Good to see this one didn't turn into a massive shit fight.
I'll add my bit for some poor sod googling/searching his way here and will try to wrap it all up in one post.
To avoid confusion, easiest way to describe a brake system that is "open" is one that returns the system pressure to ambient when the lever is in the out/off position. Air does not contact the dot fluid. There is a rubber membrane separating fluid and air. Go find a Guide service manual if you really want to appreciate.
The older avids and newer SRAM brakes all feature open system. Sadly the bladder system in the avids was pathetically small and didn't offer enough expansion room, thermal expansion and/or moisture absorption would exceed the bladders expansion capabilities resulting in pads grabbing rotors. Later bleed instructions had you pull the syringe at the lever out hard to pull the bladder in as much as possible, maximising expansion volume for future needs.
The Guides feature some innovative design balls up. Lever pistons ( aka master pistons) that would foul in the bore. It's a combination of factors in my view. Nylon pistons not machined to the right size. Nylon that swells in DOT fluid. Contamination getting into the exposed bore as there are no seals to the outside to stop dirt getting in, that scratches the bore, allowing dirt and moisture in.
A sticky lever piston, if you are familiar with the feel of the brakes, will result in a different lever feel, as there will be less resistance on the compression stroke of the lever.
I've seen some very dirty, 2-3 year old guides, I've never seen one with the bladder fully collapsed from fluid volume growth due to moisture absorption. I can think of one scenario not covered above. If the pads are worn out, someone does a bleed which fills the system and then new pads are installed at a later date and calliper pistons pushed in (to make way for thicker pads), pushing bladder at levers out, then thermal expansion will cause pads to lock onto rotor.
I own 9 sets of Guides. Vacuum bleeding is the answer. You push new fluid into the system in alternating directions, then pull a vacuum to pull the air out of the system. Surprisingly the cone shaped seal in the lever is fine with 3bar pressure and -0.98bar vacuum while bleeding.
Completely irrelevant to the above. Newest generation Guides with the Bleeding Edge callipers. Seems fluid turbulence and fluid sheering are concepts not understood by SRAM engineers. The bleeding edge banjo fittings opening is sub 1mm in diameter. No wonder the lever return is crappy.
I too love Guide's feel and modulation, and put up with the DOT fluid shenanigans.
