{Snip}
Thank you. I'm not exactly sure what you mean so I'll describe the
setup in more detail. There is a large pipe coming out of the furnace.
There are 3 pipes coming off of this pipe, each one going to a green
pump above it. One pump is for the 1st floor, one for the 2nd floor,
and the third for the garage.
{snip}

Yes, I will try to describe what I was talking about:

YMMV.....

In some HW setups, there is a large diameter pipe that feeds all the
pumps (as you described you have.) Again in some homes, all the zones
have a return line, and these returns are joined into a single pipe that
feeds the cold water return to the boiler. Thus the water can be re-
heated and re-pumped out to the multiple zones. My personal experience
is the return is about 20 degrees F minimum to maybe 40 to 50 degrees
cooler than the output of the boiler.

(Aside: again from my experience, a thermostat was attached to the water
tank on the boiler, and this thermostat controlled the burner for the
boiler to kick on or off, completely independent of the room thermostat
(s). DAMHIKT, that the boiler thermostat can go bad without the room
thermostats going bad. And you must never, ever, set the boiler
thermostat higher than it is supposed to be set to, which was 180 degrees
in my case.)

The back-flow valve is also called a check-valve. This is nothing you
can control, it is soldered into a part of the pipes. Internal to the
the back-flow valve is a one-way flapper valve that allows the water to
flow only one way in the pipe. Without a check-valve, when the 3rd zone
pump is on, water can flow backwards from zone 1 & 2 as well as normal
flow from the boiler; of course this presumes the pumps for zone 1 & 2
are off. With all three zones pumps working, there will be no back-
flow.

Wikipedia has a good write up on check-valves. Google for check-valves.

But, again, I think the most likely cause of your problem is an air
bubble in the line. There must be a small air bleed at, or near, the
highest point of the zone piping. All water has extra oxygen, nitrogen,
or carbon atoms in dilution in the water. As the water is heated,
cooled, and circulated these free atoms of gases will come out of
dilution and form gas bubbles. Small air bleed valves installed at the
high points allow the home owner to manually bleed these air gas bubbles
out of the boiler HW system. Every time you drain and re-fill the
system, you will need to bleed the air bubbles about every month or so.
You may get better instructions on how often to bleed the air on a re-
filled system from another poster on this newsgroup.

Also, there should be a small tank on your system for water expansion so
you don't blow your pipes as the water heats up. It also should have a
separate air bleed valve. Just make sure you plumber shows you this
expansion tank, it's bleed valve, and the location of the other bleed
valves.

I know, you know much of this already, and I gave way too much unwanted
information.

Phil