Series one convert front brakes to xj12?or xj6?

So the OP before converted my 72 xj6 too… I believe to be xj12 73 to 79 I thinks? So I have replaced the brake rotors and 4 pot calipers to 83 xj6 which seem to be the same part numbers? So all have been running great for 2 years besides my left rotor which is warped has I discovered spinning wheel it drags on one spot. So last week my servo started to get hard and could hear vacuum coming from rear of servo and noticed my ldle was crap. Anyways I ordered a new servo from moss not rebuilt and when removing old servo noticed someone drilled holes to compensate for a later.servo possibly xj6 the two top holes where drilled lower to make it fit, and was out at the bottom 1/2 inch so was crocked. So I’ve installed new servo and brakes work great.except they are grabbing slightly. And seem to be on a bit after applying brakes. Now I did repair my original master cylinder which is still on for series one. I did replace the little pin with spring seat unit at rear of the master. I’m pretty sure this reliefs the pressure to resovoir? My question is the master is this the problem for slight brakes sticking? And are the 4 pot calipers ok with this master? And what are the difference between xj12 and xj6 rotors and calipers. And my other question do the spindles from xj6 plug and play on series one front end. Are spindles same xjs, xj12, xj6. And should I convert master from xj6?? I plan on buying 2 new rotors and pads any recommendations for them where to buy cause last set from rockauto economy buy 19 bucks so need some direction! Thanks guys.
Happy New year to all!

Excellent brake upgrade found here:


Jeff H

Thanks jeff.much appreciated I will read that article.


Grabbing may indicate contaminated pads or discs, Sawyer - but also; new pads and discs may need some time to settle. The ‘grabbing’ may be sort of imaginary - just a wee bit more aggressive compared to the early ‘feel’…?

Two small holes commute from the master cylinder to the reservoir on later cars. First movement of the pedal moves the primary and secondary pistons to close the holes - allowing pressure build-up in the circuits. As the pedal is released; the holes open and fluid is moved back into the reservoir. The initial pedal movement also applies the pads to the discs - and increasing pressure is applied to the pads. A retaining pin is fitted to prevent the secondary piston from extraction…

As pedal is released, the only piston retraction force is from the elasticity of the caliper piston seals. This retracts the pistons only from the pads - the pads themselves are not actively retracted from the discs. But without pressure they are not exerting any braking effect - and are kicked free of the discs by even slight disc runout.

So unless the pads are mechanically preventing from moving, there should be no drag. Hve you actually checked the front wheels off the ground for drag?

Set-up is identical for the 6 and V12 - as are most brake and underbody parts…

xj6 85 Sov Europe (UK/NZ)

Are you sure the XJ6 and XJ12 setups are identical? Maybe for S2, but not S1. S1 6 does not have vented discs, so hubs, stub axle and possibly even lower wishbone are different.

This stuff is all very far back in my memory bank. Don’t trust any of the following without confirmation. I think the hub/spindle from the S3 xj6 front end is different–bigger bearings mainly. The control arms interchange with S1 but there were various geometries. In particular, lower and upper arms were changed to allow more positive caster. They were changed at different times; very complicated. See archives particularly posts from Doug Dwyer. The brake servo from S3 has different mounting than the S1. I’ve never been able to decide if there is functional difference. Total piston area of the three and four-pot calipers are identical. The masters are interchangeable; S3 have integral reservoir but either bolts to either servo. IMHO.

As a sweeping statement, Kevin - it has merit, but not gospel…:slight_smile:

Jaguar got it right first time around, and evolved later models gradually while making upgrades compatible. They may work better, but is usually a direct fit - if they do not fit it’s about the best warning you’ll get. Certainly, upgraded parts will have different parts numbers - but different parts numbers doesn’t necessarily mean they won’t work. Which is no reason to avoid using the parts with the proper parts number for the model year…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Yes the spindles and control arms were.changed to series 3.

But 2 years Frank of driving on same pads and rotors maybe it’s the warped rotor! I’m going change both and my pads I purchased seem to leave a lot of dust on wheels probably pads from china . I have a order set for EBC green pads. But like to hear from any one else using them! Not having much luck with cheap stuff I’m learning to buy smarter when it comes to brakes cause sometimes a good deal costs more in the long run!
Frank ; the pads are freeing after pedal release but seems there is a slight delay.

Robert why is there a proporsonal valve on series 3??? I remember assembling the front end its very confusing when assembling the front suspension it look off set from spindle I thought I had parts wrong way around until I photos. Very complicated for sure.

Many years ago my S1 XJ12 was involved in an accident and it looked like the front was down on one side. I had a spare front beam from an S1 6 and fitted that, whilst swapping over the XJ12 brakes. A wheel alignment was done, all good, but the car forever after “sawed” at the wheel when rounding sweeping corners, and chopped out the front inner tyres at an alarming rate, as in a couple of hundred miles. The car ended being laid up, and I never investigated much further, but thinking about it, I guess the V12 must have a different lower control arm to keep the front track the same with the wider brake assembly. Anyone know for sure? I tried comparing them, but it was a bit difficult to get accurate measurements on the car. Maybe you could set the camber

Maybe you could set the camber in the straight ahead position but when lock was applied, the camber would be off.

Don’t forget the series one XJ12, vented disc and three pot calipers fitted with spacers, same as series 3 E type,
straight swop.

As it sounds the problem with dragging and sensitivity started when the booster was replaced, I would look at the pushrod length from booster to master cylinder and the pedal stop. If the pushrod is keeping the master cylinder piston from retracting fully to open the compensation port in the master cylinder fully, the brakes will drag; this will heat the pads and rotor, causing an increase in sensitivity along with warping the rotor. Ditto on the pedal stop; if too tight will do the same.
You can diagnose by loosening the master cylinder from the booster slightly and possibly shimming it away from the booster; the pedal will be lower, but if the symptoms disappear will indicate the problem.


There is a specified distance between the adjustable pushrod and the piston in the master cylinder. At least on the S3. This means you have a little bit of travel before anything happens. I would also check if the pistons in the master cylinder come back nicely, if the rubber seals stick (from contamination) they might be harder to push back and that way keep some pressure in the lines? Just a guess. Maybe the rear seal has been contaminated with oil when the booster was replaced and has now swollen.

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After 20+ years, several older Jags, and a variety of pads, I’ve had best overall luck with Raybestos PGD9M and PGD135M brake pads. No noise, good feel, minimal dust, good bite.

(The very best pads I’ve ever experienced are Akebono ceramics but (last time I checked) they didn’t offer anything for our older Jags/calipers. Might be worth checking again, though)

Please clarify “grabbing”. You want to feel some ‘bite’ when you apply the brakes…but it shouldn’t be obnoxious.

If you mean the brakes pull to one side on application then all typical advice and repair measures apply; there wouldn’t be anything Jaguar-specific to look for

I agree that this is important to check in any instance of brake drag, especially if mix-n-match components have been used.

Plus the usual/typical sticking caliper piston(s) or internally collapsed brake hoses

Not sure/can’t remember about the Series 1 cars in this regard.

Series III V12 cars used different control arms versus the Series III 6-cylinder cars until '83 or so, when the 6-cylinder cars were upgraded to the V12 control arms. Complicated, as Rob mentioned earlier. Years ago there were a lot of postings to sort out the part numbers and changes. You’d certainly want to be very careful about inadvertently mixing things up.



Doug the brakes are obnoxious like if I just touch the pedal it seems it’s taking over applying the brakes for me! Will look in to the rabestos pads.
Cheers doug

No pulling to one side!

I will check pushed distance but it is brand new from moss should I have checked distance you think it was done from factory!

You might feel it if pushing the pedal, and if unsure, loosen the bolts and try again!

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