PS Pump Woes

[Steve Sherman]

I just replaced the PS pump in my 87 5KTWQ.  The old one was working OK,
but the tiny leak had grown to the point where the front of the engine
was constantly wet with hydro oil.  I had "cleverly" kept a "used but
good" pump on hand for just such a day.  However after putting the new
to me pump in, it seems that this pump can't properly pressurize the
brakes.  Steering seems fine - that is power assistl.  Brake warning
lite flashes (it is the low pressure switch, lites go off when I
disconnect it), and if I drive a block  are no power brakes...

So, did I just get taken on this used pump, or is there some bleeding
process that I am supposed to go thru?  Would like to know before I go
out and buy a new one.  (FWIW, I did prime the pump.before putting it
in, put oil in all ports and turned it by hand, and oil surged out  two
all ports)

Any thoughts?

TIA