I have a simple pneumatic claw with a 2 pump compressor that I built somewhat recently. It worked fine for a while, producing up to 4 bars of pressure, but then the system wouldn't increase the pressure of the system more than 2 bars maximum, usually stopping at a bit more than 1 bar and the pressure increased much slower. When I press on the the pumps in certain ways, the system will act normally, but I don't want to press on the pumps continuously. Is this due to a leak, or some other issue, possibly with the pumps? If so, how do I find out which piece is leaking? And how do I stop this from happening? Thanks.
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:
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).
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.
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.
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.
LEGO Pneumatic system could consist of different elements like hoses, switches, pumps, cylinders and T-junctions. Each one may need different approach.
I could think of several ways of testing these issues.
First one might be a little hard, but depending on the issue could be a quick solution. This would works for any pneumatic element. You need to get into a quiet area and pump pneumatic system as much as you can. Then stop the compressor and if there is quick pressure drop you should listen for any air escape.
Next option is to use water. Depending on your model you could submerge it partially to cover pneumatic connections points while system is under full pressure. If you see any bubbles - that's your problematic area. You need to be cautious aroud any electric parts (motors, batteries and cables) if you have any. You are safe submerging every pneumatic element except cylinders, pumps and switches. Although, I think switches could be safe for water since they don't have metal parts.
In case partial submerging is not possible I could advise using water drops over the hose connection points. That is where hoses connect with everything else.
Testing pumps, cylinders and switches might be a little hard since they got moving parts and seals. If these elements are hard to remove from your model I would attach a closed circle system to each of such elements to pump the air and check if it leaks the air.
Alternatively, if your pneumatic system is easy to disassemble you could try swapping one element at a time and see if that has any effect. I would only do this with pumps, cylinders and switches once confirming hoses are not leaking any air.