The dilemma with the Vacuum Pump rebuild kits is that if you buy the Piston End kit rebuild kit and the Drive Arm end rebuild kit (the part with the Bearing) you have almost spent as much as the cost of a new Vacuum Pump.
And, when you are done rebuilding it you have no warranty.

There is a thread where they speak of only replacing the Bearing. But, in that thread they said they cannot be sealed because they need to be lubed by Oil from the Timer.
So I do not think you will find one with a sealed bearing. What I believe was said is that the Older type Bearings had a Plastic Cage to hold the Balls in and I believe I read there was less Ball Bearings in them.
When the plastic Cage went the bearing failed.

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84 300D, 82 Volvo 244Gl Diesel