Reservoirs don't have stiffeners...
Regarding the conical compression spring, if memory serves, it's in the bellow that also contains the accent (or crash) bellow. It's purpose, therefore, is to prevent the bellow from closing too far. You might also notice that the tension of the 'V' springs is different in the two reservoir bellows. According to the Standard service manual, there are 7 lb. and 8 lb. springs. The heavier springs go in the bellow without the conical spring.
While there are no explanations in any book or manual concerning any of the springs in the reservoir bellows, logic dictates that there should be somewhat of a balance in terms of the vacuum level required to collapse the bellows. So, as the bellow with the lighter springs reaches full compression, the moveable board runs into the conical spring.
Regarding the accent (or crash) bellow, it only reacts to sudden changes in the vacuum level. That's why the spring that holds it open is very light weight. As long as the vacuum level remains fairly constant, regardless of how low or high that level is, the crash bellow (which is actually a valve) doesn't react.
Regarding the felt under and in front of the conical spring, it's only purpose is to prevent noise. I would not suggest removing the conical spring unless it is loose. As I recall, it's held in place with horseshoe nails, and any looseness will cause rattling inside the bellow -which would be annoying. As for the felt that the spring runs into when the bellow collapses, in the past I have replaced that with a piece of thick flap valve leather. Here again, it's only function is to prevent tapping when the spring makes contact with the moveable board.
Here are some pictures of the crash (or accent) bellow in a Standard reservoir bellow. Click on any picture to see the full sized image.
For more detailed information about the Crash Valve, read below pictures.
I just pulled a Standard Accent device from the bass reservoir and can give you the dimensions, now.
It has a tiny (1-3/16" long, 9 gauge torsion) hinge spring which does little. It's main "spring" is the pneumatic cloth it's covered in. The reason it can be sucked closed easily is because the cushioned valve is so large in diameter and so close to the hole.
The hole is 1-1/2" dia and the cushioned valve is on ave. 3/32" gap when fully open. This is ave. because it's about 1/16" toward the hinge and 1/8" toward the open end.
The length overall is 3-7/8". Overall width 2-7/8". The hole centered is 2-5/16" from the hinge and 1-1/2" from the open end. The cloth is 3/4" wide and the leaves are 3/16" thick.
It's the leverage the vacuum in the plenum minus the vacuum in the bass reservoir which sucks down the large cutoff valve and closes it off momentarily. There are then the 4-7/16" screw access holes left open to help equalize the pressure quickly inside the device whenever it closes.
But there's also a stronger spring one-way valve, held closed by its spring but not as sensitive to more direct vacuum on its surface as is the accent device. So suppose you didn't want an accent but continually more loudness. Well then, that one-way valve stays open, bypassing the accent device. So that's giving the bass reservoir the same pressure that's in the plenum. So from that increased overall pumping pressure you'd have to really kick the pedals to get an accent over the louder playing.
In other words, only a momentary hard kick of the pedal will give you an accent. When just normal loud playing is desired the heavy reservoir springs close, keeping the one-way valve loose and regulating the pressure for loud play, opening as needed, percentage-wise, to keep the loud play even.
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