I do not really have experience with this issue, but my thinking would be -

Are the mechanical parts in good order?

Is the fluid reservoir full?

Check fluid line for rust, or even a leak.

Next I would bleed the system.

My thoughts would be that both the rubber parts at each end have the same wear as they both act at the same time.

I would then dive deep and pick up rebuild kits for the two cylinders. You can compare the new rubber parts to your old ones. With brake cylinder cups, worn edges are sharp and even scored - new cups have a thin square edge.

Then remove the system and rebuild both ends - Make sure you gently hone the cylinders to remove gunk - then reassemble.

You may never have to look at that system again.

I am sure that others might have tips for you.