My HF used to lock onto zerks and I couldn't pull it off. Even pulled a couple of the drive-in types out of the machine. Then I noticed the tip is adjustable. There is a screw joint about halfway along the tip. Loosen that a bit and it doesn't grip as hard. Seemed too easy but I no longer cuss on hard zerks.

When I replace cartridges, I let the plunger rod go inward a couple inches from full out. Push the new tube on until I feel it bottom against the plunger. Then pull the plunger backward so it pulls the new tube into the barrel of the gun, but no further. When it gets where it needs to be, I screw the pump end on. No more air-locks for me