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Though the transmission dipstick tube
In the transmission.
Gear oil 90 weight
either standard power steering fluid or dexron transmission fluid.
through the transmission dip stick tube on the back of the engine on the passenger side.
It depends on the rear end gear ratio, but the top speed should be between 115 with 3.55 gears or 125mph with 3.23gears.
Dexron II originally in 1970. If you're buying fluid for it now you won't likely find any Dexron II as it has been replaced by Dexron III/Mercon fluid and that's what you'll use now.
if the noises are only in fourth gear then its an internal transmission problem. Make sure that the fluid level is ok. If the fluid is ok then the transmission will need to be pulled and checked inside.
A transmission fluid leak means that the plug may need to be replace, or the seal or gasket may need to be replace. As it could be caused by a work seal, or gasket, or a loose plug.
The exact same place you check the fluid level is where you add fluid. rear wheel drive (Sedan , Brogham)-- transmission tube and dipstick behind right rear cylinder head. front wheel drive (Eldorado) - side of left side cylinder head . Good luck
You can use any SAE certified rear differential fluid in your 1970 Chevrolet. The 1970 Chevrolet differential fluid can be checked from the differential fluid check port.
Depending on what trans type. A manual trans has a fill plug on the side of the trans. And a auto has a dip stick which is located in the engine bay