I routinely move my SEE with the air pressure below its low setpoint with the master alarm light on. I am usually backing it out of its parking space and going quite slow and am not using the brakes a lot until the pressure is coming up. As I mentioned I have at least one known leak on my air lines and the SEE may sit for long periods between use so I am usually starting with zero air pressure. I do know that on my long slow drive home the pedal went all the way down to the floor with little brake effect. Once I got the air system to work again when home the brakes came back to act like normal.
My 1300 does not allow this as it has air actuated emergency brakes, until the air pressure comes up and releases the E brakes I am not going anywhere unless I activate an emergency override by crawling underneath and hitting the air cylinders with hammer.
One thing the OP just mentioned is driving at 40 to 50 MPH. Barring a billboard smooth road, 40 to 50 MPH may be an aspirational goal but most but not all owners probably have had their fill at 30 to 40 MPH especially with the stock Michelin's. With mine, anything over 30 would start to be a problem bumps across the road would start a front to back pitching while a crowned road could start a side to side pitching. I attribute both to a short wheelbase with a big weight suspended off the front end. I did switch to Pirelli Pistas which are more of an on road tire and on my long drive over to the site prior to my air loss I did perceive some improvement in handling but didnt push it. The other issue with mine is it just runs out of power on grades. I live on steep hill on state highway equipped with run away truck ramp. When heading home and heading up the hill, I am in second gear at full throttle going 15 MPH by the time I reach my turn off. On moderate grades even 30 MPH is a challenge. The SEE may have adequate torque and lots of gearing but a typical econobox has more horsepower. 16K with 100 HP does not equate to high speed up hills.