If you did not remove U-joint ball housing.
Fill to level of plug hole if warm, fill about 1/2" below level if cold.
When the U-joint ball housing is removed and reinstalled.
1948 - 1951 Chevrolet Truck Shop Manual Read Number (5) Under Assembly
Your suppose to fill the ball housing through the speedometer drive gear.
It does not indicate how much fluid.
GM tells you to do this so the U-joint and the torque tube bushings have some
lubrication until the tranny starts funneling fluid into the U-joint ball housing.
Here is what happens.
There is a transmission rear main output shaft bearing.
Fluid flows back and fourth through that bearing to lubricate the U-joint and the
front and rear torque tube bushings. When the U-joint ball housing is removed and reinstalled.
You fill the ball housing through the speedometer drive gear and top off the tranny case fluid through the side filler plug.
After driving the truck about 10 miles, some of the tranny case fluid will be funneled into the U-joint ball housing.
Which will lower the tranny case fluid level.
Now you should top off the tranny case fluid again.
Until the fluid is leveled off between the U-joint ball housing and the tranny case.
If the fluid is continuously low in the tranny you may have a bad torque tube oil seal.
And the fluid is flowing down the torque tube into the rear axle housing.
One way to determine if you have a bad torque tube oil seal is to remove the axle housing fluid filler plug.
If fluid starts pouring out, that may be a good indication the torque tube oil seal is bad.
