I hope I understand your issue.
"Slightly Crack" the union,,,,,Do you mean allow air into the system?
Is it possible that the 3/4" line in question just doesn't have enough sap in it to pump out ?
My Shurflow acts funny when the trees are just not giving it up, poor sap day like.
Hope that helps.
Terry
2011- 35 taps, ,,,milk juggs and buckets-propane turkey fryer !!
2012 -75 taps,,,,, 40 on tubing into buckets, and 35 buckets - 3nd hand Homemade arch, brutal.
2013- Just over 115 taps and still adding more.
2014, new SS flat pan and new arch built, guessing 75-100 taps this year.
2015, 50 taps on Shurflo pump.
2016- Getting a line across my creek this year.
2017-took a year off, Jamaica was fun !
2018- 45 trees tapped, keeping it fun !