Dr Tim is right, I used to maintain a down slope but likely not a constant degree of slope back when I ran my first lines 3/16 on a hill. That hill had a 60' fall in elevation from the top tap to the mainline it terminated at. Then one day I was checking lines for leaks. I had a large limb (about 6-7" diameter) fall on one of the laterals near the upper part of the lateral in a moderate wash. It was fascinating to watch the sap/air/sap go down, under the limb (which had not pinched the lateral) and back up to the next tree and continue down to the mainline (which was still about 40' lower elevation).
While not scientific by any means, it looked to be moving at the same speed as the 2 adjacent laterals.
Yes, I'm sure it slowed some, but without a stop watch I could not detect it. The adjacent lines both had about the same number of taps both above and below that area and those laterals were 3 of about 7 or 8 that emptied into a 3/4" mainline I had run in that direction off a 1" main just to catch those laterals. As such, I think if you have enough drop in elevation after a spot that may not have a good slope and if you have laterals that are not over powered you will be fine.
What I mean by not over powered, is that the number of taps above the flatter section is not too many, and below that area you have a good number of taps you will be OK. On my laterals I try to have 25-30 taps, BUT I also have a hybrid system. My mainlines had 19" vacuum on them by a mechanical pump and a vacuum tank. That might have also skewed my results significantly.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.