I know it's been a while since this topic has had any activity but I've had the same problem with a gv I bought not long before lock down and now I think I've solved it thought this might help as Google doesn't seem to have any solutions. Having used a vacuum gauge on the pump and seeing good vacuum Ive put two servos from a breakers on it, changed the solenoid on the front of the engine and tried a few non return valves to no avail. Putting the vac gauge after the servo showed the vacuum was very slow to rise. Turns out its the vac pump failing, you can test by spraying some wd40 into the metal tube leading to it with the engine running, if the servo starts working then bingo! I've driven a few hundred miles (essential journeys only!) and servo is still working well, whether this is a temp fix or not time will tell. As the pump is buried deep in the engine if it fails again I will add an electric pump to bypass it - plenty of info on eV forums on how to do this, should be achievable for under £150 Hope this helps someone.
