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I have to change the Control Switch for the Differential Pressure unit on my 1976 BMW 2002. I've got the switch and have been holding off putting it in because I'm not sure what kind of problem I'm going to have with brake fluid leaking out when I'm changing out the switch. I just cleaned and painted my engine compartment so I don't want to get brake fluid all over the place. Can someone give me an idea of what kind of trouble I'm going to run into with brake fluid leaking out during the change?

 

Thanks...........

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US spec '76 cars (only) have a pressure sensor installed next to the master cylinder.  It senses if there's a differential in hydraulic pressure between the two circuits--which would indicate a leak or possible brake failure in one circuit.  When it senses this, it lights the brake warning light on the dash.  This is the same light that warns about low brake fluid on all 68 and later cars, and also if the e-brake is engaged on squarelights.  

 

Willie02, if your light is lit from the sensor, have you tried resetting it?  There's a reset button under the rubber cover on the housing; that may cure your problem.  But if you have to change the whole assembly, yes, it will drip brake fluid when you unscrew the rigid brake lines.  So put plenty of rags underneath to catch the drips, and install the new one as quickly as possible.  I've always had better luck when installing an object with several rigid lines going into it (like the master cylinder) by starting all the lines first, then bolt the item in place.  

 

Also, you can slow down the fluid drip from the unscrewed lines by removing the brake fluid reservoir cap and stretching a piece of Saran Wrap over the opening.  

 

cheers

mike

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Say what??? I'm changing the brake control switch on my 1976 BMW 2002 which from what I understand not all 2002's have. The Differential Pressure Switch is located on the drivers side fender and is part number 34 32 1 115 861. Within this housing is the control switch that is part number 61 31 1 360 940. When the control switch goes bad it makes the red brake warning light comes on - on the dash indicating it needs to be investigated for going out. Mine is bad and I need to replace it. Sorry not sure how else to explain it.

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...and flush the affected area with water as soon as possible after you tighten things up.

 

Fresh paint's particularly soft, and brake fluid loves that.  I think I'd try to tent the area below

the switch to catch the fluid.

 

 

t

 

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Thanks this is exactly what I needed to know before digging into it. I've tried the reset but no deal. I will come up with a way to keep the brake fluid contained and just for a safety precaution when I'm finished I'll flush with plenty of water.

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Can't the pressure unit with sensor just be removed and the lines rerouted directly to the master cylinder?  With the hand bendable brake lines (NAPA, etc....) this is almost easier, and certainly cheaper, than maintaining a redundant component with multiple couplings.

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It can be removed- and lines from an earlier car can be fitted.

 

It's not exactly redundant, however, as multiple posts of 'weak brakes- is this normal?' can confirm.

There's no way to diagnose a half- failure of the master if it doesn't leak, other than to realize your brakes

aren't what they ought to be.  And that's what this little gizmo does.  Most dual- circuit brake systems

had them, starting in 1968.  I wonder if the level switch was a 'permissible substitute' until 1976...

 

t

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Inside this device are two opposed pistons, one pushed by the front brake circuit, and the second pushed by the rear brake circuit.  These pistons have seals.  In the center is the switch.  If there is differential pressure due to a brake failure, the pistons move to one side, and actuate the switch.  

 

When things are working correctly, there is no brake fluid in the center where the switch is.  If there is brake fluid there, the seals have failed.  A new switch won't fix the problem.

 

If the light is on, it also may just mean you have a problem in your brakes.  The switch is working correctly in that case.

 

I would remove the switch, after putting something below it to catch any fluid that may escape.  It won't be a lot, since there is no pressure on the system at rest, and it would only be the amount of fluid that leaked past the pistons.    Connect your switch back to the wires, and see if your light is off.  Actuate the plunger and verify the light goes on and off.   If it works as expected, the switch is good. 

 

For more details on how these things work, search the alfabb for Brake Differential Pressure.   If you have fluid in the center, you will need to disassemble the valve and replace the seals. 

 

Harold

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