CEB Hydraulic Pressure Switch
I am using this sensor:
PS61 Switches: 
Catalog page: 
It took me quite a while to find, but is working great and is significantly cheaper than anything else comparable I found. I connected one end to ground, and the other to an arduino input pin with internal pull-up resistor enabled. a HIGH signal on the pin indicates low pressure and a LOW signal on the pin indicates pressure over the set threshold. There is a screw to adjust what pressure you want it to switch at. In the code, I have it set so that if the pressure switch detects high pressure during any of the automatic steps, it moves on to the next step. I also have something in there where if the auto cycle completes to rapidly, it stops running. This is for when the the switch is constantly detecting high pressure and it should probably stop what it is doing.
- Paul's Suggestion - continuous pressure switch -