420G changing seals on brake master cylinder


Finally looking to get to this in the week. I note on some youtube vids that the reservoirs are detached along with the BMC. What do you feel is better? My original thought was to remove fluid from the reservoirs and then remove BMC.

Let me know what you think.



the 420G has remote resevoirs…just detach all lines and hoses and unbolt the MC.

be very careful not to spill fluid on paint, plastic with towels on top is the only sure way.

I syringe out my resevoirs

Tony what DOT number are you using in your Jaguars?


not Silicone, hence my warning not to spill

Thanks Tony,

Will follow those steps


As per photo, I have detached from reservoirs, brake lines and unbolted from the Vacuum. However, the cylinder is not showing any sign of wanting to come away from Vacuum… almost as if it is not separate. I don’t want to force a greater problem and wonder if there might be a suggestion?



they are separate, probably old fluid has just glued them, tap with a plastic mallet, spay penetrating fluid

OK perfect, thanks. I assumed this but it just felt I should check first.

Thanks again


Hi Tony,

So the 2 pics are of what came out. The 2nd spring and assembly is bit of a puzzle. The manual states that one should "lift the leaf of the spring retainer … What does that mean? If possible, please let me know the location…


  • Also, I was able to locate the full Brake manual ref for XJ 6 s1 I have attached and hope helpful


BJAG XJ6 s1 10 Brakes.pdf (924.7 KB)

it might be alluding to that plastic retainer behind the spring…I can confirm this is tricky to get back together…from your pic, it may not need to come apart ?

what seals are in your repair kit ?..its vital they are oriented correctly

The seals in the kit are I suspect incorrect. The diameter on the thicker seal (looks similar to one on Gland seal #8 but the diameter of teh supplied seal is 50% of this seal and impossible to apply.https://www.welshent.com/Search.aspx?k=sp-2484-A

I just looked at Moss and their repairs seals for a 420G look completely different https://xks.com/i-7143760-jaguar-7-8-brake-m-c-kit-10754.html?ref=category:1207224

ANy thoughts??



Yes, I believe I experienced this issue, in that I supplied the identified correct kit to my sleever, which was the “later” kit, (68-70) but it would not fit, and was resolved by fitting the “earlier” kit (65-67)

This is a strange issue and appears to be some sort of worldwide mis-identification matter

Thanks for detail. Yes I may just order a new one? I just put it back together and thought I would practice bleeding the master. If you check the attached pic, only the nearest brake line is bleeding, nothing from furthest… ANy thoughts?



trace the one that isnt bleeding…is it the rear line ?

that was the issue with mine, after rebuild, I could not bleed the rear brakes, and traced it back to the master cylinder

The “so-called” correct seal kit has a “too long” seal that blocks the rear port from bleeding

The remedy was to fit the seal kit from the earlier MC…65-67…NOT 61-64, thats different again.

Note…this was done after I had the bore of my master cyl sleeved in SS, and the rebuilder sourced the 2nd kit

Traced the line and yes it’s the feed for rear brakes. Do you have the reference for the 65-67? Is it for mx 10 or another?

Thanks for info


Also, will be jacking up the back so I can get the rear wheels off etc. Is there a solid part you might recommemd to apply the jack? The jacking points are not overly solid and would like to avoid causing damage during the lift if at all possible. The car has been up on the garage hoist when at the mechanic shopbut have not jacked her up



the jack should be applied under the innermost section of the jacking channel.

That area is heavily reinforced, and if there is rust there that is a very serious problem that needs repair…dont apply the jack on the outer part of the the channel

You can also use a section of wood under the rear stiffening beam on the unibody

or a section of wood under the lower wishbone

420G is not an easy car to jack up and also insert a stand, but it can be done if you are careful

I ordered a replacement but it is not the same. Replacement was for XJX series 1

Any thoughts?

I always jack up at the differential (large piece of hardwood as to not bend the plate of course), and then use wood wherever it makes sense, as per your suggestions in this case. Keep the jack in place and give it a good shove before working on it.
Since the X/420 G should have the same rear suspension as my XJ’s I don’t know why one wouldn’t jack up there.

Thoughts: I think that is a Series 3 XJ master, and likely has metric threads. It is designed for a plastic reservoir on top (two pins through the two drillings) and while it can be used, it is the wrong part. It will not bolt in. The Series 1 has a reaction valve or what it is just like the one on the right. You have the wrong part.


An XJ6 one may work, but would probably need new pipes custom made

I recommend order the seal kit I mentioned, and hope that it has the correct seal that you need
(for MKX 65-67)…it has a different master cylinder that is not compatible with the booster.

did you have the bore closely inspected by an expert ?

I got mine re-sleeved by a world leading expert, and we had to sort that seal issue as I mentioned.

You cannot afford any errors with the brake master cylinder…if it fails, you have no brakes at all, not one circuit, but none!