I may be wrong, but:

For testing purposes or perhaps even long term, I believe you can just install a temorary bypass hose, looping the PS pump's ASD output port to the return port on the PS pump.
You should be able to drive the car.

This would keep the ASD portion of the tandem pump lubricated and in use but eliminate any load on the pump from the ASD.

You may want to unplug the ASD relay (wherever it might be) to keep the ASD valve from operating.
To keep fluid in and dirt out of the disconnected ASD feed and return lines, add a separate bypass hose to loop them together.

If easy low speed power steering returns, you've determined the ASD system was creating an excessive load on the PS pump.
Possible ASD failure points could be bad sensor(s), stuck relay, bad accumulator, bad valve, etc.

If it is still hard to steer, than you may have degraded fluid, air in the lines, a bad spool valve in the PS pump, etc.
How does the PS fluid look and smell?
Has the PS filter been replaced (if this car has one)?
Any leaks from the steering box?

ASD activates when there is a measured difference in wheel speed.
Are both rear wheels the same size?
Is there any binding of the calipers or parking brakes?

Hissing, and banging of the ASD line, I believe are not normal.
The hissing may be due to a bad accumulator.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022