That short tube makes it easy. Less fun on a QB78, but found it possible to do.
Have never tried on a long tubed PCP QB. Guess you could still do it, but it does seem that manipulating the valve stem and return spring gets difficult. Guess it depends on what type of valve and return spring set up you have, but it gets to be a bit like building a model ship in a bottle whe you're working down a long PCP air tube.