Finding Vacuum Leaks

The following information is NOT about testing any particular device in a player piano. It is assumed that you have already determined that a device or component in the player system is leaking vacuum. But, once you know it's leaking, what's the best way to find the leak? That's what this web page is all about.

Basically speaking, there are six types of leaks. The two most common types involve the bellows cloth and the leather valve facings. The less common types involve the rubber hoses, the gaskets, the wood, and the glue seals -or where the cloth is glued to the wood.

When it comes to finding leaks in wood, I've found that the best technique involves air pressure and Pnenoseal. I demonstrate the technique I use HERE.

Finding leaks where bellows cloth is glued to wood can be difficult because it's usually not practical to apply air pressure to the bellow. Most often, such leaks are found at the corners and where the cloth overlaps itself. In such cases, a 'listening tube' or a stethoscope can be useful. A 'listening tube' is just a piece of trackerbar tubing that's about two feet in length. One end is stuck in your ear and the other is used to hunt for the leak.

Another technique for finding leaks where the cloth is glued to the wood involves using a thin glue and a small brush. With vacuum applied, paint a small bead of glue at the edges and watch to see if the glue sucks into the seam. If it does, clamp the area where the glue sucked in and wait until it dries. Then test again. BTW, almost all glues that are used in player piano work can be thinned with a small amount of water.

A listening tube or stethoscope is also useful for finding leaks at gaskets. If the gasket is glued on both sides, you can also use the thinned glue technique mentioned above. However, that technique is absolutely NOT recommended if the gasket is only glued on one side.

In some cases, finding leaks in bellows cloth isn't necessarily easy. That's because it's dark inside the bellows, and the cloth might "look" fine on the outside. What you can't always 'see' is where the air-tight rubber layer has cracked or deteriorated. With small and medium size bellows that are part of a stand-alone device (like a governor, a bass or treble soft unit, an auto-rewind device, etc.), blowing air into the device can be a useful testing technique. The trick is sealing off all of the places where the air can get 'out' of the unit. Often, this means sealing off the exhaust valve port on the valve (or valves) and the connector that's used to trigger the valve/s. Once that is done, and air is blown into the device via the vacuum supply connector, you can 'feel' the air escaping from the bellows with your hand. If the cloth leakage is minor, you should be able to hear it with the listening tube. Most often, the leakage will be at the folds in the cloth, which is where the majority of wear occurs.