Date: Sun, 14 Dec 1997 09:39:27 -0500
Reply-To: Mark Hudgik <mhudgik@THE-SPA.COM>
Sender: Vanagon Mailing List <vanagon@gerry.sdsc.edu>
From: Mark Hudgik <mhudgik@THE-SPA.COM>
Organization: remodeling@homewrecker.com
Subject: Re: Air in Brake Circuit
Content-Type: text/plain; charset=us-ascii
Mike,
It's not clear whether you're closing the bleeder screw before lifting your foot
off the brake pedal, but it sounds like you are sucking air back through the
bleeder screw when lifting your foot off the brake pedal. I use a manual hand
pump attached to the bleeder, it has a clear cup on it to collect the fluid and
allows me to see when all the air is out of the line, (your can pick up a hand
pump at your FLAPS for less than $30). The pump setup allows you to bleed your
system without requiring a second person. I prefer it even when a second body is
available as it is faster and cleaner. Below is some generic information on
bleeding brakes that may be of interest....or not ;)
-------------------------------------
Bleeding is a service procedure that involves purging the master cylinder, brake
lines, calipers and wheel cylinders.
If there are air bubbles in the fluid, they will compress when the brakes are
applied, causing either a low or soft pedal. Bleeding gets the air out, leaving
only non-compressible brake fluid.
Air can enter lines when the system is opened for repairs. Air can also enter
the lines if the master cylinder reservoir gets too low.
To remove air, bleeder screws on the calipers and/or wheel cylinders are opened
one at a time. Old fluid, along with any air, is then drained, pumped, pushed
or siphoned out while fresh fluid is added to the master cylinder reservoir.
Manual bleeding is usually a two person job. One person pumps the brake pedal
while another closes the bleeder screw after each stroke to prevent air from
being pulled back into the system. Gravity bleeding doesn't involve any
pumping. The bleeder screws are opened and the fluid is allowed to dribble out.
Gravity bleeding is slow (and is less likely to get the air out) and seldom used
except on certain import applications. Most professionals use power bleeding
because it is fast and does not require a helper. Compressed air is used to
force new brake fluid through the master cylinder to push out the old.
Another method sometimes used is vacuum bleeding. Special equipment is used to
siphon old fluid out through each bleeder screw.
To get all the air out, brakes must be bled in the proper sequence. Depending on
how the hydraulics are split (front/rear or diagonally), the usual sequence is
to bleed the wheels furthest from the master cylinder, then the closet wheels.
On most rear-wheel drive vehicles, the recommended sequence is RR, LR, RF, LF.
On front-wheel drive cars with diagonally spit brake systems, the sequence is
RR, LF, LR, RF.
If the master cylinder is being replaced, it may have to be bench bled before
being installed.
-------------------------------------
Hope this helps....
Mark - 91 Carat
Michael A. Radtke wrote:
> Hello,
>
> '84 GL.
>
> Yesterday, my brake pedal started going nearly to the floor. So, I checked
> the rear drums and the brakes were in good adjustment. The disks have no
> adjustment. From this, I concluded that I needed a new master cylinder.
> No further tests were done.
>
> Today, I installed a new master cylinder. However, I am not able to bleed
> the front brakes. The first sign of trouble was when I started getting a
> fine foam out the bleeder ... almost like whipped cream. Finally, that
> quit, but I still get air. No matter how long I bleed them, I still get
> air from both front wheels.
>
> I am lost. What should I do next?
>
> Help please,
> Mike
>
> Michael A. Radtke - Z74 Voice: 602-862-4897
> Bull FAX: 4853
> 13430 N. Black Canyon Hwy.
> Phoenix, AZ 85029 Email: m.radtke@bull.com