1.5 should be OK. Realize that the smaller the vacuum line the more vac you lose to line loss (friction). This is much more critical with vacuum than with air pressure. The differential between the vac level at the pump and atmospheric pressure is not that great, thus you lose faster. The line leaving the pump, is the 1" connected to a 1" port or is there a bushing from a larger size? If the latter, remove the bushing and install the full size, then match that size or go 1 size bigger out.
Dave Klish, I recently bought 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.