1976 Chevrolet El Camino Transmission issues?

1976 CHEVROLET EL CAMINO
53,000 MILES • 5.7L • V8 • 2WD • AUTOMATIC
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EL CAMINO 76
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I've noticed that in the last 6 months my el camino has not but running in the condition I'm used to. While in drive or drive 2 or 3 the engine will rev. At 2000 rpm but the car will move at idle speed only. Only a at about 4000 rpm does the car go to about 25 mph. I've notice I can accelerate in reverse without over revving the engine.
When changing the oil I saw there was a unlabeled cap to which the contents were bone dry. But the transmission fluid dip stick had fluid on it. The fluid compartment in question is located in the front near the radiator and belt system. It is definitely not the radiator cap but I am unsure if it would be the trans fluid since the dip stick has fluid on it.
Thanks
Jun 21, 2015 at 9:07 PM
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CARADIODOC
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The most important thing right now is to not force the car to move with a slipping transmission. That is going to overheat and destroy the clutch plates very quickly. Most likely the transmission needs to be rebuilt, but there are other potential causes that are less serious, . . . IF you don't destroy the clutch plates first.

The first thing is to check the transmission fluid level. You do that by warming up the engine, hold the brakes applied, then shift into every gear, 1, 2, 3, N, R, and park, insert the dried-off dip stick, then pull it out and check the level.

If the fluid level is okay, it's most likely time for a rebuild. The filter could just be plugged too. The pan has to be removed for that. A lot of fluid will splash out making that a very messy job. If the fluid is low, first check underneath for a leak, typically around the pan on the bottom. If everything is dry, the vacuum modulator could be leaking. The fluid will get sucked up into the engine where it gets burned and comes out the tail pipe as black smoke. To check for this, pull the vacuum hose off the modulator valve and see if it's wet inside. It shouldn't be. That valve is on the right rear of the transmission, near the bottom, and right above the pan.
Jun 21, 2015 at 10:05 PM