Hahl Pneumatic Slave clock

[telechro]

i've built a Hahl pneumatic slave driver. It has a small air pump that increases air pressure and advances the slave one minute. Once advanced, a valve closes and then at the next minute the valve opens, the air pressure returns to normal and the clock advances again. This simulates the normal operation of the master. The problem is that as soon as I close the value to hold the high pressure, it leaks out. I suspect the diaphragm is not making a tight enough seal to hold in the air pressure. If anyone has worked on Hahl slave clocks, I would appreciate any advice on trouble shooting. As it stands now, it works fine as a 2 minute slave.

thanks for any help you can provide.