420 Brake system Service and Parts availability

I inspected a '66 420 today for a family to appraise it, been sat for more than 5 years since the father passed, it was (mainly) fairly well looked after before that

The wife has had it for sale but for too much, I advised them the price I thought appropriate for a 1) non runner, 2) driver, 3) most items fixed

It has a defective transmission engagement system which I said I should be able to fix for them, as it it is a BW8

Things is, it has brake pedal goes to floor, they muttered abouts spending thousands at the only 2 local experts who can work on them, both of whom I would avoid if possible, and they needed no convincing of that

The brakes consist of a complex upper master with plastic reaction valve, and a lower servo/slave system that feeds to the calipers

The servo is in the wing

I know not much about this system, but indicated the problem may well be in the reaction valve area, but I thought the whole system would need stripping down, especially as it has sat for years, and extra specially as the 2 best places in town sent them away saying they could not fix the brakes

This will be beyond them, and they offered it to me on the spot for the low estimate I gave them. I am not in the market, but may be tempted, if brake parts are readily available mainly the master/servo.slave

Is that setup the same as other Jags such as some E-types

Is it a complete bastard to get going if defective?

It has some (5yr old) brake parts in there, dont know what.

As I have mentioned to others, if you buy a vehicle that has been sitting, like a 57 year old Jag, count on dismantling every part of the brake system, and replacing any suspect parts, most especially sleeving things, which is expensive

If these brake systems are a bi##, I should walk away

I rang a Jag club member who has one, he said it was for sale, he has too many vehicles. I could probably get it for the same price, he got it very cheap from a deceased estate, its had a resto, but needs some assembly and unknown mechanical work

He advised me to walk away

The inside of the reaction valve, part 26, looks a bit like a fruitcake

It did not look original, or identical. I think the reaction valve part has been replaced

It would not be the first time I have fixed something some idiot pro could not for whatever reason, I am sure there is a tiny piston them that gets frozen at the drop of a hat

It’s the same system as an E-type. I had a '67 420 and loved it. Sold it for a Mk2 project (my deam car) but probably should have kept the driving 420. The brake master cylinder I would rebuild as I had issues with the new improved masters out there. The servo get new. The calipers are same as late E-type, early XJ.

As Tim says the servo and master are the same as in the 4.2 E types.

The calipers are like the S3 E types and again rebuild is straightforward and parts are available.

As regards the brakes.
If you were prepared to put up with a single circuit system you could ditch the complex 420 master cylinder and servo and fit the more easily obtained items from a mk2 or S type. The calipers are quite easy to reseal and are pretty indestructible.
This is the same system which was fitted to the very last run out versions of the S type before it went out of production in 1968.

Is there any reason I could not swap pedal box and master with a complete 420G pedal box/booster/master? (an eliminate the servo/salve altogether)

I can only think it may not physically fit

It does sound like the original system is not all that difficult to get parts, as it is the same as certain E-types