Is there any benefit in bleeding brakes, other than removing air from the lines?

[ichitan]

We bleed hydraulic brakes to remove air from the lines.

With car and truck brakes, we only bleed them when they have air in the lines.

Is there any benefit in bleeding bicycle brakes, other than removing air from the lines?

[Sybian]

Bleeding of brake lines is for more than removing air/gas from the system. Bleeding is also used for purging the fluid, either in part or in whole (most common), in order to remove any contaminants (debris, moisture) in the the fluid/lines/cylinders, etc. and at the same time avoiding introduction off air into the system while exhanging out fluid. If the fluid has any darkening/discoloration it should be promptly exchanged (complete purge, flush, bleed) since it is visually contanimated, but has already been contanimated before observation. Moisture in the fluid will cause discoloration. Moisture, wearing seals, etc. cause deterioration of a fluid's lubrication ability and alter its boiling point.
Everything stated applies to all hydraulic systems whether on a bike, a car, or heavy equipment.

[ichitan]

(09-09-2023, 07:01 PM)Sybian Wrote:  Bleeding of brake lines is for more than removing air/gas from the system. Bleeding is also used for purging the fluid, either in part or in whole (most common), in order to remove any contaminants (debris, moisture) in the the fluid/lines/cylinders, etc. and at the same time avoiding introduction off air into the system while exhanging out fluid. If the fluid has any darkening/discoloration it should be promptly exchanged (complete purge, flush, bleed) since it is visually contanimated, but has already been contanimated before observation. Moisture in the fluid will cause discoloration. Moisture, wearing seals, etc. cause deterioration of a fluid's lubrication ability and alter its boiling point.
Everything stated applies to all hydraulic systems whether on a bike, a car, or heavy equipment.

I am not saying you are wrong.

From what I have seen with cars and trucks, work is done on the brakes when something fails. The part is replaced, and the brakes are bled.

Do you know of mechanics who replace the brake fluid in cars and/or trucks, when nothing has failed, and there is not air in the system?

[enkei]

(09-10-2023, 02:53 AM)ichitan Wrote:  

(09-09-2023, 07:01 PM)Sybian Wrote:  Bleeding of brake lines is for more than removing air/gas from the system. Bleeding is also used for purging the fluid, either in part or in whole (most common), in order to remove any contaminants (debris, moisture) in the the fluid/lines/cylinders, etc. and at the same time avoiding introduction off air into the system while exhanging out fluid. If the fluid has any darkening/discoloration it should be promptly exchanged (complete purge, flush, bleed) since it is visually contanimated, but has already been contanimated before observation. Moisture in the fluid will cause discoloration. Moisture, wearing seals, etc. cause deterioration of a fluid's lubrication ability and alter its boiling point.
Everything stated applies to all hydraulic systems whether on a bike, a car, or heavy equipment.

I am not saying you are wrong.

From what I have seen with cars and trucks, work is done on the brakes when something fails. The part is replaced, and the brakes are bled.

Do you know of mechanics who replace the brake fluid in cars and/or trucks, when nothing has failed, and there is not air in the system?

Normal automotive brake fluid is supposed to be renewed approximately every two years. I realise that most people don't do this, but most people are pretty ignorant, so I wouldn't use their practices as a useful guide.

I think it's easier to treat bleeding and flushing/replacing as different jobs/goals, because even though they often happen together, they're used for different purposes.

Yes, if your brakes are performing well, you don't normally need to bleed them. However, you still need to change the brake fluid regularly to maintain the health and performance of your brakes. This is primarily the case with DOT fluid, which is hygroscopic (absorbs moisture) but even mineral-oil bike brake fluids get contaminated, so we might as well treat them the same.