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Hi All,
I'm considering what to do with the brakes on my MK2 Jag, one thing I wanted to consider was converting it to dual circuit for improved safety however, i've looked at this and there are no commercially available kits to do this.
The 420 had a system which was partially dual circuit but even then the link between the master cylinder and the servo was single circuit and hence a single point of failure.
I have considered fitting an X300 pedal box, master cylinder and servo and altering the bulkhead to suit. I'm not entirely sure on how this would affect the system hydraulics. I would also need to move the battery and considered either putting it in the boot of possibly on top of the front crossmember under the front wing where the vacuum reservoir would normally be.
The X300 master cylinder appears to have a bore size of 23.8mm (around 15/16") whereas the brake master cylinder for the MK2 is 7/8" which I understand in theory will make the brake pedal harder to press. I haven't been able to find the servo ratios yet.
How do you work out the difference as presumably the X300 master cylinder effectivly has two 15/16" pistons moving fluid and the MK2 only has one 7/8" piston.
Does anyone have any experience of this? I'm using the original brake callipers at the moment which are 2 1/8" at the front and I thin 1 3/4" at the rear.
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This seems a major exercise for little obvious benefit. Dual circuit braking only provides additional 'safety' if the vehicle is so badly maintained that its brake components are allowed to corrode or otherwise deteriorate over decades, and then fail in service. I could see the merit for an offroad vehicle used as intended but not for an adequately maintained road-going car, especially a cherished classic. I've left my 420 with its single point of failure. If I were thinking of safety upgrades then cunifer hard piping and Aeroquip hoses would be my first port of call. On a MkII I might think about later disc and caliper options, but ultimately the limit to to braking performance will be your tyres and the driver!
I'm curious what has spurred your interest in this 'upgrade'.
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Last Edit: Jun 4, 2020 15:19:35 GMT by theoldman
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I believe the smaller the cylinder diameter, the higher the pressure is but also the stroke will be longer. Looking at the differences in diameter which are small, i believe you would hardly notice it Peter
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This seems a major exercise for little obvious benefit. Dual circuit braking only provides additional 'safety' if the vehicle is so badly maintained that its brake components are allowed to corrode or otherwise deteriorate over decades, and then fail in service. I could see the merit for an offroad vehicle used as intended but not for an adequately maintained road-going car, especially a cherished classic. I've left my 420 with its single point of failure. If I were thinking of safety upgrades then cunifer hard piping and Aeroquip hoses would be my first port of call. On a MkII I might think about later disc and caliper options, but ultimately the limit to to braking performance will be your tyres and the driver!
I'm curious what has spurred your interest in this 'upgrade'.
There are couple of reasons i'm considering this:
It will provide an additional degree of safety having two circuits. There is less to go wrong as the servo has no hydraulics within it. The X300 master cylinder are reasonablly inexpensive compaired to original components.
I had an S-Type around 15 years ago and the professionaly rebuilt servo failed while I was doing 50mph coming up to a sharp bend, I nearly stuffed it into the back of the mercedes in front but fortunatly the handbrake managed to slow me down enough. There was no warning that this was going to happen, ever since i've been concerned about single circuit brakes as you have no safety measures in place and with a big heavy automatic you can't even use the gearbox to slow it down.
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May be worth having a look at the dual circuit system used on the Volvo 140 series, and the final year or two of the Amazons. They ran both circuits to each front caliper (operating different pairs of pistons on the Girling four pot units), and then each circuit took one of the rear brakes, to create two triangular circuits, complete with a dash warning light to warn if one circuit had gone down.
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Sept 30, 2020 17:37:23 GMT
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I've started to look at this and so far have purchased a pedal box, servo and master cylinder from an XJ6 (X300). This looks like it can be made to fit but the pedal is cranked 50mm over to the left which moves it around 100mm from where it needs to be.
I looked at bending the pedal but was concerned about doing this so came up with another plan, the LHD car uses the same pedal box but with a pedal cranked the other way. I've measured it up and a left hand drive one seems like it would do the trick so i've orderd one from a suppier in the USA.
Next comes the metal work, I have read that the Beechams MK2 Jags (convered to XJR power) used a 5mm plate for the new box that the master cylinder fits on with 3mm plate between this and the existing bulkhead.
Whilst i'm not doubting that what they did was strong it does seem overkill when you consider that the MK2 jag bodywork is only 1-1.2mm in this area and the original XJ cars are probably similar. Does anyone have any thoughts on this?
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It's not just about strength it's also about stiffness, a couple of mm deflection between the pedal pivot and master cyl can easily end up as 20mm of lost pedal travel, you shouldn't need 8mm of steel if you include stiffening flanges etc. though.
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How will you arrange the circuits?
Modern stuff is mostly diagonally split. But to stop them pulling violently towards the working front side they tend to have large amounts of caster angle, which you won’t want on a classic, even if it were achievable as it would make the steering unreasonably heavy amongst other things.
Older dual circuit systems tended to be front/rear split, which doesn’t raise the pulling issue. The drawback is that if the front circuit fails, the rear brakes alone tend not to be very good at stopping you and can easily lead to a spin if braking really hard. As already mentioned there were a relative few models (Volvo, Austin Princess) with true dual circuits retaining some braking effort on all four wheels. Clearly the best from the safety perspective but complex.
Nick
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1967 Triumph Vitesse convertible (old friend) 1996 Audi A6 2.5 TDI Avant (still durability testing) 1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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How will you arrange the circuits? Modern stuff is mostly diagonally split. But to stop them pulling violently towards the working front side they tend to have large amounts of caster angle, which you won’t want on a classic, even if it were achievable as it would make the steering unreasonably heavy amongst other things. Older dual circuit systems tended to be front/rear split, which doesn’t raise the pulling issue. The drawback is that if the front circuit fails, the rear brakes alone tend not to be very good at stopping you and can easily lead to a spin if braking really hard. As already mentioned there were a relative few models (Volvo, Austin Princess) with true dual circuits retaining some braking effort on all four wheels. Clearly the best from the safety perspective but complex. Nick I plan to just go for front/rear split as its only for use in the event of an emergency and i'm not planning to drive the car that hard.
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It's not just about strength it's also about stiffness, a couple of mm deflection between the pedal pivot and master cyl can easily end up as 20mm of lost pedal travel, you shouldn't need 8mm of steel if you include stiffening flanges etc. though. I think i'm going to go for 3mm plate and then put some stiffening webs in underneath. I think 5mm is just too much, especially when it will only be welded to the thin bulkhead.
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