Distribution Block and Switch

[bollybib]

This morning I had some fun with this doohicky and then I felt like writing so I thought I'd share, here goes....

For trucks with dual circuit master cylinders and front and rear drum brakes, a brake warning light is provided on the instrument panel. The light is activated by a switch that senses the differential pressure between the front and rear hydraulic systems. According to GM, "In the event of fluid loss in either the front or rear brake system, the indicator on the instrument panel will illuminate red when the brake pedal is depressed".

The switch is contained in a part described as "Brake Pipe Distribution and Switch Assembly" in the 1969 Service Manual. It is not a proportioning valve!

The switch can be tested on the vehicle as described in the 1969 Service Manual, below.



If the switch is not tested periodically, and/or the brake fluid becomes contaminated, the internal mechanism can stick. The test described above may then fail to activate the switch and turn on the warning light.

At this point, there are some options.

1. Obtain another switch (a new one runs around $90).
2. Attempt to free the switch mechanism.
3. Leave it alone.

I have a distribution block on my truck now that failed the test above. I was unable to test it until after I had replaced all cylinders, repaired lines and flushed and bled, so I chose Option 3 because I didn't want to mess with brake stuff any more.

However, I recently acquired a booster setup from a 69 C30 that came with a distribution block. I wanted to know if this one worked. I found that the following procedure worked well.

1. Flush the passages with clean brake fluid and blow them out with compressed air.

2. Clamp the block in a bench vise.

3. Connect a continuity checker between the switch stud and the body of the distribution block.

4. Find a clean 7/64" drill bit or equal.

5. Rig up a means to apply controlled pressure against the drill bit.

6. Insert the drill bit into one of the inlet ports of the distribution block. You will feel a circular flat surface at the end of the passage. This is one side of the differential pressure switch.



7. Using your rig, apply controlled pressure on the switch surface with the drill bit.



8. If it is a new switch, you should be able to feel the switch move without resistance against spring pressure.

9. If it is old and stuck, keep applying pressure against the surface. You should feel a little snap when it breaks free. If the drill bit bends, there may not be much hope. Use your own judgement here.

10. If the switch is good, the continuity checker will now show zip ohms and the switch is OK in that direction.

No Pressure Applied


With Pressure Applied


11. Stroke the switch a few times and then reverse the orientation of the distribution block in the vise. Follow steps 6 through 10 for the other port. This ensures that the switch works in both directions.

With any luck, it will work. Once installed, you will now be visually warned of brake system catastrophe.

I verified that this distribution block works. I was so happy, I had to share it. Have a nice day !

[lolife99]

Thanks for the great write-up!
I'm pretty sure no one has ever provided this info.

Now throw some disc brakes on that truck and make it stop! Ha.

[bollybib]

Thanks, I had fun with it. Seemingly simple items like this are usually overlooked in detailed writeups.

I think I'll be keeping my drums for quite a while longer though. The booster should help a lot.