Assuming it holds air ok, the valve, its return sring, the hammer and the hammer spring are ok, there are a couple of o rings which may be the cause. One seals the pellet probe (Bolt) the other seals the action to the top of the valve (transfer port). A leak of either will give you low power and in this case the gun will sound very loud. You'll need a chrono to check it. If it is a later gun it'll have anti-tamper on it which will need removing for most repairs.
It will be less than 80 bar when it dumps the air, probably 40 bar. This is a problem if you use a pump as it can be impossible to refill; no problem with a bottle as you can shock it shut with 200bar. In any case when this happens, make sure you cock the action before refilling as the pressure of the hammer on the valve will not allow it to seal.
Your average gunsmith wont know what to do with it.