The ecu uses direct feed to the engine. The brake booster is on a separate circuit that has a check valve that holds highest vacuum because engine vacuum fluctuates as your engine speed and load change

 

It's in the booster itself. There is reserve valving. You don't
use up all the vacuum on any particular stop.

Remember being able to pump the brake pedal 4 or 5 times before
getting a hard pedal when the ignition was off?

If you still have a 1973 F-body manual the description should be
in there. Failing that, a 1968 Ford manual.

 

Plums
Apologies if you knew this, but the Lockheed vacuum servo on the XJS has that feature, as AFAIK, do all such Lockheed servos on all UK cars of the era. The XJS extra reservoir is in addition to that provided by the Lockheed unit itself.
Greg

 

Thanks.

Yes, I was aware of there being a 'reserve capacity', so to speak, to the brake booster.

Earlier I thought you were referring to some sort of separate reserve storage.

Cheers
DD