Re: can't bleed brakes if AST???

["Ron Buchalski" <rbuchals@xxxxxxxxxxx>]

Don,

The reason the notice/warning is provided in the Bentley manual is that the 
dealer uses a computer to cycle the ABS pump while bleeding the brakes, 
insuring that the entire system is purged.  There are many people who bleed 
the brakes themselves, without cycling the ABS pump, and haven't had a 
problem.

I've heard people say that they bleed the brakes, drive the car and engage 
the ABS several times, then come back and bleed again.  I'm not sure that 
this clears fluid out of the pump, since the brake system is closed (i.e., 
the fluid doesn't circulate from the master cylinder, through the ABS pump, 
down to the calipers, and back to the master cylinder), so I don't 
understand how this alternate method would flush fluid out of the ABS pump.  
The ABS pump merely modulates the pressure put onto the hydraulic system, to 
prevent wheel lockup.  Why would it pump fluid out, into a closed tube 
(brake line to caliper)?  I don't think it does.

When the dealer performs this automated bleeding operation, the ABS pump is 
engaged while the bleeder screws are opened, so the fluid in the ABS pump 
has a place to go...out of the system!


Ron Buchalski
BMWCCA #76387
1995 E36 M3
1993 E34 525iT
1999 Mazda Miata

>
>Date: Tue, 18 Mar 2003 19:37:54 -0600
>From: "Bredle, Donald L." <bredledl@domain.elided>
>Subject: can't bleed brakes if AST???
>
>I'm reading the Bentley service manual before bleeding the brakes on my '96
>328i.  It says in a warning box that on cars with All Seasons Traction 
>(AST,
>ASC, or ASC+T)  special BMW service equipment is required to properly bleed
>the ABS/AST system. It goes on to say ...'for safety reasons.'
>
>Does this mean I can't or shouldn't bleed the rest of the system except for
>the ABS? Or that I'm wasting my time if I do because there will be old 
>fluid
>stuck in the ABS reservoir?  What is the 'safety' issue they are referring
>to?
>
>Thanks,
>Don Bredle


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