Well, almost one year later, I've still not changed anything, but a couple of "trouble" to enable engine starting this past winter has made me more determined to try to sort this out - luckily we have 2 cars and so I've been getting concerned about taking my car on long trip this past winter, really in case I end up not being able to get it started in cold weather.
Firstly
@Simon , I've read that the official or original clutch slave cylinder has a "retained" or "captive" push rod, so it is not possible to just swop push rods - does this tie in with what you found when you swopped your push rods over?
Next, I felt the need to quantify this issue by finding the location in the ENGINE controller, where these 2 sensors outputs get enabled, and to physically measure the points in the clutch pedal movement arc where these sensor outputs change or get enabled, and also just how much clutch pedal movement is left - ie is spare, after that point has been reached.
So, I bought a cheap 1000mm aluminium measuring rule, found it was a bit flexible and so prone to giving erratic readings, so I taped a length of plywood offcut to it to keep it stiff. Next I bought a cheap silicon cooking splash guard from TKMAX, I chose this as it just spans the steering wheel rim and has 2 lifting lugs on its outer steel frame and so can be firmly secured to the steering wheel rim using a couple of Velcro straps, the actual silicon membrane has enough "give" to allow it to deflect up enough to clear the centre boss of the steering wheel, I cut a small slot in its membrane to allow the measuring rule to pass up through it - and used the taut silicon membrane as a reference surface for taking readings.
I first tried this on my wife's 2015 VW Polo 1.2TSI really just to get an idea of how a good working system or car measurements look like, then tried it on my Feb 2011 B8 S4, if anything, the "spare clutch pedal movement" beyond the "ENGINE START ENABLE" point is more on the Audi S4 even scaling the readings for the Polo's shorter clutch pedal "span" still left the measurement taken on the S4 being more ie better, than on the 2015 Polo - which was not what I had expected. Ambient temperature was maybe 8C when I made my first set of readings.
Anyway, I'll probably try swopping push rods or even fitting a new Sachs slave cylinder (over time I felt the need to buy a second or back up clutch slave cylinder, just in case I messed up the first push rod modification - and Audi used Sachs clutch slave cylinders in the factory, and then take a full set of measurements which are:- clutch fully up position, first sensor enabled position, second sensor enabled position and clutch pedal fully down position. Really my only reason for still doing this is to attempt to never ever end up with the clutch slave cylinder piston hard against the C-clip on the end of it.
As my car is still low mileage and has its factory fitted clutch fitted, after carrying out my measuring of the clutch pedal movements, I getting round to thinking that my own issue is really more down to the shape of the floor area below the clutch pedal than an actual failure of the clutch master cylinder piston to reach the final sensor in its bore due to the slave cylinder piston already reaching the end of its cylinder - ie the C-clip at the end. That along with wearing thicker soled shoes in winter so not being able to feel and work round the footrest, aided by the knowledge that this can be an issue with these cars that have bigger engines, quattro and manual gearboxes!
Hopefully I'll get round to swopping the push rods or complete slave cylinders soon!
