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M_Motorsports

72' Distribution Valve

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Having problems with a brake upgrade with my '72 race car.  Have newly installed Arizona Z front brakes, 15/16 MC from AZ but still running with the oem drums.  Have pulled out the proportioning valve from the rear area of the car due to prior owner installing an after market PV in the interior cabin.  Adjusted the pushrod from a new reman. booster to what I believe a correct length. On the initial pump the pedal goes to about 2 inches from the firewall and soft.  Pedal needs to be pumped two times to raise up the pedal position and make firm.  Have bled the brakes at least two times.  Same results.  Stumped.  Can the distribution valve located under the MC be malfunctioning?  Should I just remove it and plump the brake lines with a straight and a T unions?  Would like a solid pedal with just one push. 

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The device under the MC is called a "differential pressure switch".  In the event of front or rear pressure loss,  it shuts off fluid going to the circuit that has the lowest pressure i.e, a leak.

 

It also serves as the distribution block for the front and rear brake lines.

 

When a leak occurs a shuttle inside the valve moves to the lower pressure side of the switch to stop fluid flow and the shuttle touches  an electrical contact that turns a red light light on your dash.  That is it. Leave it alone. See pictures.

 

The calipers you installed require more fluid hence the longer pedal.

 

If you have to pump the brakes then you still have air in the system.

 

Recheck all of the mechanical linkage adjustments including the MC push rod and re-bleed the brakes.

 

 

 

 

 

 

brake differential switch.png

Brake switch.jpg

BRAKE SWITCH OPENED UP.jpg

Master Cylinder Push Rod Gauge.jpg

Push Rod Adjustment.jpg

Edited by Miles
added picture of MC push rod adjustment

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Miles,

 

Thanks for the details.  It's pretty complete about the operation of the switch.  I will assume the switch is a no fail operation therefore will rule out doing any by pass of the switch and look else where for a solution to the soft brakes.  I will look at additional bleeding of the brakes first before anything else. 

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On 12/15/2017 at 10:08 AM, M_Motorsports said:

 On the initial pump the pedal goes to about 2 inches from the firewall and soft.  Pedal needs to be pumped two times to raise up the pedal position and make firm.

 

You have air in the system.  Sounds like the calipers based on how far the pedal travels.  The best way to understand where the bubbles are is to examine the caliper castings closely and figure out where the hole from the bleed screw enters the piston bore.  Often the hole is drilled at an angle and if you put the bleed screw at the top the entrance to the hole will be below the high spot.  You can bleed them a hundred times but the bubble will never exit the caliper bore if it's above the bleed port.  Position the calipers so that the entrance to the bleed hole is at the highest spot.  Sometimes you have to unbolt the caliper and angle them correctly before bleeding. 

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20 hours ago, M_Motorsports said:

Can a reverse bleeding tool work where you force brake fluid from the calipers back to the MC?  

 

 Can this eliminate the air bubbles that are somehow trapped in the calipers back thru the system?   Yes, as long as the bleeders are pointing up so air is not trapped.  It will not help if there is still air trapped in the MC.

 

Have one available from a friend who uses one when he bleeds his brakes alone.

 

1. Set all of the mechanical brake adjustments, including the push rod,  correctly. See push rod length adjustment above. You can do fine adjustment to the rod after the car is safe to put on the road without removing the MC.

 

2. Bench bleed the MC. Take your time and get all of the fine bubbles out.  Make sure none of the fittings or plastic lines are sucking air. Transfer the MC to the car with the outlets  plugged. Attach MC to brake lines loosely and then to the booster. Tighten the brake line fittings after the MC is attached to the booster.

 

3. Bleed brakes with a helper or use speed bleeders (check-valve style) or use a Motiv pump bleeder (what I use).  The pump bleeder works well, but can be messy.

 

On some calipers you have to remove the caliper, block the pistons, and shake/rotate  the caliper while bleeding to get the air out. Be sure to block the pistons. If you don't block the pistons you will be very sad.

Edited by Miles
spelling

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Spent the day reverse bleeding the MC and 4 corners with success.  I think the greatest improvement came working with the front Arizona Z Wilwood calipers.  I removed the calipers from the frame and turned the calipers on their side.. These Wilwood calipers have 4 bleeders per caliper (?) and the ss brake line enters thru the side when mounted.  To get all the air out you really need to turn them on their side and rotate them so the entrance of the ss line is on top.  I used a reverse bleeding gun so forced any air back thru the top, thru the system and out the reservoirs.  I guess if you use a more traditional method of forcing the fluid out thru the calipers you would need to position the bleeders at the top.  Now have a much more solid feel to the peddle. 

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