We've always used pneumatics, with both Festo and SMC solenoid valves.
It does help to have a supply you can swap out suspect parts with (older robots end up with any leaky parts).
Our system this year holds pressure seemingly indefinitely, I'll have to send you our pneumatics lead. I think he'll fit in a crate...
At one time or another I've seen just about every possible part arrive with a manufacturing flaw: regulators, pressure release valves, solenoids, pressure sensor, damaged actuators, damaged fittings. It helps of course to assemble your pneumatics system outside the robot first making it easier to locate leaking sections, but it's probably too late for that unless you have a lot of spare parts.
- Square tube cuts (there's a special cutter for this), also rough, scratched tubing ends won't seal well either
- Use as few fittings as possible (I wouldn't follow the Pneumatics Manual example layout)
- Check for SMC gasket damage or flaws
- Push the tubing all the way in until it's firmly seated
- Don't use teflon tape twice. If you unscrew a fitting at all then completely remove the old tape and replace with fresh
- Debris such as flakes of teflon tape (don't ever wrap threads all the way to the tip) will lodge inside the solenoid valves to cause a leaky valve
- Check for barely visible nicks in long (and short) lengths of tubing
- Look for damaged threads from cross-threading, leaving them to rattle around in a large bag of loose metal parts
- Push-on fittings can be damaged by yanking the tube out by force, rough tube ends
Isolate the pneumatics system a section at a time working from upstream to downstream and eliminate all leaks in each section before moving on. Just temporarily dead-end the tubing with a plugged brass fitting or a loopback T-fitting. You should be able to isolate the problem to a specific part that way.