Generator Issues

I've recently purchased a Baja which has been sat for a while, and it seems that the generator is misbehaving.

I've replaced the brushes.
I've replaced the Regulator
I've polarised the generator, amd it spins OK when current is applied to it.

After polarising the generator, I hooked the fanbelt back on it and started the car, I earthed the DF connection and got a voltage reading off the meter (15+volts) for a very short time between D+ and ground. however, this doesnt last. and after about 5 seconds, drops to 0.

Do I have a bung generator?, is there anything I can do to bring it back to life?

Thanks
- Adam

go here:

http://www.nls.net/mp/volks/

there is a bit about testing generators.

i guess if it passes all the tests, it is ok?

Thats where I was getting the info for my tests from initially.

It passes the 'Generator Testing' one for only a short time after polarising, then it drops back to 0v.

Once its been sat for a while and had all of its magnetic magic go out, does it need to be run for a period of time to regain an sort of magnetic longevity?