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General Test for Stack Leakage

Below is letter and my reply to a not-so-uncommon problem. Since it covers a relatively simple way to determine the general condition of a player piano stack, lower section and air motor combined, I offer it along with a link to other tests.

Dear Mr. Tuttle,

Hello. My name is Bob Orlikoff and I live in West Windsor, NJ. I bought an old Lenox player piano last week (along with several original rolls) and had it moved to my home from Clinton, NJ. Although I checked to see that the player mechanism worked correctly when I purchased the piano, I now find that -while the piano plays well manually- it will not play when pumped. The rolls move, but the rotation appears to be a bit uneven. Occasionally there is some slight key movement, but no sound is produced. I assume that there is some problem with the bellows, but I don't know where to proceed. I did check your website to see if you had any suggestions, but your advice to seek professional assistance is probably a wise one. Overall, the piano appears to be in good shape and I know that it has received servicing in the past. What would you suggest?

Thank you for your assistance.


Bob Orlikoff

Hi Bob,

Since you stated that you checked to see that the player mechanism worked before you bought the unit, I will have to assume that the unit has an electric vacuum pump as well as the normal foot treadles. The only other possible explanation is that you don't know the difference between the player action and the piano action or that the two are separate. All player pianos are also regular pianos and perform like any ordinary piano, with the exception that they can also play themselves (so to speak).

In any case, you undoubtedly have leaking valves and/or bellows. There are about 100 bellows and 110 valves in the average player piano. There are at least three major components, and more commonly four, that work together to 'play' the piano using a paper roll. Determining which of these is responsible for the majority of the problem you're experiencing requires the testing of each component in the system.

Over the past 15 years, I've been creating various pages devoted to helping owners determine the general condition of each major component. It's a challenging task because there have been more than 100 different player mechanism makers, each with their own patents and, ergo, slight differences. So the information I have provided is on the general side.

However, in your case.. which is not uncommon.. the electric vacuum pump is able to overcome the leakage that normally comes with aging leather and pneumatic cloth, whereas the manual foot pedals cannot overcome those losses. Considering that the average electric vacuum pump can produce three times the amount of vacuum needed to make the unit play, that is not surprising. It's also not surprising that the pump was installed instead of finding and fixing the problem which made the pump necessary in the first place. Typically, it's money.

Replacing all the old leather and worn bellows cloth is not inexpensive. It requires hundreds of hours of patient, meticulous craftsmanship costing in excess of $3000.00. Compared to the cost of a pump (around $400.00), it's easy to see why most people choose to electrify instead of rebuild. But sooner or later, even the electric vacuum pump will become ineffective.

One of the things I'm good at is pinpointing the major problems in a player system in short order. And it sometimes happens that the main culprit can be rebuilt or even conscientiously repaired for way less than $1000. But in order to enjoy effortless foot pumping, like when the unit was new, you must rebuild the entire system. It's simply a matter of efficiency verses dollars.

For some tests you can perform, see:

Testing the Player System

I'm still working on more tests for the Stack. But without the proper test equipment (which most owners do not have), it's been difficult to come up with tests that will be of any real help. You see, in order to determine the degree of efficiency of the stack, you must have a constant vacuum source AND a very accurate vacuum gauge that measures vacuum in inches of water, not inches of mercury. So for now, that test still has to be performed by someone with the right equipment.

Here's one test that will at least give you an idea of the amount of vacuum loss occurring in your system.

1) Put a piece of scotch tape over all the holes on the tracker bar.

2) Put the unit in Rewind. Grab the rewind drive spindle (above the tracker bar on the right side) and prevent it from spinning as you foot pump. After about three full pumps, you should feel a significant increase in the pedal pressure. (In a rebuilt unit, the pedals basically lockup after about three good pumps).

3) Note the amount of pumping you have to do when nothing in the system is actually operating. For in the Rewind position, the only component that is in service is the roll motor, which you are stopping by holding the spindle, AND the lower section (which produces and stores the vacuum).

4) Now turn the Tempo to '0' (or minimum) and place the unit into the Play position. Start pumping again, just as before. The difference between the amount of pumping gives you an indication of the amount of leakage in the stack.

Now... with all that said, there is another possibility. Inside of every electric vacuum pump system I've ever encountered is a Check Valve. This valve is suppose to 'feel' that you are foot pumping your unit and close off the large vacuum tube leading from the vacuum pump to the lower section reservoir bellow. If it is stuck open, you have a 1-1/8" hole allowing air into the system. In other words, you have ONE HUGE LEAK!

The Check Valve is typically located at the output flange of the vacuum pump. To test it, simply remove the hose connected to the reservoir bellows (leave it connected to the pump) and suck on that hose like you were gasping for air. The valve should instantly close. Start again, but this time start sucking softly and increase the suction (rather quickly) until the valve closes. This action simulates the build-up of vacuum in the exhauster assembly as the pedals are pumped. Sometimes a quick pump on the pedals is required to get the check valve to seat. However, it should only take a small level of vacuum to get the valve to close.

Obviously, if the valve never closes, you found a major problem. Fixing it isn't difficult. So, I shot the video below to show you what to do. But if the valve did work smartly, the problem is in the player system.

Below is a video that shows how you can use your lungs to check various devices in a player piano.

Testing/Repairing the Check Valve

Best of luck,

John A. Tuttle

This page was last revised on August 30, 2021 by John A Tuttle.

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