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Originally Posted by wuzerk
I am friendly with the owners of an MOT station who say that they have received official notification that the sinking pedal on a Rover 75 diesel is NOT a failure point when the engine is running. It is a failure point if the pedal sinks when the engine is off.
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That tallies with my research on SPP.
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I don't profess to know why, and they didn't elaborate, but it may have something to do with the fact that the diesels generate the brake vacuum by way of a pump? i.e. anything powered by a pump will have a fail safe valve which might be the reason? OR,at tickover only, the pump cannot maintain the vacuum?
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The diesel vacuum pump explanation of SPP is a myth IMO. The source of vacuum in the servo is immaterial. I've yet to read a convincing explanation, so until I do, here's my own,superficial theory (copied directly from another discussion on SPP).
It's not a 75/ZT exclusive behaviour, many other makes of car do this including Ford and Jaguar. Personally I don't go for air in the ABS module as the explanation. I'm not saying that air in there doesn't require a T4, what I'm saying is SPP isn't directly related to air in the ABS unit. SPP is far too widely reported to be explained by air in the hydraulics. The brakes wouldn't operate as well as they do if that were the issue. Again, I personally think it's an effect of the electronics trying to 'understand' what's going on when you have the engine running, you're pressing down the brake pedal, but the wheels aren't rotating. How does the ABS system interpret that? And many MOT stations are indeed aware of SPP and do not regard it as a fault. As stated above, SPP is only apparent when the engine's running. That also would require explaining.
EDIT: It would be an interesting experiment to use an external vacuum pump to run the brake servo and see if you get SPP without the engine running and with the ignition off. My gut feeling is you would not. You'd need something like a small Edwards Speedivac pump to pull the servo pressure down enough.
TC