alan_m used his keyboard to write :
Plenty of Youtube videos showing what is inside
Example:
https://www.youtube.com/watch?v=6vNOYmCp4xQ

There are other videos showing more dis-assembly and indicate that they
probably could not be easily modified.

Thanks - I have found a way to modify one....

https://www.youtube.com/watch?v=pWzzttw4SuE

Its not for continuous use, just long enough to pull a bit of vacuum in
a demi-john or a 5 gallon container for a few seconds, then release.