How to locate pneumatic leak
According to various sources, LEGO pneumatics is not designed to work above 3 (or maybe 4) bars of pressure, so your setup might have been damaged by the overpressure.

If you can "press on the the pumps in certain ways" then I would recommend starting your investigation there. As to actually locating the cause of the leakage, I have five main methods in mind:

1. Vision

The easiest and fastest way is to check all components for loose connections, punctures or cracks. In my experience, when pneumatic hoses are subjected to overpressure or excess mechanical stress, they usually fail by slipping off their connection points. So go over all your connections and ensure that all stiff tubes are pushed into the flexible hoses as far as they can go (4-5 mm at minimum, or about one "arm" of a pneumatic T-joint).

If that fails to reveal any problems, check the lenghts of the hoses for any punctures or hairline cracks, while the system is under pressure (as the cracks might close and seemingly disappear in the absence of a force that acts to open them from the inside).

2. Hearing

It can be hard to hear the sound of air hissing out of a tiny hole, especially with a compressor running, but if you can find a way to remove the compressor from the setup, you could be able to spot the point of failure.

Ideas to remove the compressor:
 - Let it fill up a pneumatic tank, which is isolated from your system via a valve, then turn off the compressor and throw the pole of the valve open.
 - Add a long enough hose between your system and the compressor so that you can set up the compressor a bit of distance away.
 - Apply pressurized air to your system via some alternate means, like e.g. by blowing into a tube, or via an inflated balloon.

3. Feeling

If the hole is too small to see, the escaping air might still be felt on sensitive areas of skin, like fingertips or your cheeks. The hairs on the back of your hand might be helpful in amplifying the weak air movement into an observable feeling.

4. Bubbles

If all above methods have failed, you could consider submerging your system in water. Only do this, if you can remove ALL electronical components (motors, wires) to a safe distance, and only if your setup does not include pistons with metal shafts. After doing so, you only need to watch for the emerging bubbles to see where does air escape from. Note that your compressor pieces must remain above the waterline, or else they will try to pump water, which can damage your parts from the inside (or just stay there for a long time to come).

As an alternative for localizing holes in certain areas, consider applying soapy water to the suspected parts (Tip by JPhi1618 in comments).

5. Deconstruction and reconstruction

In the improbable case that even the submerging method has failed, you could deconstruct your system, inspect each part individually for their ability to produce/transport/direct/use pressure without loss, and then reassemble it with proven flawless parts.