BA compressor not reaching the maximum pressure ?

If the compressor is continually running and not making full pressure on a reciprocating compressor that is usually a sign that a valve has broken in the head (top of compressor) would require strip down and valve plate replacement kit fitting

External leaks are easy to find, internal only by dismantling and put it clean together, look at all of the gaskets well, For a BA Compressor always good to do that, but this wasn't they answer you looked for, no, no smart tips

Very smart answer , but I want also the leak causes ? 

Start elimination procedures, possibly there is a leak of pressure.

One of the seals, gasket, pipes or hoses.

CHECK THE SEALS