420G fluid type and size of bleeding hose

Hi all,

Hope everyone is safe and as well as can be expected.

Hoping somebody could confirm brake fluid type for 1967 420G and what size tubing required for attaching to the bleed valves?

Thank you


I’ve been using 1/4 inch inside diameter clear aquarium tube for bleeding. Being clear it lets you see what is coming out, fluid or bubbles. As to type of fluid, the standard is DOT 4 brake fluid but I have successfully been using DOT 5 silicone for decades, but there is much controversy on using silicone.

Or you can go to DoT5.1 and stay away from the silicon and benefit from the higher boiling point.

perfect, thank you.

Also could not locate rear bleed valve?

There is one on the top of each caliper. A bit hard to access and very close to the exhaust pipe that runs through the suspension subframe. At least that is the way it is on my 3.8S.

OK thanks John,

Will try to have a look under tomorrow… will be quite a squeeze!

I recommend almost evacuate the resevoirs first ( I use a huge syringe)

replenish with clean fluid, then bleed, avoids sending old fluid into system, and makes sure you have fresh new fluid that has not absorbed moisture from the atmosphere

rears in particular are very hard to get fluid flowing…and get a spanner on…or a tube

you wont get a full resevoir through them

fronts should be easy, if not the front rubber lines need replacing

I dont worry about brake fade in a 420G, they have good brakes, and that would only come into play if they got hot…or your fluid was contaminated

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Thanks Tony,

Will give it a go in the coming days…

The feel on the brake pedal is simply zero resistance to the floor so likely the issue is the master cylinder… However I assume it will not harm flushing the system ?


Something like this will save you hours of frustratoin in the future. It will fit the big sedans:



unless the fluid has leaked out somewhere, almost certainly the master cylinder.

They can be fitted with a new seal kit, but the more may be pitted


Yes I have seal kit on its way. Do you feel I should wait, reseal and then flush fluid?

yes, because there will be no pressure to bleed…rears need pressure…fronts should bleed just by gravity.

There are 2 types of seal kit for a 420G, very early, and later.

(The early one has a booster with a steel band around that allows splitting the booster)

Its not easy to assemble either, so pay a lot of attention to how it comes apart.

Typically a re sleeve is needed, you may be lucky

Hi Tony,

Yes that makes sense.

I just had my manual out for review and page 14 & 15 are missing!! Fairly tragic as detail the removal and replacement…

Would you be able to shoot these two pages across?

Sorry to bother!


Hi…I put this up as it shows a picture of the MKX brake booster/early 420G

It is misleading though…and there is nothing in the 420G supplement for the later item

The MKX and 420G booster and master are different

Assuming you have a later 420G, just carefully remove the brake union nuts,
then remove the 2 half inch nuts that hold it on the booster.

You will need a 1/2" allen key to get it apart

put a lot of plastic and rags under it so no fluid gets on the paint, and fluid doesnt drip back out of the disconnected lines

Thanks so much… I assumed they (Mk10 & 420G) were the same.

Yes I will preceed very carefully.

Thanks again



So I have not detected any leaks at all from likely areas.

I have from Welsh (see attached) replacement seals for the MC. I am hoping this will do the trick, we’ll see.
Is anyone aware of a youtube link where this is detailed or similar?



your master cyl should look like this one…a S1 XJ6…if you have a 420G

you need a 1/2" allen key to remove the tipping valve assembly…part 3

its difficult to reassemble the seals and parts associated with spring 14

its essential to pay close attention to the orientation of each seal…and measure its length.

Its a known problem for one of the seals to be too long, meaning the rear brake circuit will be blocked, in that case the earlier style seal kit has that seal

Thanks Tony,

Yes I have 1967 420G, all original. We are having a few “colder” days here in Canada so will wait a few days and get with it then.

Thank you

Hi Tony,

Indeed the MC is the one as detailed in the diagram you kindly attached . I ask one favour: is it possible to resend a larger copy of the diagram? I cannot increase the sise without distortion. Or perhaps where the diagram is taken from?
I have manuals for Mark 10 and recently purchased one with 420G additions but even these do not detail teh MC as per your diagram. When you have a moment please let me know



s1 master cyl-crop-crop

thats the biggest I can make it…you should be able to “save a copy” and enlarge.

It is from The XJ6 S1 Manual (same MC and booster as 420G)

be aware to take careful pics and measurements of all seals…if oriented backwards, or too long, they will not allow the re-kitted MC to work.

I would recommend MC bore inspection, preferably by a very experienced brake man.

The bore is slightly stepped, and any pitting at all requires a re-sleeve

ensure the master cyl itself will bleed prior to connecting the rear lines