We have a 10 year old ASKO DW, and we mostly love it. It has recently
developed a problem draining, and I discovered that the inlet valve/
solenoid had a slow drip. I replaced that, and the problem was
reduced, but not eliminated. What happens is that water builds up in
the bottom catch pan, and then triggers the float which prevents the
unit from filling.

I cleared the pan, and ran a cycle. Perfect. I ran another cycle.
This time when it had finished, there was water left inside the DW, in
the bottom, but the dishes were clean. I ran the cycle again, and the
water from the bottom of the DW ran through the overflow into the
bottom pan, again triggering the float. Ok, that's why is shuts off,
but my question is:

What would cause the DW to not pump out all it's water sometimes?

Other info:
The DW has a electro-mechanical rotary controller, no fancy
electronics.
The drain is looped up and then down.
No vacuum break has been installed. The original installation
instructions did not call for one, but later installation instructions
did! Note that the drain hose runs from the pump, up to the plastic
hook, then down through the floor at the back of the cabinet into a 2"
ABS pipe, trap, horizontal run of 2", (total 15') then into a vertical
2" pipe, into the basement floor. This final drop appears to be
correctly vented, but I haven't tested it.

Some previous posts indicated that a slow drain might cause an
overflow. I'll try to put a fish tape down the drain to see if it is
clear. What specifically does the 'vacuum break' do, and how might it
affect my installation?

Thanks for your help!
Rob