Pump runs for about a minute then loses pressure
I am new to hydroponics and just built a continuous drip system with 6 buckets. I have a 300gph TotalPond fountain pump at the bottom of my 18 gallon reservoir. It comes up a 24 inch riser and into a 7 foot horizontal 1/2 inch pvc manifold with 1/4 drip tubes. When I power up the system, I get great pressure, equal to all tubes, and then after two minutes the pressure drops. If I bump the pump on the bottom of the reservoir (holding the riser) then it pumps again for 30 seconds and drops again. I am pretty sure the system is air tight, and cannot figure this out.
I had a larger 550gph pump which was doing the same thing. I returned it thinking that it was defective, so I don't think the problem is the vertical rise. The current pump is rated for 6 feet and I only have a 3 foot rise.