Brake fluid change / brake bleeding

[RubEric at aol.com]

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)