Pneumatic leaks

My team is dealing with a leak on the input fitting to one of their solenoids.

Any suggestions on resolving this sort of leak?

The leak seems to involve the fitting and / or the solenoid. We replaced that section of tubing without success.

try determining if the leak is between the tubing and the fitting, or between the fitting and the solenoid.

if the leak is between the tubing at the fitting, make sure the tubing is cut perfectly square, and/or replace the fitting.

if the leak is between the fitting and the solenoid, make sure the fitting is tightened all the way, and try using pneumatic teflon tape to make the threads absolutely airtight. (the only components I’ve found this to be necessary on are the valve fittings on the reservoirs, but it can’t hurt to try them on a leaky fitting. )

if that doesn’t work try replacing the fitting and lastly the solenoid. I doubt you need to replace any of the components though, it’s probably due to something not being super tight.


Is the tubing a nice square-cut?
Normally this will solve most of the basic leakage issues.


Well, it might hurt. The small fittings that connect to the pneumatic cylinders and to the solenoids seal with a tiny washer. Teflon tape can often get in the way of that washer sealing properly.


interesting, I assume it’s not a problem if the tape is only present on the lower portion of the threading?


I found the issue. It was actually not the tubing, or the fitting.

There is a small metal bracket on the bottom of the solenoid. My students had zip-tied the solenoid to the part being moved by the pneumatic actuator. This had managed to loosen the screws holding this bracket to the bottom of the solenoid by a turn or two. These screws also hold the bottom assembly of the solenoid TOGETHER…

Once the screws were tightened, no more hissing. The system is pressurized overnight for a more serious test.

Just another thing for coaches to check when weird problems arise…


Yeah the tape has to stay off of The mating surface between the fitting and the cylinder where the washer seats


I don’t think it would help here.

Teflon tape is used to seal a fitting using a tapered thread (that seals by an interference fit IN THE THREADS). The NPT (national pipe thread) fittings on the reservoir are in this category.

These fittings use a standard M3 (3mm metric) thread that is not tapered and is not supposed to seal in the threads. It is supposed to seal at the base of the threads on a gasket. The threads are not supposed to be an interference fit at all (should easily go in by hand until it stops very suddenly as the gasket hits the valve).


Glad you found the leak. If you didn’t - then there is a “Leak Detection Spray” sold online, which you spray and it bubbles up where there is a leak.

Not sure if we did a wrong thing, but a few years back we used the liquid they sell as refill for bubble toys and it worked to find the source of the leaks for us.

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The traditional method to look for leaks (dating back to bicycle tires) is dish soap (the liquid kind used for hand-washing dishes) and water. Bubble solution like you used sounds like a really good substitute.

The one thing I would be wary of is to make sure you don’t douse the electronics. Getting electronics wet is bad. Getting them wet while they are powered is much worse. If things get wet, let them dry out THOROUGHLY before you plug them back in.


Make sure the bronze colored insert has the black washer on the threaded side. It will leak if not there