Most recently the pump was working nonstop for about a month. This incarnation of the pump has the waste valve with parts salvaged from the original PVC pump and the check valve with an elastic as a substitue for a spring.
First, what caused it to stop? When I try restarting, the waste valve sticks in the UP (closed) position. But occasionally when I try to manually restart it doesn't go up all the way, ie it never gets into the closed position. This latter problem is either too low a water flow or the valve itself not sliding smoothly into place. Since if I restart it several times it usually does get into the closed position, I assume that it isn't sliding smoothly into place when it doesn't close. So why when it does close does it not open again?
The only two possibilities I can think of are:
1. that the water level in the collection tank has gotten higher or lower than it has been over the past month when the pump was working
2. that some moving parts have worn/changed/broken/shifted causing it to stop
Some specific causes could be
1. the valve sticks in the up position because it doesnt slide smoothly back down
2. the decrease in pressure isn't enough to allow the valve to drop down again. this might be caused by a malfunctioning check valve which might be down to the elastic or the gate itself not seating properly or the valve not sitting level.
3. not enough/too much air in inner tube?
5. waste valve too light (the new rubber washer i put in is lighter than the previous)