GMC Repair: th400 transmission on 87 gmc diesel suburban 4wd 3/4ton, vacuum hoses, suburban 4wd

Question
My transmission does not shift into high gear for the first 2 or 3 miles after the vehicle has been sitting for a couple hours. I can make the transmission shift into higher gear in those first 2 or 3 miles if there is no load such as a level stretch of road or down hill. I accomplish this by letting my foot off the throttle at about 35 mph and then slowly give it more pedal. Once the vehicle is "warmed up", it has no problem  shifting in any circumstance.
So far I have changed the transmission fluid and filter, checked the vacuum hoses for obstructions or leaks and replaced the transmission modulator. Changing the modulator helped somewhat by allowing me to give more throttle without the transmission "kicking down" to lower gear for the first 2 or 3 miles. The vacuum at the vacuum pump is 24 at idle. the vacuum at the modulator from the vacuum regulator valve is 18 at idle and 8 at about 3/4 throttle. When I changed the modulator, about a pint of transmission fluid leaked out of the transmission. I don't know if that has any significance? My plan now is to replace the vacuum regulator valve. What would you suggest?

Answer
When you removed the vacuum modulator did you also remove the modulator SHIFT VALVE that the modulator moves back and forth as this valve has a tendency to stick and a simple light sanding with some emry cloth makes it slide back and forth very smoothly.

Did you also consider removing the GOVENO to see if possibly some of the teeth are stripped off the plastic gear and or the govenor valve is stuck ?.

Are you POSITIVE you installed the CORRECT modulator ?.

The diesel vacuum regulator valve has also been a major source of problems and MUST be SET EXACTLY to specs.


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