>The vacuum diaphragm can develop a leak. If that happens then the cruise
>control might set and hold the speed for some time however if the leak is
>larger than the supply line and modulator can add vacuum to the system the
>system will slowly lose control and the vehicle will slow down. This can also
>happen if the vacuum line to the diaphragm is cracked or loose.
>Finally, the linkage that connects the diaphragm to the accelerator linkage
>can fail. Some aftermarket cruise control systems use a short length of what
>looks like fat key chain -- bead chain. I have seen several units fail when
>the chain simply breaks.

The better systems don't use vacuum. And the van's speedo is driven
from the left-front wheel, not the gearbox.

The Rostra UltraCruise is available with vacuum or all-electronic.
I'd go for the latter. Saves complexity, less to go wrong, more
sophisticated response. Easier to fit.
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
<andrew.grebneff@stonebow.otago.ac.nz>
Seashell, Macintosh, VW/Toyota van nut