Air throttle air leak.

[somewhereinusa]

I have a Williams air throttle that has developed a bad leak on the inside. Air is coming from the dust cover. I assume there is an o-ring of some sort  around a spool? I can't seem to find any info. From what I can tell Williams doesn't make them any more, at least it isn't listed oh their website. My bus is a 91 Bluebird, it's a through the floor model. Are they all alike or are there different models? Can I do the repair from the top without taking it out? It's a REAL bear to get out. Anyone have a breakdown picture or service manual? Where can I get parts?

Thanks,
Dick