420G Brake problem

Brakes have been awesome until these last few days. When I back out of our garage I always test my brakes several times because the driveway is very steep and ends in a ravine! When I start the car, for the first minute or so, (whilst the hisser is working), and back out of the garage testing the brakes and the brakes work once or twice as normal, then need massive foot pressure to stop like there is zero vacuum for two or three attempts then they go back to normal.

I have been for several test drives and the brakes work well all the time, I can get a lock up, the car stops okay, its just during start up that this happens. It happens if the car has been sitting for a few days or a few hours, same thing.

Though my car is a 1967 420G it was built in August 1966 so the system is like a Mark X Tony AWG in another thread describes it as this : 4.2MKX 66-67 had a booster with a band clamp that holds the clamshells together, and type B master cyl, Mine is this clamshell variety.

I tested all my vacuum lines for leaks with starter spray and no increase in idle.

What other checks can I do to discover the issue and or easy fix before condemning the booster?

If it is the booster is there a repair kit for this type and if so from where?

Are there any booster shops that can refurbish this type?

Should i simply go ahead and install a XJ S1 as Tony suggests and get it over and done with?

Thanks for your advice

I have not personally worked on this type of booster/master combo, but I do know the way the booster connects to the master cyl, as referred to in MKX FSM shows that the master cyl has its tongue poking out, and this needs to be adjusted right.

It may be this is jamming, or not correctly adjusted.

there is also possible a tiny piston in the booster (as in older units) that regulate internal air pressure…on older models, these get stuck.

I have rarely heard anyone describe work on these units, I believe the 420 may also have had them

Not much help, but I have never heard about this problem, and Ihave owned one G since 1988 and the other one since 2010. My 4,2 Mk10 (has the same setup) but is more recent. So, itś not a common problem.

Ok I think it is a mental problem:exploding_head: rather than a mechanical one, or possibly both.
We recently bought a BMW 128i rag top for the missus and I get to drive it occasionally. It’s Auto transmission as well but unlike the Jag, the Hyundai and Dodge Ram that we have, on the Beemer you have to step on the brake pedal to get the car to start. So unwittingly I am doing this on the Jag too, which I never did before (and with handbrake on and in park I don’t need to). I might have even taken my foot off and then back on again whilst I adjust my seat belt and secure my coffee cup, every time I depress the pedal with the engine off vacuum escapes.

Ok, here is where I need some help. After switching the engine off when the system is at max vacuum (Servo and reservoir) if I depress the pedal it takes only 2.5 pedal presses for all my vacuum to be gone, (pedal rock hard). How does this compare to other 420G/Mark X owners who have the same system, that is, Dunlop Clam Shell type?
Q. Do you also have only 2.5 pedal presses?

I checked my one way air check valve and it was ok, just in case I had a spare new one and put that in anyway and there is no difference so air check valve is ok.

Thanks guys

As a note to my MK2, I get ONE push on the pedal and it becomes rock hard and I basically do not have any more foot brakes… Any maneuvers in the driveway must be taken with great care knowing I have limited foot brake action. I always keep a hand on the parking brake moving the car around the garage. Interesting our Ford Fusion has a electric parking brake, which I just love. It will activate at any speed below 2 MPG with a jolt! I have for many years put all cars in neutral, applied the parking brake, let them roll and settle then put into park or gear for safety. I am Appalled when I see people throwing their automatics into park when parking and knowing how much harm they are doing to their parking pawl. That pawl isn’t designed to support that weight or keep a vehicle stationary on a hill. On my MK X I have heard that parking pawl click when the parking brake wasn’t applied fully and the vehicle moved down hill a few feet.


Gerard, after turning off the engine on your Mark X how many pedal presses do you get before all the vacuum is lost? Also is your servo a Dunlop clam type like mine and do you still have the vacuum reservoir?

out of interest, I will check my 420G to see how many presses I get, think it is a bit more than that

altho 2.5 seems a bit low, as long as it is consistent with monitoring, I would not be overly concerned, and seeking info as you are

One thing I did alter and largely eliminate is the vacuum booster system that supports the plenum door, just completely useless

I think the system you have is called a “Dunlop 90” (will check my ref later and correct if wrong)
As its entirely likely the booster itself would benefit from an OH, I would ring Barrats to discuss whether they have that kit.

Be aware that almost everyone will lack knowledge on your system, and possibly sell you the wrong stuff

with these big heavy cars, you dont want to lose booster pressure, it takes very heavy leg pressure to stop them (in a driveway rolling situation!)

I have completely over hauled every part of my brake system.
My Girling Supervac 100 booster cost $400 to be rebuilt

think we need a sort of semi register thread to ID some Zenith owners

I´m all for a ”Zenith corner”, as I have 3-4 and like them tremendously, though this summer the Mk9 has been a daily driver, almost, as it blocks tge entrance to the garage (where 10 other old Jaguars live).
Coming back to the brake booster issues, do we have any relevant pictures to identify the variants of MC and booster?
I used to have a binding front left caliper, and was told that repair kits are no good as the O-rings are of incorrect size (and these calipers are unique to the 4,2 Mk10 ang 420G). One G had the conversion to 420 calipers done while it lived in Chicago. This necessitates the switch to 15” XJ S1 wheels and normal 70 profile tyres. I happen to love the 82 profile XWX 205 VR14 tyres.
Anyway, another, current, issue is that the hoses from the MC into the inner wheel arch build up a layer of tiny spots of water or DOT 4 fluid. Takes about 3 weeks of inactivity, and is easily wiped off.
This might be related to the lack of tightness between the brake fluid bottles and the alu lida, where the threads of bottle and lid are so bad. I tighten the lid, but it wont tighten up, and I can just lift off the lid…
I did try another vendor, but they all have the same supplier.
This in turn means that the humidity tester for the DOT 4 blinks red after just 6-8 months. (The plastic bottles with plastic lids on my BMW coupes from the early 70s, hold the DOT 4 perfect for 3 years).

But other than our brake issues, the cars are fantastic. I drove one of them for 375 miles on Thursday, going from Stockholm to my summer house in the S of Sweden. The road holding is far superior to my fatherś old S-type, or the overrated Mk2. In the Zenith we sit in the car and not on it, a bit like a gokart😅. I do 120km/h at 2500 with the ZF 4-speed box with OD on 4th. That box was used by Jaguar in the XJ40. It transforms the Zenith to something much more modern in driver friendliness.

From all I have heard on these forums, the replacement bottles are very inferior

Mine are original, and dont leak…I have taken certain maintenance to preserve them

be aware the master brake cyl rubber hose ID is 1/4" not 5/16"…that will certainly cause a leak

there are 4 slightly different braking systems on MKX-420G

very early 3.8, later 3.8, 4.2 MKX, 420G

as far as I know, the caliper kits for the respective piston sizes are still obtainable, but you must know what size pistons you have

Just in case you wanted a forum entirely dedicated to the big Jaguars: http://etype-forum.forumotion.com/

Not sure just how active it is lately.

I do not recall on my MK10 how many pumps to deplete the booster, its been several years since I last drove it because of a massive transmission fluid leak. Money has been showered on other cars in the intern. I was referring to the booster of my MK2.

Like Peder, I love the ride from my RHD even here in the USA as a co pilot helps spot when I might error and cross over any traffic lines.