Won't Upshift or Downshift Under Throttle

[codyw20]

Read through all these and my problem seems just a little different, but maybe we are just explaining it different so I wanted to post here.

I actually just got a 94 K1500, my first gmt400. When I drive normally everything seems to shift and drive mostly fine, sometimes hard shift but seems fine for older truck maybe. But, if I have to give it a big bit of gas (not fully WOT) it will seem to hit a false when it should shift up between gears. If I let off it will then shift and go into gear, sometimes clanking hard. Also, If I am at hwy speed and need to pass, it again looses power or a false neutral, right when it should be shifting down to get a lower gear for some passing speed. Transfluid looks really good actually, lots red so looks pretty fresh. The truck does have 265K and I don't know any history of it other than it was a one owner vehicle and it seems like it was owned by a guy who would have taken care of it, not thrashed by any means, some upgraded motor parts. Any ideas? I hope it is not just old and slowing going out, but I guess it could be. Maybe TPS if I am lucky? Other?

 

[RedLeader289]

Check your vacuum lines for a leak somewhere.

 

[eyesi]

try this,

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if it applies

 

[codyw20]

Thanks guys, I will see if I can find a leak in the lines. I think I see them, two of them going from the trans up to the TBI. I guess those are them. I have the 4l60e so I guess the throttle valve doesn't apply. Thanks for responding.