Rebuilt one yesterday.
It's different from even the late ur-quattro (RR) pump in that it only has one piston in the front (high-pressure) part. There was no piston in there (and no cap) when I got it, but on dismantling it I found only one of the two opposing chambers had a feed hole drilled. The high-pressure constrictor banjo bolt also has a smaller hole than the ur-quattro version. I suspect this may be for noise suppression - the two-piston pump on the RR certainly overdelivers and is the major cause of engine bay noise once the tappets are sorted out.
Anyway - it has its benefits. There are still two one-way valves fitted so if the delivered pressure is ever inadequate you can just swap them over.
But my old diagnostic needs revision - an S2 pump (certaily from 1993) should only resist turning ONCE per revolution, not twice.
It's different from even the late ur-quattro (RR) pump in that it only has one piston in the front (high-pressure) part. There was no piston in there (and no cap) when I got it, but on dismantling it I found only one of the two opposing chambers had a feed hole drilled. The high-pressure constrictor banjo bolt also has a smaller hole than the ur-quattro version. I suspect this may be for noise suppression - the two-piston pump on the RR certainly overdelivers and is the major cause of engine bay noise once the tappets are sorted out.
Anyway - it has its benefits. There are still two one-way valves fitted so if the delivered pressure is ever inadequate you can just swap them over.
But my old diagnostic needs revision - an S2 pump (certaily from 1993) should only resist turning ONCE per revolution, not twice.