It's a little difficult to explain how one can tell if the internal flap valves aren't working until they understand how they are suppose to work.

Their purpose is to prevent the vacuum that's stored in the reservoir bellows from escaping back into the exhauster bellows. In other words, they are "one-way" valves that allow air to be sucked out of the reservoir bellows but prevent that air from going back into the exhausters at the end of the pumping cycle. So, as long as you are pumping the bellows, air is being sucked out of the reservoirs, but as soon as you stop pumping a bellows, its associated flap valve closes. Once the flap valve closes, the exhauster bellows return spring pushes the exhauster back to its starting point -fully closed. This cycle repeats itself each time an exhauster bellows is pumped. It doesn't matter if you pump shallow pumps or deep pumps. As long as vacuum is being generated by an exhauster bellows, its associated internal flap valve is 'open'. As soon as you stop pushing the exhauster further open, its associated internal flap valve 'closes'.

At the same moment the internal flap valve closes, the external flap valve opens, and the air in the exhauster is 'pushed' out by the exhauster bellows return spring as the bellows closes.

Therefore, as you push down on the exhauster pedal, you 'feel' the tension of the pedal return spring plus the tension of the vacuum being generated. But, as soon as you stop pushing down on the pedal -and allow it to start returning to its resting point (fully closed) all you should 'feel' is the tension of the pedal return spring.

Now, if the internal flap valve that's associated with the exhauster is not working, what you will feel as the pumping pedal comes back up is the tension of the pedal return spring PLUS whatever tension there is as a result of the vacuum level in the reservoirs. Obviously, if the vacuum level in the reservoir is 'zero', all you will feel is the tension of the pedal return spring. However, if there is vacuum in the reservoir, and the internal flap valve is not working, the springs inside the reservoir will try to 'suck' air 'into' the bellows "through" the exhauster, and you will 'feel' that additional tension under your feet.

The easiest way to determine if the internal flap valves are working is to do the following:

Push one of the exhauster bellows half way down and hold it there. Then, push the other pedal all the way down. If the internal flap valve for the pedal that you are holding half way down is malfunctioning, it will feel like something is trying to resist your effort to hold the pedal stationary. Or, it will feel like something is trying to push the pedal back up.