It looks to be an interpump box jet Turbo 10/100 type design so no issues with parts etc. Could be an unloader issue - in which case you can try with the machine running turning the black numpty on the unloader all the way down and back up again a few times as it sounds as if the unloader is sticking in bypass.
Also worth checking the jet for any blockages and the GEKA fitting on the inlet too as these leak hideously with age.
The 10 seconds of pressure is the bit which makes the issue a bit odd, if it were water seals or check valves the issue would be instant.
Hope this helps
Ben