Brake fluid change / brake bleeding

[Kneale Brownson]

Eezibleed system (using maybe 10 lbs pressure from a tank or spare 
tire):http://www.rdent.com/pages/tools.html


At 12:01 PM 11/22/2000 -0500, RubEric at aol.com wrote:

>You wrote:
>
><<Bleeding ABS cars is no different to bleeding other cars.  Make sure the
>master cylinder is open (yard broom against pedal and head rest) and
>pressure bleed - the MityVac is difficult to use on Audi's brakes.>>
>
>I am confused by your recommendation to place a broom against the pedal,
>which I assume is to depress the pedal.
>
>My understanding is that when the brake pedal is in the released position,
>the pistons in the master cylinder are retracted and the ports in the bottom
>of the master cylinder [the compensating ports] are uncovered.   As a result,
>there is continuity of the fluid between the reservoir and the entire brake
>system via the pressure side of the master cylinder.  When the brake pedal is
>depressed, the pistons move forward and the piston and/or piston cups move
>past the compensating ports, thereby closing them. Brake fluid is trapped in
>the forward portion of the master cylinder and the fluid is forced to the
>wheel cylinders or calipers via the brake lines to apply the brakes.
>
>When pressure bleeding, pressure is applied to the fluid in the reservoir by
>any of several means. This pressurized fluid moves from the reservoir through
>the ports into the pressure side of the master cylinder, through the brake
>lines to the wheel cylinders or calipers, then out through an opened caliper
>bleed screw carrying (hopefully) any air entrapped in the lines, etc. If the
>pedal is held in the applied position with the aid of a broom, it seems to me
>that the ports would be closed rather than open and brake fluid in the
>reservoir pressurized by the pressure bleeder could reach the brake lines, or
>have I misunderstood?
>
>I don't have a pressure bleeder as such.  I use a reservoir cap to which I
>have added a schrader valve and blocked the  vent hole and I apply air
>pressure with a bicycle tire pump. I don't know how much pressure one would
>dare apply before blowing the reservoir off the master cylinder so I apply no
>more than about 10 psi.  Since the reservoir is small I have to pump the air
>frequently to maintain pressure, and check the brake fluid level often.
>
>Incidentally, I have seen a pressure bleeder on the www that is quite unique
>in that is bleeds from the BOTTOM up instead of the top down.  It uses a hand
>operated pump resembles a Mityvac that applies pressurized brake fluid to the
>CALIPER instead of the reservoir. Fluid (and air) moves upward through the
>system back to the master cylinder. Interesting.
>
>
>Eric
>(84 Olympic, 95 S6, 33 CQ, XK120, MkIV, 93 RT10)
>