Airspeed Indicator problems

The original Schweizer airspeed indicator in my glider sticks.
Once on the ground I blew into the pitot and the indicator shows an
airspeed but then it takes 10 seconds to slowly go back down to 0.
On the back of the instrument there are 2 vertical ports.

I disconnected and blew through the top tube which runs to the static
ports. Blew just fine, no blockage.

The pitot line runs to the bottom port on the back of the instrument
but not direclty. It has a tee that routes it off to another
instrument also. I would have thought that the pitot should have an
uniterupted tube going directly to the airspeed indicator.

The diagram in the opertors manual shows a dedicated line going from
the pitot to the top port on the back of the instrument
and the static line going the the bottom port. Just opposite of my
particular installation.

Any advice would be appreciated.

Stan