Brake bleeding

[Barry]

Did my annual brake fluid change today. I usually vacuum bleed but it tends to draw air in around the bleed nipple threads and you never know if some of it gets entrained inside the caliper.  So I decided to try a new method I'd recently used on the car called gravity bleeding.

Suck out the majority of old fluid from the reservoir and refill with new.

Attach a short clear plastic tube to the bleed nipple

Open the bleed nipple half a turn and watch the fluid flow slowly but steadily through the tube into a container.

Keep an eye on the reservoir and top up when required. It took less than 10 mins for two reservoir fills to flush through the system.

When you you are happy close the bleed nipple and the jobs done.

Nothing could be simpler and provided you keep an eye on the reservoir there is zero chance of getting air in the system. Gravity bleeding is the same in principle as pressure bleeding except the pressure is lower being limited to the head of fluid which at 2 ft is approx.  0.5 PSI.  As I expected it worked better on the bike than the car because of the greater head of fluid and shorter hydraulic system. The rear brakes on the car took a long time while the fronts were not too bad.

[BPT]

Thanks for this write up, Barry. I need to do this but have been putting it off for fear of screwing it up. I don't have any kind of bleeding apparatus.

Two things: when you say "suck the majority" of the fluid out, are you doing anything more than opening up the bleeder and squeezing the lever?
Do you need to run a certain amount through to flush out the old, like until it runs clear? Or is it sufficient to just fill it back up once drained?

        Thanks

[Barry]

Two things: when you say "suck the majority" of the fluid out, are you doing anything more than opening up the bleeder and squeezing the lever?
Do you need to run a certain amount through to flush out the old, like until it runs clear? Or is it sufficient to just fill it back up once drained?

       Thanks
                   

I use a syringe to empty most of the old fluid from the master cylinder reservoir then top it up with new fluid. That's pretty much standard practice whatever bleed method is used as it means you start off the flush with mostly new fluid.

Old fluid tends to darken so it's easy to decide when enough fluid has been flushed through as you just stop when the fluid coming out of the bleed nipple looks clean. At least 2 or 3 fills of the reservoir are likely to be needed as a minimum but brake fluid is cheap so there is no harm in using more volume.

To clarify: with the gravity bleed method there is no need to touch the brake lever during the whole process, just open the bleed nipple and allow the old fluid to drain while keeping the reservoir topped up with new fluid.

I'm puzzled that I've spent 45 years bleeding brakes without giving this simple method a try.  Perhaps it just didn't occur to me that the brake fluid would drain out without any assistance.

[Justin B.]

Another trick I learned somewhere is that once you think you have it bled and can't get a good hard "pedal" if you squeeze in the brake lever as far as you can and secure it to the grip with a zip-tie before you call it a day the next morning you'll have nice hard lever.

[BPT]

Ok, now it's more clear to me (your additions plus my espresso!).  I wasn't paying attention when you wrote to fill with new fluid THEN open it up to drain. I had it in my head to drain andclear it first.  Now it makes sense and I assume that the draining of the old helps syphon through the new.

Makes me think I can handle this more easily now. Last time I did anything with brakes was about 15 years ago on a VW Beetle. Kind of a pain and took two people.

So did we determine from one of the other threads: DOT 4? If not changing any components, just the fluid.......

[mrclubike]

This method also lower the risk of causing the master cyl to start leaking after bleeding
What happens is the extra piston travel during bleeding causes the piston seal to run across corrosion that it normally does not cross over

To empty the MC without a syringe you can just gravity bleed it down until the master cyl is almost out and the soak the rest out with a clean rag and then top of and then bleed again   

[Barry]

This method also lower the risk of causing the master cyl to start leaking after bleeding
What happens is the extra piston travel during bleeding causes the piston seal to run across corrosion that it normally does not cross over

I had that in mind when deciding to do it this way as the master cylinder is the original 38 year old one. I've been vacuum bleeding for years for the same reason that it doesn't require the brake lever to be stroked.


BPT,

Yes Dot 4 is fine.

[BPT]

Thanks, gentlemen.