<VV> RE: Bench bleeding

[Ron F Hinz]

Good, logical reasoning there!
RonH

----- Original Message ----- 
From: "Dennis & Debbie Pleau" <ddpleau@earthlink.net>
To: <virtualvairs@corvair.org>
Sent: Wednesday, November 24, 2004 4:48 PM
Subject: Re: <VV> RE: Bench bleeding


> My feelings about bench bleeding.
>
> It is required for dual master cylinders (although there are ways around
it
> that are probably more trouble than bench bleeding).
>
> When you put a dry dual master cylinder on a car, hook up the lines and
> start bleeding, this is what happens.  You fill if with fluid and have
> someone pump up the system and you open the bleeder on the passengers side
> rear, you let some air and get some fluid into the rear circuit.  and you
> repeat until the line and cylinder of the right rear circuit is bleed and
> full of fluid, and then you do the left rear until it has no air.  Now you
> move to the passengers side front and try to bleed it.  Since the rear
> circuit is completely full of fluid the pedal only goes part way down and
> you get some air and fluid out of the right front.  You do this a few
times
> and all you get is fluid, so you move to the left front and you bleed all
> the air you can out of it.  Now when you open any bleeder you get nothing
> but fluid, but the pedal still feels like there is air in the system,
> BECAUSE THERE IS and no matter how much you bleed one wheel at a time it
> will never be bleed out.
>
> The air in my description is trapped in the front brake circuit bore of
the
> master cylinder (usually the back towards the firewall).  When you bleed
> the back brakes, the piston went full travel in its bore because you were
> compressing air in the front brake circuit and air compresses and the
front
> brake circuit did not stop the master cylinder travel while you were
> expelling the air in the rear circuits.  When you switched to the fronts
> after the backs were properly bleed, the hydraulics pressure of the back
> brakes stopped the piston movements in both font and rear circuits before
> they bottomed out.  Consequently some air is still in the front circuit
> bore of the master cylinder.  Since you will never be able to bottom the
> piston in the cylinder of the front circuit bore, you will never get all
> the air out resulting in a spongy pedal.
>
> When you bench bleed, you filling both the front and rear circuits with
> brake fluid and getting all the air out both circuits, this way when you
> have bleed you have no air trapped in the master you can't get
> out.  Remember the proper way to bleed a master cylinder is to put it in a
> vice and bend tubes from the outlets until they are in the reservoirs
under
> the fluid level.  This way when the air is expelled it rises out of the
> fluid.  When the cylinder starts its return stroke and sucks back its
> sucking back nothing but fluid.  On a bench you can get all the air out of
> the master really quickly.
>
> One way to bleed a dry cylinder on a car is after both reservoirs are
> filled, have your assistant pump the brakes and hold them.  Open the
> bleeder on both the right rear and right front before they let off the
> pedal and then go back and close both bleeder before they let up on the
> pedal.   Now both pistons have gone full travel in there cylinders.  Do
> this three or four times and you have all the air out of the master
> cylinder and then you can bleed normally.
>
> Some day I'll make a few drawings and submit this as a tech tip to the
> Comminque.
>
> Dennis
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