[NoVa-Corvairs] Fwd: <VV> RE: Bench bleeding

[Bryan Blackwell]

Since we were just talking about bench bleeding, here's a great post on 
the subject from VV:

Begin forwarded message:

> From: Dennis & Debbie Pleau <ddpleau@earthlink.net>
> Date: November 24, 2004 7:48:46 PM EST
> To: "virtualvairs@corvair.org" <virtualvairs@corvair.org>
> 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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