Clutch pedal loses pressure at WOT or above 3500rpm

[Weisjacob1$]

My clutch pedal intermittently loses pressure during daily driving. Under 3500 rpm and driving lightly the pedal retains normal pressure, shift points, and feel. Above 3500 rpm or at WOT the pedal loses pressure to the point that it will not shift or go into gear without firmly placing the pedal all the way to the floor, holding the pedal down, and placing into gear slowly. Attempting to quickly shift without cautiously placing into gear results in a grind. The shift point also moves from directly off the floor and has very little throw. Pumping the pedal or returning to light driving will return the pedal to adequate pressure. Rev matching into higher rpm seems to increase the loss of pressure. It’s like the pedal is only dead above 3500 rpm and progressively loses more pressure as the rpm climbs. If anyone has had an issue similar to this before any information would help. Also any information on where to start: clutch master, brake master, slave cylinders, throw out bearing would be appreciated. Lines have been replaced with steel braided and reservoir is full. No leaks or loss of fluid.

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[jacknifetoaswan]

Sounds like a vacuum leak. There's a vacuum port on the front of the intake manifold that goes to a check valve (not exactly sure what the part is called), and then the lines run to the master cylinder. That check valve could have failed. It looks like a 90 degree elbow with two ears coming off it. I'll grab a photo when I head out for my alignment.

JR

 

[TG_GT350]

I have a similar problem- how did you diag? smoke machine?

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