Even if you can have them serviced the smart thing to do is get rid of them and use safer 20" rims, even tubeless 22.5" wheels are out there if you like.

You'll find some good info here about different rim types and some options.
http://www.stovebolt.com/techtips/wheels/

And here's a good cutaway picture of an RH5* rim.
http://rides.webshots.com/photo/2903721360080251109pPFWcO

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-