Following on the previous posts, both the valve board and pouch board have been rebuilt or refurbished for optimal performance. Now it is time to test your work, to see how it performs.
This will be a lengthy post with several steps, but stick with it!
To bench test the valve chest, you will need a suction source.
Although it is possible to generate a decent amount of suction with the human lungs for a short period, it is much better to have an electric powered suction machine.
In order of preference, the following devices can work: 1) reproducing piano rotary pump with electric motor; 2) player piano generic suction box, of the “PP Co” type 3) small shop vacuum cleaner, available at big box stores.
It is also important to have an instrument to measure the suction level (e.g. a vacuum gauge), so that you know you have enough suction power, but not too much (so as not to damage your work – a possibility with a vacuum cleaner).
Assemble your secondary chest and devise a way to hook up suction in a secure and leak free way to the chest. In my case, I took a scrap piece of hardwood and made a junction block, which could be attached to the suction hose and then clamped to the intake port of the chest. It doesn’t have to be fancy, just so long as it does what it needs to do.
The suction source should be set to a low-ish setting (7-10 inch water column) and the vacuum gauge also needs to be in the mix to measure what you have.
Depending on if the chest is a single or double valve model, testing procedure will vary.
For my double valve chest, I taped off the ouch board edge with the input holes, with pieces of tape covering 3 holes per piece.
First, with all input holes taped, we want to be sure all the valves are off, and seated so that there is no leakage.
This is also the time to calibrate the overall tightness of the chest; having measured prior the suction level on the gauge with the hose completely stopped by hand (before hooking up to the chest), now look at the gauge again and compare.
In a perfect world, there is no difference in the gauge. In practical terms, however, a good target to strive for is a 1-2″reduction only in suction level. More loss than 2″ and you may have a problem with leakage, which will have to be found and corrected.
Let us return to the valves. With all the valves “off”, first pass your ear over the face of the valve chest to listen for any obvious hissing sounds. If you have difficulty hearing (if the suction machine is too noisy), try using a piece of tracker tubing, placing one end in your ear (not too hard!), and the other in the pneumatic channel which is next to each valve. This allows you to get right down to the seat plate, to hear any potential leakage.
If the vacuum level drop is significant (no measurement) and you hear no audible leakage, it may be so severe and diffuse that you have to take a step back to diagnose the problem.
In troubleshooting, we must use the process of elimination. You could start by taping a strip of airtight material over the whole top of the valve board, taking the valves out of the equation and evaluating only envelope of the chest and the perimeter gasket.
If that checks out okay, but still hear leakage problems, return to the valves and consider the following points (most common errors), which were compiled by Tim Gautreaux (from MMD):
- the “inside valve” has been pressed too hard between the retaining collars and cannot flex [wobble] and seat against the metal plate;
- the buttons are not screwed down enough and the pouches are holding the valves open [insufficient button clearance];
- the pouches, if old, have shrunken and are holding the valves open;
- the metal seats haven’t been sealed properly;
- the metal seats haven’t been cleaned and coated;
- the valves have too much or too little travel;
- all the felt from the fiber [stem] guides is gone;
- the gasket in the L-shaped board has not been replaced.
Once you believe that you have eliminated these potential problems (including the often overlooked issue of “wobble”), fire up the suction box again take a fresh look.
To reiterate, when the valves are off there should be no explicit hissing or leak at the valve seat. The vacuum level loss should be minimal.
If these conditions are met, we must now repeat the test, but with the valves in the on position (pouches up).
Before starting this test, tape or cover over the pneumatic channel ports and screw holes, and open all the pouch input channels. Turn on the suction, and again listed for the telltale hissing leaks.
This time we are listening right at the top seat of the valve well, which is essentially at the outer holes on top of the valve board.
With all valves open or “up”, with all other holes and ports on top of the valve board taped off or sealed, there should again be no audible hiss.
You can get the “big picture” by opening them all at once, but it is also valuable to test them one by one in sequence, actuating them as quickly as possible and stopping in both the on and off positions.
It is undeniably tedious to go back and correct faulty valves, and some valve designs are worse than others for this.
However, this is the best time to do it, and it is absolutely necessary to correct errors as soon as they are noticed. This round of testing is just the first of several cumulative tests to be performed, and you can’t build good work on a bad foundation – it’s really that simple!
Once these valves are all working well, and your chest is tight, as a final test you should even be able to test this with your mouth by connecting a 3/4″ (or larger) hose and trying to suck the atmosphere out of the chest.
With a big huff, you should encounter fairly immediate and significant resistance.
The longer you are able to hold suction with the same amount of force, is a good indicator of how tight your chest is!
Upon successful completion of this work, you have achieved a nice milestone in your restoration.
Now set aside the secondary chest, and get back to work!