After driving my ancient beasts for many years, the first time I drove a small, modern car and touched the brakes, I would have hit the windscreen if I hadn’t been wearing a seatbelt. So massively over assisted, very difficult to get any feel from the soft pedal. In contrast, I drove a race prepared E type around Donnington without any servo assist at all. Rock hard, progressive pedal. Such a better experience that I now have a servo bypass brake line for my Alfa.
ABS light did NOT come on, just the red brake light? Add a little more fluid to the reservoir. I’m sure when you slammed on the brake, the fluid got sucked in and juuuust let the low fluid sensor think it was low.
I don’t think it’s limited to small modern cars. Even the giant land cruisers can have overly touchy brakes. I’m not entirely sure what the intent is there. Perhaps someone feels that the proverbial little old lady doesn’t have enough leg strength to stop the car. Unless she’s wheelchair-bound, though, she presumably has enough leg strength to stand on one leg, as one alternates from one leg to the other while walking. If it were me, I’d design all brakes to require about 100 lb force on the pedal to lock up the wheels. And when the driver is wheelchair-bound, making sure the brakes can be properly applied should be one of the required modifications to the car. We shouldn’t all be subjected to touchy brakes, whether we have ABS or not.
Float? There’s one in the main fluid reservoir? 
Must be, for the fluid level indicator.
Unless the newer cars have a different system?
Yes indeed. My Range Rover is also very touchy. Very good brakes though I must say.
And I guess that with the advent of ABS and the elimination of the possibility of blocking the wheels and start doing donuts, the old lady won.
I like the hard brake of the Jag, it gives you more of a feel of the forces involved to stop a 2 tone car.
On the other hand, on the Range Rover, I feel that it’s not me that stops it but it. It’s part of the “servitude” aspect of it all. Soon enough there will be no brake pedal at all. And no steering wheel either.
Autonomous cars…rue the day.
Regarding the float, I think there’s just a set of contacts. Submerged=good. Exposed to air=bad.
Interesting: I drive two flavors of Hyundai, plus have driven a number of other various marques, and none have anything like "over-assisted* brakes.
For giggles, I once disconnected the vacuum on Tweety’s brake assist bellows… and it became a LOT less stoppable.
I’m guessing that this was an official justification of British engineers who were forced to fit this type of servo into XJS v12.
Honda CR’V has bigger diameter booster fitted, there is a reason for this.
If you’ll look closer into the space under v12 bonnet especially around the servo itself - everything becomes clear. There is no way to fit something with bigger diapraghm in there. Jaguar was trying to make it longer to compensate on the absolute pressure generating surface, bu this won’t work effectively.
We have 12 pots there so the piston diameter in brake pump must be quite big (lowers force transfer ratio). The other factor is the vacuum itself - when was the last time you’ve checked your v12’s inlet for accurate value…
Which brings to mind the rationale behind the vacuum reservoir used on the V12 brake system…
Cheers
DD
I never understood that, either, DoubleD … It seems to be a triple system on the XJS (and some of the other models) - standard brake fluid system + vacuum assist + accumulator assist … Does Jag just want to make SURE our XJSes stop when they need to?
Well, Jaguar engineering decisions are sometimes inscrutable. Adding a vacuum reservoir suggests, to me, that Jaguar anticipated (or experienced) conditions where vacuum supply was not sufficient…or perhaps too easily depleted.
It’s a fair bet that other cars employ a reservoir as well. But, in my observation most do not, even some known high performance models.
To this I’ll add that both my Jag V12s have produced less vacuum than the typical 18-20". More like 15-17". This would suggest some sort of problem yet both have run really, really well. Occasionally over the years I’ve heard others mention the same. Is there some tie-in here? I dunno.
Cheers
DD
I consider my ‘88 to be running very well and I agree that with the OE vacuum advance system and base timing 15-17” of vacuum is all you will see.
If you run the valve clearances at .012-.014, you will see higher vacuum and a smoother idle due to the camshaft profiles. Also if you ditch the vacuum advance delays and reducers, you will see 17” with the normal valve clearances.
Setting the timing at 18 degrees at 3000 rpm results in the base timing being about 0-4 degrees, which is why the manifold vacuum is low compared to most engines; vacuum advance would normally address that, but is limited in the Jaguar scheme.
Vacuum is highest during coast at higher rpm (such as coasting to a stop); Jaguar takes advantage of that by incorporating the vacuum reservoir. Many modern engines suffer from low manifold vacuum at times due to variable valve timing and incorporate an electric vacuum pump to supplement manifold sourced vacuum for the brake booster.
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