Is degassing DOT brake fluid before a bleed still a thing?

Question

I recently bled old AVID Juicy 3 hydraulic brakes and followed the procedure described in this AVID Bleed Procedure video (starts at the relevant section). While preparing the syringes, the technician emphasized that degassing the fluid would lead to the optimal brake bleed. This made perfect sense to me, but I was still curious to watch a more recent bleed procedure video from SRAM and also compare to what Park Tool had to say. To my surprise, there were no more mention of degassing the fluid during the syringes preparation.

Is degassing an outdated step that we should not spend time doing anymore?

Answer 1

I don’t think the principle of degassing is necessarily outdated—it’s been a while since physics last received a software update ;) Most likely, it’s due to improvements in hydraulic brake technology. Specifically, it's a combination of how brakes are now more tolerant of air ingress, and that better power/modulation means it's less necessary to extract every last percentage point of performance. Art's answer addresses some of the technical details of these refinements. If one can get a very good bleed without degassing the fluid, the manufacturer and Park Tool may choose to omit the step from their instructions, which is especially helpful for a novice brake technician.

For what it’s worth, I don’t try to pull a vacuum on the fluid or anything when bleeding my brakes, but I will let it sit there for a few minutes to let the bubbles (caused by agitation while transporting the fluid) float to the top, and then I draw fluid from the bottom.

Answer 2

TLDR: No, degassing is still relevant and important.

Modern brake lever designs are very good at tolerating small amounts of air in the system. It will naturally travel to the highest point in the system (which is the master cylinder if the bike is kept upright) and will get cut off by the piston action and therefore will not get compressed. This is true so long as amount of air is small and bike is not turned upside down.

While degassing is a good practice when performing a brake bleed, I would argue that it is equally important (if not more) to not get loads of air into your brake fluid in the first place.

When bleeding my brakes I always take care to draw [miniral oil in my case] slowly to reduce amount of air bubbles and then leave syringe in vertical position for a few minutes to let the bubbles congregate in one end of the syringe.