I've been working on my 1973 S3 V12 for about 8 months now. Don't have a lot of time, but reasonably pleased with progress. I now have the engine running after sitting idle for about 7 years.
My problem now is getting the brakes working. The back brakes seem fine - no air when I bled them. The front brakes are a whole different story. When I try to bleed them I get a continual stream of aerated brake fluid - sometimes more air than brake fluid. At first I just assumed that there was a leak in the pipework, but after several attempts to find a leak, came up with nothing. I also experience a lot of smoke from the exhaust after trying to bleed the brakes!
On looking at the brake circuit in the manual, it looks like there are effectively 3 circuits - 2 hydraulic and one pneumatic/vacuum - and that the pneumatic circuit is linked to the engine through the inlet manifold. So now I'm assuming that I have a leak from the hydraulic side to the pneumatic side in either the master cylinder or the slave cylinder.
Can anyone confirm my assumptions of the circuits in the system and the likelihood of the location of the leaks?
How easy/difficult is it to overhaul the brake cylinders, or is replacement a better option?

 

It sounds like your brake booster, (UK= servo) is knackered. The servo works using engine vacuum and there is a vacuum line to the inlet manifold with a one-way valve in it. There is also a vacuum reservoir tank. If hydraulic seals go in the servo, the fluid enters the vacuum chamber where the piston operates to boost the brakes, and is then sucked into the engine via the vac line to the inlet manifold. Boosters of this era are normally serviceable with new seals, but doing the job DIY can be problematic. I did manage to do one of a Mark 2, but on that one, I needed a caliper gauge to measure the depth of the operating rod.


It's probably easier to swap out the old booster. Have a look at the parts here: -
SNG Barratt - The Ultimate Jaguar Parts Specialist

 

Ditto the above, either the booster seals or diaphragm are gone. I strongly recommend replacement as the booster has lots of plastic parts, all of which have gotten brittle with age and the heat in its location above the exhaust manifold.

 

The V12 engines throw off a lot of heat !