There is a "blind plug" just below the modulator to connect a guage. Incorrect modulator pressure CAN'T cause a loss of 4th gear.

You measure modulator pressure with engine running & NO vacuum at modulator. That trans should have a black modulator & have 4 BAR of pressure.

No 4th gear is ALMOST always a problem with the 2 "teflon" seal rings on the K2 clutch guide flange at the inner rear of the main trans case. Can also be leakage from the K2 "lip" seals in the clutch pack.

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