Take a look at the transmission modulator on the transmission, there is a vacuum line running to it from the engine. Pull the line off and see if any transmission fluid runs out. If it does, you have a bad modulator and it needs to be replaced. However, if no fluid comes out, check to see if there is vacuum pressure to the modulator. If you find no vacuum, trace the line up to the intake manifold and check to see if its connected properly and make sure there are no cracks, kinks, breaks etc. If these things all check out, you're transmission will need to be rebuilt.