I have 72lbs of pressure. As long as you currently have flow, I can't imagine it being a problem as the water is just going to go right through - there's no blockages to build pressure.
Flush it first with the garden hose to take care of 99% of the flushing, then flush the "bad" water out with air to get rid of most of it. I seem to remember just blowing it out by mouth. Then pour AF through - just keep one end higher than the other and gravity can do the work for you (use mouth/air tank pressure to help if you want).
1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor