I have a 54 Powerglide, and I assume that the 50 is similar. There is a needle mechanism that meters AFT from the transmission into the U-Joint area. And there are bushings and at least one seal to keep the ATF from the differential.

In fact, my metering needle is stuck, and I need to fix - if you find any info on the needle assy please forward it to me!

Loosening the U-Joint ball housing should release some ATF if the lube system is working.

From what I've read, an 'oakie bushing' is the preferred bushing replacement. I figure to replace this as well when I get the driveline apart to fix the needle.