Bleedin' Confused (Shimano Hydraulic Brakes)

AxleAddict

Where possible, I like to carry out my own bike repairs and the time has finally come to attempt to bleed my hydraulic disc brakes and replace the hydraulic fluid. As you would imagine, there are plenty of YouTube videos out there to illustrate the process, although I did notice that some procedures had more steps than others.

For example, here's a GCN video which explains the process whereby you inject the new hydraulic fluid into the system via the brake calliper (with the old fluid being ejected into a receptacle attached to the bleed port on the shifters) - after that its pretty much job done.

https://www.youtube.com/watch?v=dXpLvtQ9aDA

Then you have the Park Tools video (which are typically very thorough) which also describes a similar process initially - but after the old fluid has been replaced by injecting new fluid into the system via the calliper, they then describe an additional step whereby more clean fluid is allowed to flow through the system from shifter to calliper (via gravity), with excess fluid being captured in a waste disposal bag attached to the calliper. The Shimano manual for my brake callipers also describes this additional process - but no detailed reason as to why its required. Seems a bit wasteful of hydraulic fluid to inject it into the system only to let some of it drain out again.

https://www.youtube.com/watch?v=mAEeAKmCLFU

Just curious if it is necessary to bleed the system in both directions?

Mr. Cats

I found this video helpful

https://m.youtube.com/watch?v=JXQKx4S_eoI

My understanding is that the critical goal is to ensure no air bubbles and by sending the fluid back through the system via gravity, you are reducing the possibility of bubbles/trapped air introduced in the injection process. In reality you can reuse the fluid so the amount lost is minimal.