Just finished replacing the original input and output unsealed bearings in my Model A transmission with modern double sealed bearings. The original rear out put shaft bearing was allowing gear oil to run down the torque tube and over filling the rear end. The sealed replacement bearing I selected were Torrington-Fafnir P/N 208PP (Front) and P/N 306PP (Rear).
A lot of steep hills where I live. I purchased both sealed replacement bearing on e-bay for less than what one bearing would cost at NAPA. These were not made in China.
The rear gearbox bearing is unsealed to allow lube through to lubricate the universal joint and driveshaft roller bearing. There is a seal behind that roller bearing to hold that lube in place and prevent it migrating down to the diff. Sounds like you should have replaced that seal. Also...these modern gearbox bearings run narrower snap rings than the originals, this allows more endplay both directions.
The universal joint is lubricated by a grease zert on the bottom of the main shaft bearing retainer ,take s about half tube of grease to fill cavity. Snap rings fasten to case not on bearing . See page 131 snap ring Snyder's current Cat. Also see page 130 for transmission assembly diagram .
There is a large flat washer that controls how much gear oil gets into the front and rear bearing. They almost touch the outer races.
When I use the bearings that are sealed on both sides I remove the inner seal so the bearing can get gear oil, as originally indended.
As stated above, you must be sure to pump some grease into the u joint once in a while.
I did a trans for a guy that drives his A at least once a week. He has a nasty steep, but short driveway.
He goes up forward and backs down.
I would not leave the inner seals on the bearings, they both run engine speed when the car is running in third gear.
After a few years the bearings may otherwise dry up from lack of fresh lube
I has not lost gear oil in 1 1/2 years since I did the job.
As others have said, the front and rear bearings need to be open to the inside of the transmission also don't forget to put the slingers back in place they not only prevent the lube from getting out they also act as spacers. When you are almost done, make sure the vent hole in the tower is open, it's about half way up on the rear side. When you grease the u-joint, take the speedometer drive off and fill the housing till the grease starts coming out.
The bearings that I listed are dimensionally the same as the originals. I did leave the slingers in place and packed the universal joint "Full" of grease just like my model T. Ford placed a grease fitting for keeping the u-joint greased. There is much discussion about keeping or removing the inner seal.....I chose to keep the bearings sealed as they were designed to be. Out side Sealed Bearings used in modern day rear ends and drive axels are sealed and last many thousands of miles at hwy speeds far exceeding what a model A transmission shaft turns.
I think the quality of bearing you are using is a major factor in bearing life.
Lubrication is the major factor in bearing life. If you want to get the down low ask on Fordbarn in the Model A section. I am sure that Tom Endy will let you know if you are on the right track.