When I did my Seascape workshop, Nic taught us how to clean the script liner brush so it won't split and can potentially restore those who have just started to split. Take the Bob Ross conditioner and put just enough in your palm to create a little puddle in the center of it. The roll the script liner just like you do when you load it up with thin paint for small detail work. You'll probably have to do this for 3-5 minutes. This will remove the goo that has accumulated at the base of the bristles. It needs to be done almost every time after each painting session. If you don't think a full cleanup is needed, ALWAYS clean your liner in clean paint thinner. The particles of paint in a dirty thinner also contribute to the splitting issue of the script liner.
Here is what I got from a friend of mine. He has been an artist for over 30 yrs and dealt with all the issues we are experiencing. There is another way to try to save your all of your brushes (including a stubborn split liner brush) and give them all a new life. Use warm water and put some "soil love" in it (about 1/3 soil love & 2/3 warm water). Do circular motions and some presses with your brushes to help the removal of the hard goo stuck at the base of the bristles. This might take some time but well worth it. Once done, wrap your brushes (specially the bigger ones) in paper towel and use masking tape to tightly keep the bristles together (washing oil painting brushes with water will flare out the bristles). Once dry, you may have to put something greasy to have the bristles stay together. Once the greasy product have been thoroughly applied by massaging your bristles, you can use clean paint thinner to remove the excess. And voilà!
I get that it can be quite extensive, but it's only when needed that the "soil love" cleaning process should be done. I did it with one of my 2" brush and it really gave it a second life.
⚠️ IMPORTANT NOTE ⚠️
DO NOT LEAVE YOUR BRUSHES IN THE WARM WATER & SOIL LOVE FOR MORE THAN 5 MINUTES. The water with then get into the wooden part of the brush and totally ruin it for good.
Let me know if you have any questions. 😃