The gasket on the speed control mechanism needs replacement because there are only two speeds possible.
Too fast when the control is tight.
Too slow if it is loosened a bit.
What materials will work for a replacement gasket?
The material must slow the passage of air.
The material must slow air passage more when tightly compressed,and allow more air when less compressed.
The compression of the material controls the speed at which air escapes.
[2 Control dial removed.
Dial showing deteriorated gasket. The gasket deteriorated gasket material looks a bit like tar.