Type 2.2d SE Manual Estate. Zurcon Blue Champagne/Mocca, F/R Parking sensors, ,Auto Lights, Auto Wipers, Headlight Washers, Sat Nav 2012 update, 17 in Wheels, 116K Mods:- Electric Autofold Mirrors on Locking
    Simple Autofold Mirrors on Locking,http://www.jaguarforum.com/showthrea...irrors+locking
    UPDATE YOUR SIGNATURE http://www.jaguarforum.com/showthread.php?t=91945

  5. #5
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    Like it!
    Could equally have been from between my ears.

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  7. #6
    Member Shug's Avatar
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    Thanks for the replies and the good humored banter.

    J Green - have you been out in the car since the swap of the servo - did it make a difference, how does it feel now.

    rchiv - Interesting about the weight transfer, it does make a lot of sense, at least for some of my concerns. Riding motorcycles it is easy to get the same affect, hence why they typically have monster brakes on the front with fairly small on the back.

    Thanks
    Jaguar XJ6C 1977 4.2 (HIF44 carbs) UK RHD ZF4HP22

  8. #7
    Senior Member rchiv's Avatar
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    On motorcycles the front brake is the main brake and the rear is just for stability. Else you would be locking the rear wheel all the time.

    Roger
    HEY I THOUGHT I HAD RETIRED, Ex Auto Electrician
    2006 X Type 2.2d SE Manual Estate. Zurcon Blue Champagne/Mocca, F/R Parking sensors, ,Auto Lights, Auto Wipers, Headlight Washers, Sat Nav 2012 update, 17 in Wheels, 116K Mods:- Electric Autofold Mirrors on Locking
    Simple Autofold Mirrors on Locking,http://www.jaguarforum.com/showthrea...irrors+locking
    UPDATE YOUR SIGNATURE http://www.jaguarforum.com/showthread.php?t=91945

  9. #8
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    In my opinion & experience (XJ6 S3) if no hiss or whisle heard from the servo + pedal is easy to apply no hard wooden feel, then the servo is good.
    In this case I suspect the master cylinder 1st. 2nd check front calipers/discs/pads
    3rd replace both flexible front hoses they are cheap and widely available.

  10. #9
    Member Shug's Avatar
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    Hello, and thanks to everyone for their time in feeding back some information

    Based on the pedal feel I have to assume! that the servo is working correctly. The brakes do work well, I guess some of my issue is comparing them with modern car brakes and then putting my "how do I improve things" glasses on.

    Underlying all of this is I'm currently running with 16" wheels with 225/55 modern tyres, maybe not good performance tyres but more modern that 1977. So with may swap I've probably increased the front tyres ability to brake, which will increase the weight transfer to the front making the rears more prone to locking oops! the green stuff pads may! even have exacerbated this. So to get rid of the rear locking = oversteer, which isn't too good if I'm braking in a bend, I have choice of lowering my C of G , reduce my rear braking performance or increase my front braking performance. I'm currently happy with the ride height so this is now heading towards a brake upgrade which appears to be a minefield and beyond my initial questions. I have found StopTechs web site interesting reading http://stoptech.com/technical-support/technical-white-papers/brake-system-and-upgrade-selection
    I need to review my options
    Again, thanks for the feedback.
    Jaguar XJ6C 1977 4.2 (HIF44 carbs) UK RHD ZF4HP22

  11. #10
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    the rear brakes should not lock before the fronts in any circumstances. This would have been an original design consideration under approval testing (even in 1977)

    I think I would be looking for master cylinder issues first and then in front calipers

    A good MOT-style roller brake test would also give you some figures to work with

    As it happens, I'm also in Newcastle and have a lot of design experience in braking systems, so would be glad to help if necessary

    your worst case would be two occupants, full braking, empty fuel tank and coming down the steepest hill - and it should still not lock the rears under those circumstances.

    your wheel/tyre package is unlikely to have changed the weight transfer, but I'd check to see that your front calipers are original design and that you're not missing a G-sensitive deceleration valve anywhere

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