In our early systems the valve served as an overflow to avoid overpressure of the oxygen compartment. During over-inflation, the valve serves as an over-pressure release.
Use of high altitude with an over-inflated oxygen compartment, mixes overflow oxygen with the reduced oxygen air, creating air with a “slightly” reduced oxygen concentration, usually at about 19% O2 saturation instead of 14%.
This reduces the desaturation potential, and you will not be able to challenge your respiratory system to potential.
The fix is simple: work out on oxygen for 5 minutes first to reduce the pressure inside the oxygen reservoir. When the oxygen compartment is full, it is firm to the touch.
Five minutes of rich-oxygen workout reduces the internal pressure to prevent oxygen release through the valve. This stops the overflow.
At normal pressure, less than overfill, the high altitude simulation will be at about 14% saturation which is equivalent to about 10,000 feet. You should be able to desaturate after spending a few minutes exercising at this altitude.