4

I have been trying to understand about the bleeding process. While there are many videos and explanations online how to bleed, my question is simple:

If the oil from the pump can travel to the caliper & vice versa, why do we need to bleed both when doing bleeding? For example, why can't I just bleed a larger amount of oil through the pump? Why do we bleed both at the same time if both essentially access the same "resource"?

3 Answers 3

6

You bleed a hydraulic system to get rid of air bubbles, water and other contaminants that may have found their way into your hydraulic fluid. Since the entire system is connected (caliper - hose - lever - expansion tank) contaminants may be floating anywhere in that system. This is why you need to flush everything to remove old contaminated hydraulic fluid and replace it with the clean one.

3
  • 1
    I see. So I guess it is related to the air-bubbles - as it wouldn't be possible to flush them out when using only one syringe at one spot (lever /caliper), because when we push the air bubbles out, they need to leave the system from the other end. And if it is sealed, it is not possible. Did I understand correctly?
    – yuvalon
    Feb 18 at 22:50
  • 1
    Sounds like you did. The bleeding process is exactly that. You introduce new hydraulic fluid from one end of the system and allow old fluid with contaminants escape from the other end. Feb 19 at 0:47
  • 1
    Thank you very much for the help @art
    – yuvalon
    Feb 19 at 7:27
2

A common problem with bleeding is that while one pushes the liquid from one point at some limited rate, the air bubbles travel somewhat with the flow and somewhat wherever they feel like, e.g. upwards in the system, allowing them to escape your efforts.

If one has a system where the pump is at the lowest point and the tubing goes only upwards, a single bleeding point at the highest point would work. Bicycle brakes, unfortunately, work the other way round.

It is usually even worse in cars where one has 4 wheels, usually 2 braking circuits and an overall of 5 to 10 bleeding points (all of them rusty).

2
  • 1
    "If one has a system where the pump is at the lowest point and the tubing goes only upwards" - would it not be the case if I installed the bike on a mount and turn them so the pump is at the bottom, and the caliper on top? And then we could do a single bleeding point at the highest point?
    – yuvalon
    Feb 19 at 16:52
  • 1
    @yuvalon could pretty much work, depending on how exactly the pump is fed. And there is another radical approach to bleeding, also popular in motor vehicle maintenance - just vacuum the system before introducing the fluid.
    – fraxinus
    Feb 20 at 8:09
2

On a practical note, since other answers have established that you're really trying to remove air bubbles, which have a tendency to 'pool' in inconvenient places on their upward journey, I've had good success with removing the caliper from its mount, chocking it open with a piece of wood or a tool handle, and then bleeding it off the wheel, whilst rotating it strategically to allow air bubbles to float upwards towards the bleed nipple (or towards the reservoir on the handlebars/foot brake on my motorcycle)

2
  • By choking it open you mean keep the pistons all the way out and put a tool to make sure they don't fall off? Then you mentioned rotating the caliper to allow air bubbles to float upwards - but even if we do that, how will the air bubbles will go away after their "travel" upwards? (there is no way for them to escape the system as it is sealed from the other side?)
    – yuvalon
    Feb 19 at 16:56
  • 1
    @yuvalon It doesn't really matter the extent that pistons are extended. if you can chock them open wide enough that you then can slide them back over the disk, that is convenient ;) the chock is to prevent them from popping out entirely. I typically perform at least the initial bleed with the reservoir cap open, and after that, the reservoir diaphragm has a small hole allowing for air to escape upwards. that can become blocked, which presents as an inability to release the brake after it's applied. Feb 19 at 17:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.