Recently at yard sale I purchased a Prentiss #26 vise with a self adjusting rear jaw. My question is how tight is the rear jaw supposed to be?
To move mine I have to use a dead blow hammer or put something in the jaws and close them with the screw. If it's normally this tight that's fine, but otherwise I'm thinking the rear jaw might be a replacement that wasn't machined to fit this vise and I might try to polish it.

The rear jaw should swivel on it's own as you close the vise on any non-parallel piece in the vise.

The rear jaw is very closely fitted on a spindle that is tapered and sits in a matching tapered hole in the base of the vise.

The 'bed' that is machined into base of the vise to accept the rear jaw w/ it;s spindle is also closely machined for that rear jaw.
Usually a rib on the bottom of base of the jaw matches a slot in the vise base. That takes some of the pressure off of the tapered pin when the vise is closed up on a work piece.

Likely the rear jaw, it's tapered spindle,,the machined bed it sits in and the tapered maching hole for the spindle in the vise base all need a clean-up then lite greasing.
IIRC, the jaw justs lifts up and off of the base.
Might need some 'help' in coming off the vise base.

It doesn't take much rust and crud inbetw the parts to jam them up, especially that tapered pin and the matching tapered hole in the base it sits in. They can sieze up very easily.

Thanks for your reply. I think the design of my vise is different from the one you describe. The rear of mine has two "tracks" or circular slots machined in it. One on the top that's facing up and the other one faces forward. The adjustable jaw rides in these. The drawing on this page shows it. You can see the rear jaw in figure 1479 and 1480. Prentiss Vises [1896 Catalog Excerpt] : Prentiss Vise Co. : Free Download, Borrow, and Streaming : Internet Archive