I just bought a new to me glock 26 and just finished cleaning it I have not fired it as yet.
I am new to glocks and I am puzzled about something.

when I pull the slide back while no mag present, it locks back

when I pull the slide back while empty mag is present, it locks back

when I pull the slide back while a full mag is present, it locks back

does it only not lock back when it is fired or is it sick

Sounds like the slide stop is installed wrong. The slide stop spring should be under the locking block pin. I bet your spring is above the pin. Sorry i don't have any pictures to show it to you. Take the slide off and lift up on the slide stop. it spring back down on it's own.

ok, did the field strip, spring appeared to be where and how it was supposed to..
but, the slide stop appeared to be a bit high in relationship to the pics I found on line
so I was able to flip it over the rail. bend it down a bit, reassembled pistol and it worked fine as far as testing an unfired weapon is possible..guess tommorow or wed I will go run a few hundred through it and see what happens.,,
in looking at the weapon from the side, the slide stop is sitting lower to the frame and no longer touching the slide..

slide stop seems to be a flimsy little thing, and should probably be replaced.
the manufacture date is 10/99 and previous owner stated he bought it new..
but, has less than 500 rds through it.
by design there doesnt seem to be alot to the slide stop..
should I replace it?
guess I will if this is not the cure.
thanks for the input so far

Ok, you don't have the pin in this photo but you should have the spring in just about the same place.