I have a problem with my liner brushes for oils, I have three of them all with nylon bristles. After some time the bristles loose their shape and they split. It is now very tough to make small limbs and other details with them.
This is what I have tried:
Clean them with thinner
Clean them with baby oil
But it didn't really help. Does anyone have an idea how to bring them back to shape? I have no problems with my liner brushes I use for watercolors or acrylics.
OMG Tom, I was gonna post about this very issue as well. I have the exact same problem.
Not sure if there’s goo that get stuck at the attachment (base of the bristles) and over time it get so much that it’s spread apart the bristles… I also tried everything and always hang down my brushes without success.
it would be nice to hear the magic trick to keep them like we we first got them.
Common problem i think, be sure to reshape after cleaning
I have trimmed mine with a scissors in the past and it worked for about 5 paintings. Then I just break down and buy another.
Luckily they can be bought for very little money, i recently got three for £4 but i guess this is a wasteful mindset 
I did trimmed them too. I like having the option to make very thin line work with them. And I guess until there is a magic trick to keeping them to their original shape, I'll keep buying new ones. 
Thanks for all the suggestions. I have tried it with hot water using the suggestion I found on YouTube, URL: https://www.youtube.com/watch?v=4xue6wRDrvc and it worked. My liner brushes are back in shape again!
Tom0779 Thanks Tom, I will give it a try.
I usually just replace them. Check out Hobby Lobby or Michael’s whenever they have them on sale it’s a great time to buy
Has anyone tried coconut oil? It would have to be warmed to be a liquid, shape the bristles, and it should dry firm and train them. 
I dip mine in safflower oil and then roll the bristles on a paper towel to shape.
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. 
NTZArt Interesting. Ordering some now. Thanks for sharing.
Oh, so glad it worked for you!!! Yay!!! I'll let him know.
This is for oil brushes I paint in oil. After my brushes have been rinced out with solvent and cleaned with marseille soap and water, I do this everytime...... I put a bit of the same soap on my brushes and into the palm of my hand I comb them out gently to bring it back to it's normal shape, dry and store for the next use. When I reuse my brushes I rinse the soap off into my solvent. The marseille soap or brush soap conditions the bristles. Other good soap is dawn dish soap. Before I used to comb out my clean brushes with safflower oil, bad idea. My brushes became hard because the oil oxydated.
Thanks for sharing that video!!