Like a lot of you, I’m sure, I’ve bled plenty of brakes systems on cars and motorcycles. Seems like there are two ways to do it — by force or vacuum — but the common denominator is that bleeding goes one way only. Whether the fluid is pushed or pulled out, once it leaves the system it stays out.

I’ve noticed that Shimano brakes advertise one way bleeding. That says to me that Shimano engineers are paying attention to the way that brakes are engineered on cars and motorcycles. IMO it’s ridiculous that the bleed valve on the caliper should allow expelled fluid to flow back into the system, but it seems like some other brands operate this way. I recently had to bleed a Promax brake and it was kind of a pain to use two syringes just to make sure the expelled fluid stayed out. It would be a lot easier just to pump clean fluid through the system until the old fluid and air were forced out.

Anyway, to get back to the OP’s original question, it sounds as though the SRAM brakes don’t have a one way bleed valve. That necessitates the extra syringe to maintain vacuum in the system and keep expelled old fluid (and air) from re-entering the system.