I have been trying to diagnose my shifting issues on my 87 f250 6.9. It has a c6 and basically when i'm driving normal it won't shift with out nearly hitting redline. So, I pulled the vacuum hose at the modulator and while idling, it has no vacuum there.

What should I check?

Thanks, Kyle

 

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Sorry I did not notice it was a Diesel.

 

Does your power brake assist work? If not you probably need a vaccum pump. If it does follow the lines around that that go to the modulator looking for leaks.