I'd try bleeding all four wheels.

Even without opening the fluid system on the brakes, brake fluid
gradually grabs moisture from the air which is why it changes color and
gets darker with time.  To keep brake performance at top efficiency and
get maximum life from calipers and such, it's a good idea to fully bleed
the brake system each time you put new pads/shoes on (brake shoes that
is, not the ones for your feet <g>).  Brake fluid doesn't compress, and
neither does water, but other impurities that collect in the fluid can
and bleeding the brakes will usually help with that.  Also if you have
antilock brakes, it's not a good idea to just push the pistons back into
the calipers as the contaminated fluid (tends to be more deteriorated at
the wheels in part due to heat from stopping) can damage the ABS
actuaters.  It's generally recommended with ABS to open the bleeder
slightly as you push the piston back in so the excess fluid doesn't get
pushed back up the brake line, and then to bleed the system until you
get fresh clean fluid out of the bleeder at each wheel.

IMHO, telling you to just drive it with the pedal going to the floor to
stop the car borders on criminal negligence on the part of the shop that
did the work.  That type of behavior means SOMETHING is wrong with the
system, and my opinion on brake issues like this the car should not be
driven at ALL in this condition.  I say this ESPECIALLY if the brakes
were not like this when you took it in (I'm assuming this is the case
since I would not expect you would have thought it strange if it had
been this way when you took it in).  If I am correct, I would presume
they did something to damage the brakes until/unless the could prove
otherwise to me (then again I always do my own brake jobs since I don't
trust most shops).