"it kept slipping off when I got quite a bit of pressure on it. Any ideas?"

Paul, try putting a clamp across the jaws to hold them together. Then put some tension on the puller bolt and tap the bolt with a hammer. The shock will often separate the parts.

Another trick that works is to tension up the puller bolt than hold a heavy item like a sledge hammer head against one side of the pitman arm at the shaft hole and smack the opposite side with a hammer. I've removed stubborn ball joints that way.

You can do it!

Ray