<VV> RE: Bench bleeding

[Dennis & Debbie Pleau]

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