Brake fluid change / brake bleeding

[isham-research.freeserve.co.uk at pop.pol.net.uk]

> 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.

[Theory snipped]

I haven't got involved in the theory - all I know is that this is how
Audi state it must be done.  I can't give you the service tool number
because the bulb in my microfiche reader has blown.  Audi's tool
actually fits onto the front of the driver's seat and has a mechanism a
little like a caulking gun's ratchet.  The pedal remains depressed
throughout the procedure.

> 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?

If it makes it clearer, Audi also specify that the master cylinder,
proportioning valve, and the lines in decreasing order of length should
be bled in that exact order.  I've been doing it for years and it always
works.  There are a lot of people around who _can't_ do it properly - I
know that by the number of people who turn up with appalling brakes
having been told by 'experts' that they can't make them any better.

> 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.

The Gunson device uses tyre pressure and an external reservoir bottle -
otherwise that's exactly how it works.

> 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.

But not safe on Audis.  Do it how Audi say.  Please.

This includes MityVacs.

--
 Phil Payne
 http://www.isham-research.freeserve.co.uk/quattro
 Phone +44 7785 302803   Fax: +44 7785 309674