Brakes confusion
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  1. #1
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    Brakes confusion

    I am having trouble getting my ex Jag XJS brakes working right on my Hotrod. The front/rear balance is wrong and the pedal without servo needs far too much effort. They seem to work pretty well with the servo.

    I question whether I have got the brakes pipes installed right. I have a diagram from (I think) the right vintage which definitely shows the front brakes operated from the far end master cylinder port and the rears from the inner port which is exactly what Enoch says in his post 05/09/2014. However, as far as I can see from the diagram he posted, it is the other way round.
    Also I am a bit confused by the markings on the cylinder. The inner port has a P marked – is this Primary? – it certainly operates first when the plunger is pressed. The end port is marked S (secondary?) and operates later. Shouldn’t the primary braking be on the front? Could this be my balance problem?

    The master cylinder operates freely on the bench.
    The PDWA (on the bench) allows fluid through.
    All callipers seem free and all pipes are new.
    The servo obviously works but with the master cylinder disconnected the foot pressure needed to operate it without vacuum is considerable – is this normal?
    I am considering removing the PWDA for simplicity and it may be causing my problems. Lots of conflicting views about whether it does any more than provide the warning light and it seems that many XJSs just didn’t have them. Got the following from a Jag Workshop online manual:-
    “NOTE The brake system pressure differential warning actuator (P.D.W.A.) unit has been deleted. This deletion in no way affects the performance of the braking system as the conventional split system is retained”

    Any answers to my questions or ideas would be greatly appreciated.

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  3. #2
    Off the Xmas card list kapri's Avatar
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    Jag also used a diagonal split system for awhile . Front of m/c furthest from servo is conventionally front , the proportioning is done internally by seal position giving a 52f/ 48r split. It's about 14 years since I used a Jag m/c so can't remember which way but I'm pretty sure it's per convention.

    The bleed off can be seen as bubbles in the m/c reservoir at full travel .This is normally a sign of a shagged inner seal but NOT o a Jag unit ( learned the expensive way and involved speaking to Lockheed brake designers who didn't know how it worked but the blueprints showed the split).

    With no fluid in system the rear WILL operate first once bled they operate in tandem unless one circuit fails. Best to bleed rear system ( nearest servo) first to ensure enough movement to make front circuit operate

    Yup, Jag brakes are ****e without servo as m/c has large bore to provide the fluid requirements for an all disc system. However large bore = poor line pressure hence the need for a servo . Depending on servo size it can double or treble line pressure.
    Last edited by kapri; 14-11-2015 at 19:43.

  4. #3
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    Update. Servo now working perfectly with 18"hg. Got rid of PDWA as it is not a proportioning valve just a pressure inbalance warning.
    On testbed the brakes work well with no servo (as required by IVA) but very hard pedal. Balance across axles is perfect. However, with servo, fronts now lock with the slightest pedal pressure.
    On the road, in the dry, you need very light pedal pressure otherwise you almost get thrown through the windscreen. Haven't tried it in the wet!
    So how do I correct this imbalance?
    I think I need an adjustable proportioning valve but IVA does not allow them. They are available. Anybody know of a nonadjustable valve that might work?
    Perhaps if I reduced the servo vacuum (pedal sensitivity) it might help - after all it works fine with non servo (but pedal too hard for comfortable driving). An adjustable vacuum relief valve is available from USA but can't find one here.
    All new pads, discs, hoses and master cylinder rubbers.
    Any ideas please.

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    perhaps a smaller servo? v12 xjs must weigh considerably lmore than a T. how small tyres are you running at the front?

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    Off the Xmas card list kapri's Avatar
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    Common problem on a rod where the engine sits behind the axle centreline as opposed to over it in a Jag. You need to reduce either line pressure (larger bore m/c or smaller servo MAY do it ) or change front calipers for ones with a smaller bore . You CAN fit an adjustable proportioning valve , it just has to be immobilised from adjustment once you are happy with the balance.

  8. #6
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    Thanks. See what you both mean about the weights and wheel sizes,
    T weight is about 1300kg (700Kg front, 600Kg rear) vs 1900kg for the Jag and engine IS well behind front axle
    Fronts 265 x 50 x 15
    Rears 31 x 16.5 x 15 (big!)
    I have fitted the servo under the dash (mainly for aesthetics)and adapted the pedal so it's complex to change. Could change the m/c but going to be guesswork to find the right one.
    I think the immobilised adjuster may be the way forward - are you sure about this? It says:-
    "Manually adjusted valves (other than to permit presetting the automatic function of a valve) are not permitted to be fitted even if they are rendered un-adjustable"
    I read it that I couldn't do it

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    Off the Xmas card list kapri's Avatar
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    Seems unreasonable if so as they allow you to fit adustable twin m/cs that are rendered non adjustable afterwards . I can think of a way round it anyway based on that wording , simply use a load adusting sensor from the rear of a suitable vehicle but in the front line . That wording means you can adust a pushrod initial setting on a weight biasing valve.

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    change of pedal ratio perhaps?
    the amount of fluid is going to be correct for the calipers as they are off the same car its just the weight of the pedal, could the pressure (vacuum) in the servo be limited in some way to provide less assistance, I must admit to not knowing how to quantify the assistance a servo gives but smaller servo seems logical could you rig up an adjustable length pedal and experiment? personally I like my pedal to be pretty heavy and prefer no servo perhaps too much for a heavy v12 but what would no servo longer pedal be like?

    sort of thinking out loud and writing it really so just suggestions you could try

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    Off the Xmas card list kapri's Avatar
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    Servo assistance is governed by bellows area o a combined servo =m/c . On an inline servo it is done by bellows area and bore of secondary m/c on remote.

    O my last T I had to do similar to derate the brakes as mine was sliding all over the place at the hint of a breath on the pedal. I went up a size on the m/c to reduce line pressure brakes were still good but not over sensitive.

  12. #10
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    Lots to think about!
    Due to the cramped space it's going to be very difficult to change the servo and the pedal is about as long as it can be. Very little scope to shorten it either.
    I wouldn't feel confident with no servo - the pedal is very hard and leg angle (again due to space) is not good for exerting heavy pressure. My first thought was to reduce the vacuum but can't figure how to do it. There is an adjustable vacuum relief valve (which may work???) available in the USA but can't find anything here.
    The adjustable bias valve in the front line sounds exactly what is needed but once set can I disble the adjustment enough to satisfy the IVA. Has anybody ever done this? This is available, again from USA

    Shop Brake Proportioning Valves - Free Shipping @ Speedway Motors

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