<VV> RE: Bench bleeding one more time (for me anyway)

[srmarti]

The point I tried to make before is that if you want bleed the master
cylinder, you can just disconnect the lines at the master cylinder. Connect
the same sort of bleeder lines used for bench bleeding and bleed it.  When
done,  hook the rest of the system back up and bleed it.  Sure, if you got a
brand new, bone dry, cylinder do it on the bench first.  Personally, I
wouldn't take it back off the car just to bleed the master cylinder.

Steve

>
>
> > 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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