mystery pump - need to fix soon?

[Tessie McMillan]

There is a pump attached to my engine ('88 5 cyl), on the power steering
side, just below the cam cover. I've been told it has a piston that runs
off the camshaft to do its work, but folks I've talked to can't remember
what it does...  vacuum assist? ABS pump? Anyway, I have a "thwocking"
sound -- which sounds to me like what a rattly piston would if it were
whacking away on the cam -- which is supposedly the sound this pump makes
when it goes bad.

I've also been told I can remove the piston and leave the pump as-is,
and everything will run fine without repairing the pump.

My two questions are: 1) how critical is it to get this done (am I
damaging the cam?); and 2) how do I do this? Sorry to be so
unknowledgeable, but do I try to open the pump, and pull the piston out
that way, or do I need to take the cam cover off? Third question comes to
mind: what the heck IS this pump?

Thanks!

Tess
'88 80 Q, Bellevue WA