Mk10 Brake booster and engine compartment questions

pretty big hmmm there Ron

the problem is bad design…the heater box sits down slightly in a recessed double skinned flat area of the firewall…if the car has ever been left to sit in non-dry conditions, this area can rust out. Metal touches metal…mine looked perfect but was a long job to repair

To open the scuttle vent, best I recall, just lever it up

take the following precautions…place cloth around a thin screwdriver , and work it under the front lip, do the same with another, and wedge it up gradually.

It is actuated by a vacuum valve behind the glovebox…access can be had there, and the con rod undone if desired…I do not recall this is needed…the return spring is very strong though, its surprising the vacuum valve can actually open the door

I recommend remove resevoir and Jag vacuum valve, install one-way valve to manifold

the inner windshield flange can be inspected for leaks and rust by removing the triangular wood pieces

I have done the back, the front screen is one of the last bits on my car to be done…it leaks

Windscreen gasket is deteriorated and will need renewal (backscreen also) so that would probably be the time to get into this. From my past experience with Jags and other vintage cars, this is the kind of issue that kills vehicles by taking them off the road for a fix that never gets finished. This was a central Florida car, and it shows evidence of a lot of sun exposure, but not much water, generally very solid except corners with a few areas of rust typical of a dry coastal climate. The key-fob is from Istanbul! While supposedly until recently a one-owner car, the story is unreliable and it is what it is. Although your insight into this Mk10 problem is invaluable, I’m not going to tackle the heater box for now. When I am eventually brave enough to go there, I’ll give a full report.

my car had 3 genuine local owners before me, and looked totally original

turns out it had a accident down the entire PS side, which had a professional repair many years ago…but using filler for the dents…its been fun fixing up that…as you say, they are what they are

I had a matchead size bit of rust showing under the rear screen seal…when I took it out, the water had tracked down and rotted the junction of roof, C-pillar and rear parcel shelf.

I had to remake these sections, including a section of inner & outer windshield flange, and MIG weld it all back in…there was about 2lb of lead covering the whole area before

not trying to scare you, it is evident from your posts you have a lot of experience

Prior to extracting that heater case…make a post, and I will give you some tips that will save time and swearing

Hi Guys. Ill interfere your discusson, if I might to. My 1967 420Gs brake booster is wrong, and its above mentioned Dunlop P90. Cant find any service kit too. I just recieved a spare Dunlop P90 unit but Im thinking of upgrading the brake booster though to a later, more easily servicable one. Just now, I can find offer of XJ6 series I brake booster as well as later XJS/XJ6 series II or III brake booster. Im quite confused of all that booster units, cause their shape is a bit different, but still so similar. So - a simply question. What is a difference, if theres some? Does all these brake boosters fits 420G and its master cylinder or not? If so, does it really matter which brake booster will be put inside the car? Should I go for series I brake booster or better a later one? I read, that you sugested Girling supervac 100, but those brake boosters are not labeled by its name, only by a jag type… And, supplementary question: are brake boosters from different English car of that era the same/exchangable with jag ones?

Btw, just wondering. Ron, dont you bought your Florida jag from James Love? Dark interior, opalescent silver blue exterior?

My best answer is that as far as I know;

No Jaguar brake booster I know of will match of will match your existing master cylinder
(if it has the engagement portion protruding forward from the master cyl front ('65-'67)

an XJ6 S1 brake booster will match your, firewall mounting, and fluid containers without alteration

I recently saw that new S1 boosters are available (or an old one can usually be rebuilt)

In the instance you acquire a used S1 master cyl, it should be sleeved and re-kitted before use.

Ron is going thru this now. He is not the first

therefore upgrade to a later 420G or S1 XJ6 booster/master combo

later ones will work, but need more mods

Thank you for your reply, to be clear, this one is the current


And those are the options. First one is later 420G?
2020-11-18 00_49_51-Jaguar Mk10, 420G Brake, Master Cylinder, Servo And Pedal _ eBay - Opera
This one should be from Series I, but it seems to be the same as above
2020-11-18 00_49_34-JAGUAR XJ6 XJ12 BRAKE POWER SERVO BOOSTER ASSIST MASTER AIR 246247 11963 11963E
Both of them are Girling supervac 100? Service kit for those should be avaiable, than re-kitting is much more available than witch current Dunlop. So it seems it doesnt mind which one from those two options I choose… It seems to me that master cylinder is the same as current one, is it? Which mods are needed? Just a brake hoses change, as the booster is shorter, but brake pedal body and fluid containers should fit, right?

Hi Marek,
Yes, I bought from James, quite an interesting guy.

I have some very good information for you, as I am in the midst of fixing my 4.2 Mk10 brakes right now. What you need is a series 1 XJ6 master and booster. Both are rebuildable and will bolt up. I did note that there were hammer marks on the inner fender well next to the original booster on my car that must have been done at the factory, as there is no way the P90 would have fit in otherwise.

I used a series 2 setup because it was given to me by a local friend, and the booster can was changed to series 1 by my rebuilder (Steve at Booster Dewey in Portland Oregon did a fantastic job, but I think you are outside the US). The internals are the same, but the series 2 bolt pattern is wrong for the Mk10 pedal box. The series 1 master has banjo fittings for the fill tubes rather than plastic, and the series 2 has metric outlet fittings which requires adapting the pipes. Lazar at Apple Hydraulics sleeved and rebuilt the master. So, easiest just to start with series 1 XJ6 stuff.

One difference that you will have to deal with is the fact that the push rod for the P90 is removable from the booster, but the XJ is not. This requires you to hook the pedal to the yoke from inside, difficult but possible to get the pin in - impossible to get a cotter pin on. My solution was to remove the pedal box (easy, 4 nuts on studs) and bore a 1" hole in the side (step drill) so that I can slip the pin in from the outside and then have room to put the washer and cotter on from inside. I’m planning to reinstall the box and booster tomorrow, and I’ll send photos. For now, here is what the modified box looks like including a hardware store plug. Note that the parts book shows a hole there, but my car did not have it. Let me know if you have questions.

thats weird, mine have the hole pre-drilled, with a rubber plug, and you can reach in and disconnect the cotter pin (with some difficulty)

kudos to you for coming up with the same solution as factory engineers

I’m suspecting that all the 420Gs probably have the factory hole, being later than my car built in '65. Btw, the XJ6 pedal box does not fit the mk10. I am also planning to use the nice fluid reservoir from the series 2.

My calipers, which are the hybrid version (half Girling half Dunlop) are off being rebuilt. By the time they return, I should be finished with the front suspension refurbishment and complete cooling system replacement. Then it will be time to drop the rear suspension to get to the rear brakes. Goal is to be driving (transmission willing) by the end of this year. I also am in the process of trading for a 700r4 o/d transmission that could be grafted (with body mods) if my leak persists. Tony, thanks again so much for your frequent help and encouragement.

thats interesting…I think mine have the seal in the caliper, so just need new pistons and seals (which I have, waiting to do). The caliper bore itself does not have to be perfect ?

Are yours different ?

Its good to see a few ones coming along

after many years as the only one, I have just found out 2 have come into our local club

I believe you are correct about the Girling side of the hybrid caliper, although I did not disassemble them. I will ask Lazar about it when he calls me for payment. On the Dunlop side, you will need a good bore or to have it sleeved. Much simpler to just mail the Dunlop cup, as a pair of calipers is 25lbs. I just have too many other things going on right now and didn’t want to tear them down. I probably should have.

I have never seen an operating MK10/420G at anywhere in the Pacific Northwest other than the Flueguar hotrod a number of years ago.


I expect that there are a few functional vehicles around though (British Columbia, maybe?), and would love to see them and meet their owners when it becomes safe to do such things here. Of course, I am not allowed across any border for the time being due to the skyrocketing Covid outbreak.

Ron, thanks a lot, give me a better understading, that when I decide to upgrade, Ill be looking straight for S1 servo booster. Im not sure about the one I intended to buy, as I find out that one bolt to secure pedal box is broken and a new bolt has to be welded to. Im not sure how secure that will be and, moreover, I found out that new S1 servo boosters still can be found for +/- 250 USD, so theres no need to hurry up at this point… Ill investigate more closer my Dunlops first and maybe contact a local company, as Im suspicious, that they renew a Dunlop booster in the past (but I suppose it will be a bit costly). Ill give it a bit more time and decide later on, as my plan is to get my 420G back to the road in 2-3 years, so Im having a bit more time than you do :smile:
But was interesting point with the pedal box. I investigate mine, and the result is, that original LHD 67 has no hole in it, the spare one I bought (should be RHD from 66) has the hole in… but the quite big difference is in the pedal boxes themself, as the one I took from my car has much broader lower part than the spare one I bought…it brings me to a question, if those differs just because its LHD/RHD version? Otherwise i thought, that MK10 and 420G pedal box is the same…


And regarding James, as Im from EU, I talked to him just via messenger, he seemed to be a very nice guy, willing to help and truly Jag enthusiastic. After that I saw him at Jay Lenos show, and yes, lets say I can understand that word “interesting” :grinning: To be honest, I was really close to buy your jag, it seemed to be overall in very nice condition, no rust, drivable and I really love the opalescent silver blue colour. But finally after two weeks of torturing myself, I decide to buy another one. Wasnt an easy decision, but finally I stayed with my original plan to seek for 420G, not a MK10. So i bought a more expensive one in a much worser condition just because of ribbed engine and clocks in the middle of the dashboard :grinning: Well, its not easy to fulfill mans dreams… :grinning: So I wish you all the best with your jag and I hope it will be a great and reliable car…

The one from your LHD '67 looks the same as mine, and I believe the 420G was the same. I think the other is different because of RHD, but Tony would know. I recommend that you consider the S1 XJ booster, as the Dunlop parts are rare if you find them - you may get stuck down the road. Plus, I don’t think the Mk10/420G master is rebuildable due to no parts. I don’t think the old master is compatible with S1XJ booster, but not sure. Anyways, better to upgrade. If you get a used booster, be sure it has the rectangular bolt pattern to fit your pedal box, not the square one used on S2, maybe late S1.
So, I did install today, although will pull the master off to prime later. I found it MUCH easier just to put the pin on the booster yoke before mounting the pedal box in the car. Otherwise, it’s a two person job. When you drill the hole in the case, try to put it as close to the stud support as you can. You will see what I mean when you look at it.
My car is very solid, but nowhere near a driver although the po did drive it a bit. I really like it a lot, but already have large hours in it and will have much more time and $s before it’s back on the road.
Let me know if you still have questions about the brake upgrade.

I dont have any at this point, I think you gave me a lot of usefull information already, many thanks. Maybe Ill have some other when I do my brakes… Now, Im working on my wire harness to be working completely, afterwards I plan to renew my cooling system, fuel distribution system, and brakes are fourth on the row. Regarding master cylinder, it seems to me that early S1 has the same as 420G. Previous owner told me, that master cylinder was already rebuilt just before he sold me the car…so well see, Ill give it a try and if it doesnt work, Ill purchase later one.

Sounds like an excellent plan. Helpful diagram showing presumably early series1/420G, later S1, S2 masters on the same page as the S1/2 booster, at least implying that they work together.

This added nothing, deleted. Keep metrification in mind.

Yes, I thing so. Its from the MOSS Europe webpages. And when you check my earlier post, with pics of available boosters, you can see S1 servo booster with the early master conected… so I firmly believe, those fits together.

as far as I know, 420G and S1 XJ6 masters are the same, but may have different part nums

the kit is the same

LH and RH have pipe exits on opposite side…so are different part nums

There is master cyl kits for the 65-67 Dunlop Master…just no brake booster kits for the Dunlop P90 booster (that I know of)

Interesting…I have a '66 RHD wreck, that has been upgraded to entire S1 XJ6 brakes
It has the later booster and master fitted

As well as XJ6 front calipers…(never be tempted to consider that mod)

So, guys, i continue to work on my car in my own tempo… slowly and unsure :slight_smile: Finaly came to master cylinder, so i add here my notes. Late 420g and S1 master cylinder are the same (left is from my Dunlop P90, right is from the S1 brake booster). I do believe it was originaly on the car. LH and RH master cylinders are the same, brake pipes just go under the cylinder from the left side, as seen on picture… Need to rebuild it fully, already bought wrong kit, im looking for the right one… Its not easy with the parts, costs and shipping time here regarding Brexit…


But, mainly, as im rebuilding my brakes completly, id like to ask… Dont you know how to get the pedal out of the box? Service manual is not much helpful. It should be just a pivot, but i cant move it in any direction and im bit scared to use extra force to the aluminium box…