brake bleed vacuum pump sucking air?

[parkinator]

Hi.

After completing a front brake tear down on Suzuki bandit 650 ABS due to binding pistons, i have bled the system the traditional way but still had spongy brakes. After reading that bubbles can get trapped in the abs system i purchased a vacuum bleeder. Now the strange part after bleeding the closest brake to the abs system, i got a few microscopic bubbles but then doing the other side furthest from the master cylinder. I had a massive stream of bubbles even after bleeding solid for 30 minutes with very little fluid coming out.

to clarify i cannot see any leaking fluid, the brakes do work but are extremely spongy and fluid is flushed through the system. I have re used the same seals after cleaning out the grooves, and wire wheeled the pistons as there was a little light corrosion.

Is it possible that the vacuum pump is sucking air through the seals, as they will be slightly looser than brand new seals.

I would really like some peoples opinion on this, as i love learning skills and hate giving people money for something i should be able to do.

Cheers.

[Mississippi Bullfrog]

I've never used a vacuum bleeding system on a bike but I use a pressure system on cars and the trick there is to use very low pressure. Maybe your vacuum pump is putting too much force into the system?

[parkinator]

Hi.

Thanks i never did more than 0.5 negative bar with a hand vacuum but could be.

[TimR]

Some abs systems can trap air in its actuators and can sometimes be purged by manual means or some systems need dealer reset diagnostics

[Mr Fro]

I use both hand and electric bleeding systems.

Those bubble could well be coming from the bleed nipple where the air is coming through the threads. This usually happens when the fluid flow is low or there is a restriction in the system.

Have you had a look through the system for any additional nipples? Quite a few bikes have additional bleed points - usually at the lever.

Some systems are just buggers to do and like to be bled backwards i.e. pushing fluid up through the caliper.

[parkinator]

the bleed nipple thread is sealed with PTFE tape, so bubbles should not come out of there and the only bleed nipples are on the calipers.

[JRH]

How bad was the corrosion on the Pistons? Has the wire brushing put groves on them?

We're the seals suitable to be re-used? It may be time for new seals as a minimum. Just a thought.

[Stu]

sounds to me like air is getting dragged in from somewhere be it either the nipples or the seals

also as mentioned the master cylinders can be a twat for trapping air

[Guest]

Mine does the same if you use lots of pressure, I believe it's due to it drawing air through the threads of the bleed nipple. The manual actually warns of that (Mityvac). Doesn't affect the bleeding.