I realize some parts remain available, but your system is obsolete and you should consider replacing it. The brass valve has scrap value...
If you are financially able to purchase a new softener with a modern, metered digital control valve and streamline your installation I would do that. Shop around for a local independent shop that uses Fleck control valves.
If my 1st suggestion is not in the budget, consider the digital upgrade control valve that RainSoft sells, Fleck makes it.
Have them do all of the work and guarantee in writing that it will provide you with 0 grain water.
This thread is chock full of helpful information and excellent images that can help others.
jberenyi, no matter what you end up doing, please do us a favor and post pictures with details including the cost.
Thank you and good luck