A picture of the cap end would help. I haven't seen a dht splitter so I am just going to speculate. The cap might just screw on or in, or could be held by a c-clip or by the use of a spiral lock wire. Look at the cap, does it have two holes drilled into it for the use of a spanner wrench. These little holes could be either in the end of the cap or in the sides. If not, then most likely the cap is held in with a c-clip. If this is the case, you will have to apply some pressure to push the cap farther into the cylinder and remove the c-clip. The cap should then bump out, but might need a little help by blowing air pressure into the hose port. Sometimes the use of the spiral wire is used. If so, you will need a small drift punch. The spiral lock wire should have a small tab just protruding out the side of the cap. Use the drift punch to start tapping the spiral wire out. You will have to keep tapping the wire tab until the wire completely comes out of the cyl . It spins out so don't try to just beat it, with care it can be reused. After the wire is removed, apply some air pressure in the cap end hose port and the cap should blow out. You can also apply air pressure to the base end port, but be prepared for the cyl rod to come out with the cap.
Now my bet is the cap screws into the cyl barrel, as that is the way most of the china rebranded cyl seem to be. There should be the two small holes drilled into the cap end for the use of a spanner wrench. You may have to borrow a spanner wrench or make one from some flat plate and bolts just big enough to fit the holes. You will find that using just a little heat with a small propane torch, heat gun, or I like to use my wife's hair dryer, helps. Just warm the barrel around the rod cap while applying pressure to your spanner wrench.