The S1 is since sold, but also did it with my 420G, which has the same Servo (Girling Supervac 100), and master cylinder
The issue, as I understand, is the inside of the shell becomes pitted, and wont fully seal the big rubber diaphragm. The result will be boost is diminuished over time. If your Servo has never been O/H, its probably time
It was expensive ($400) to have O/H, by a specialist, using “not Chinese cr@p” parts
I found my S1, S2 and 420G brakes very acceptable, but require much more pressure for hard braking than modern cars…shouldnt be hard to lock up brakes though,have you tried this exercise?
other possible causes of hard pedal;
- how old are the 3 rubber hoses?..they become restricted
- are the rear brakes seized, dont laugh, check the disc pad swept area, should be very shiny
(my 420G rear calipers had been seized for an unknown time)
I overhauled everything, so cannot say for sure what aspect improved what.
IMO, the S1 XJ had well engineered brakes.
I would only consider the need to change anything if I drove the car very hard indeed, (insanely hard lol), or a daily driver in city traffic perhaps. Experimenting with pads would be something to consider
btw, the removal of Servo alone is awkward, now that you have done it the other way, you would probably be quicker!