Could it just be the plastic non return valve right before the booster that is broken. This would be an easy fix. Perhaps the gasket from the valve to the booster is weathered?
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I guess you should unplug it to confirm whether the fault comes from it or not first of all, itās a quick and easy test. Then it will be a case of replacing or repairing it, depending on the fault.
Hmm, I will try that. Gotta put the machine into the garage and put it on the mini lift and remove the wheel and hire three people, etc!!! Ha
On further reflection, I donāt see a way to connect the vacuum hose to the booster without the check valve in place. This also would not give different results from having used vice grips on the hose which gave me some braking but certainly not assisted.
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Sounds like a problem with the internal valve (poppet valve)in the diagram above. What the diagram doesnāt show is that with brakes off, thereās vaccum on both sides of the piston/diafragm. When brakes are applied the valve opens and allows air into the pedal side of the chamber, and thus pushes the piston and MC rod. If the pedal is then held in a certain position the valve closes as the seat moves forward, restoring vaccum to both sides of the piston, and holds it in that position. As the pedal is pushed further, the process is repeated and further braking is applied. Whatās happening with yours is that the valve is not closing after the pedal is pushed a bit, allowing full atmospheric pressure on the pedal side with full vaccum on the MC side it goes into a full braking mode.
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(such a nice safety feature ā automatic emergency brakes didnāt appear until much later on other brands)
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What would the solution be? New booster? I can order one today for $200 USD plus shipping and get some other gaskets I need also. I already have the re manufactured on that the vendors are showing so installation should be straightforward as far as Jaguars go!!
G
Is it the remanufactured one thatās giving trouble? A new one would obviously be a good choice, but if itās an economical decision, as it would be for me here in South Africa, Iād opt for the remanufactured one & try resolve it with the vendor if its faulty.
The current re-manufactured on is at least 15 years old; itās had a good life! Weāve even driven this car on a 1,500 mile trip from Colorado to Yellowstone and Devils Tower, USA Sheās a one family owned car which I am desperate to keep rolling. Perhaps some of you remember my dad bought this new at the Coventry factory in the UK when I was just 6.
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I see you mention the poppet valve; could the problem also lie in the brake vacuum check valve. This would save a bit of money and the R&R of the entire unit. I called and Moss does not have these in stock but SNG has both the valve and the entire booster in stock at this time
I was in the process of ordering a new booster from SNG and the technician said I should check the check valve on the āreservacā first before ordering the new booster. Another contact is Jack White @ Jaguar Spares in Virginia, USA. He has a massive number of Jaguars and he has supplied me with parts over the years. He has been in the Jaguar dismantling business for 40 years if you need something special.He too recommended the reservac valve be checked first. So, itās back on the lift and I hope they are right as this piece is MUCH easier to replace than the entire booster.
Worth a try, donāt know how accessible it is, or if thereās some way of adjusting it. Maybe someone on the forum has done it. Seems to me the seat isnāt following the poppet as the pedal is pressed. If you have to dismantle the booster to get it out, probably will need some special equipment to open the booster. Iām not an exoert but I did once dismantle a 420 booster, which was extremely dufficult. I think you need some special clamps to hold the main body & twist the outer cover off. Then to re-assemble to compress the rim of the diaphragm enough to twist the cover back on. I had to bend open the tabs clamping the 2 halves together then try to clamp them tight again, which left them a bit distorted & unsightly but it worked ok!
Got car on the mini lift with tire removed. All parts seems accessible I will see whats loose or broken when my wife gets home. I donāt like being without my tool helper. It is such a hassle to keep going back and forth to the tool box when in need of something bigger or small. I donāt have Whitworth so we have to rely on SAE or metric sizes. Those darn people using Whitworth to design a car
I will absolutely NOT attempt a R&R on the booster; that is well above my skill level and could result in death or injury should something be assembled incorrectly. Not sure if youāve been to Colorado? A trip here would be ideal for you to visit and help out!!
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Sounds interesting, but on my meagre retirement income Iād have to sell the E-type! 
The lovely Angela, from memory? The lovely Rosanne enjoys the fruits of my labour, but her attention span on the tools is about 10 minutes, even if Iām working on the XJ6 she bought before we were an item.
Ah, and a good memory you have indeed! I swear she has Royal Purple 20-50 running through her veins. She does most of the wrench twisting under my tutelage as I am physically unable to maneuver and contort my body anymore.
She has changed a clutch slave cylinder and changed oil in two separate rear differentials to name a few of her projects. Although sometimes we argue; itās a true bonding experience and as she always says: āitās not a project well done without some one bleeding!ā
She and I curse the engineers in Coventry as they made everything so difficult. Working on the GTO is childās play compared to the MK2.
For those thinking our Jaguarās can get expensive to work on, there is a A.M. Lagonda for sale on B.A.T. Youāll need a second mortgage to cover the repair bills on this electronic nightmare of a car.
i have a few Whitworth spanners but all the nuts & bolts I have worked on on my Daimler v8-250 (same body as the Jaguar) are AF. There may be a Whitworth hiding somewhere but i havenāt found it yet. 
Took the time last night to remove the reservac. what a joy that was; a combination of 7 and 8 mm nuts and bolts with a variety of washers in odd places. Obviously its been apart before. I notice some JB weld type of epoxy around parts of the air check valve. Not sure who did this, but no doubt to stop a leak.
I found that I can blow into one nipple and air comes out the other nipple, blowing into the other does nothing. Sucking on the one nipple did pull air from the canister. DO NOT do this, as I got a mouthful of 50 year old gas fumes. From these tests, I conclude that this valve is good?
Iām going to connect both vacuum lines from the booster to the manifold together and I think I will have good brakes but no doubt just for one pump of the pedal or a set of staggered pumps to pressurize the booster?
These would be temporary measures to resolves the source of the problem. I still have the option of buying a new booster for about $200 USD. I am of course cleaning the shield and the reservac tank in hopes these steps will make the car faster and more attractive!!
I am also going to cut the piping about to a length 2" coming out of the intake manifold which terminates at the booster by going through a maze of rubber and metal hoses. I will route this more intelligently through a vacuum home alone the right front fender well. This should also provide a sure file way of eliminating any leaks in the factory routing.
Gerard
Using this pipe fitters cutting tool I cut of the curved metal pipe coming from the inlet manifold. turning the brass nut about 1/16th of a turn 7,000 times using Kroil (rust penetrate, that is the boss!) About a 15mm wrench size.
In my case, this was connected to a steel pipe than a rubber hose and then back to a steel pipe and then to a rubber hose to the reservac. This was all between the engine block and the carburetors. Perhaps this mess was done years ago as the hose clamps seem newer. I am next going to route a new āvacuumā hose from the intake manifold stubby piece in front of the battery (RHD) along the frame and connected directly to the reservac. This will eliminate 3 potential leak points. I did bleed, so its a job well done!
Whilst fault finding you can run the vac line directly to the servo and ignore the vac reservoir. When you get the brakes working properly you can decide to reinstall the reservoir.