Perhaps the sensor was broken off to hide the fact that the master cylinder is only working on one half it's circuit. Replacing the sensor might reveal the issue. Also, I've read about a recall on Workhorse brake systems, you need to check and see if your chassis is subject to the recall and if the work has been done. You might send a private message to Jon Brazel
I'm sure he can check using your VIN number.
There are a number of reasons for pulling to one side, but eliminate a faulty master cylinder first. If you remove the cover on the reservoir, both sides should have equal levels of brake fluid.