I've had plenty of four wheel drum cars, from a '70 'Cuda 440-6 with 4 wheel manual drums to a '70 Nova to my current '66 Le Mans with 4 wheel manual drums and a single chamber master cylinder. Each of these cars could/can stop fast enough to cause whiplash when the brakes are cool. Proper adjustment is key, and is evidenced by the high pedal described above.
I have dismantled a distribution block. It is a simple device. As designed, the piston hovers between two high pressure circuits, and when pressure in one side drops it is supposed to slide to the lower pressure side and set off the warning lamp by contacting the switch.
Our 'bird distribution blocks are iron with a brass piston and rubber o-rings. Like all rubber components, the potential for failure after close to 40 years is significant, and binding between dissimilar metals is a possibility. If the o-rings are bad, fluid or residue will be evident if you remove the switch from the side of the block. If any fluid is present in the switch chamber, at least one of the seals is leaking. If both leak, fluid can fill the chamber and at best you have effectively a single cylinder master cylinder as fluid passes from front to rear lines or vice-versa; at worst you have a leak at the switch, which is not designed to hold pressure or fluid, and you lose line pressure or let air in.
Another possibility is that the piston slides in the bore and sticks on residue (we do all flush our brake systems regularly, don't we? ) or that it travels to the end of the chamber and for lack of good pressure on the defective side, fails to return to center. This can be visually checked by looking into the switch hole, or mechanically checked by passing a straightened paper clip into one of the front line seats; it should pass straight through without hitting the piston. For the rear, which does not pass through the block, see if the paper clip goes more than half the width of the block. If it is stuck and will not return to center but is not leaking, remove the block and replace it or if you are lucky, a soak in an alcohol bath and careful depression of the piston through the seat on the end may be enough to put it back into place. Do this with the switch out, then visually confirm that the piston is centered. You can also use compressed air if the block is removed from the car. Rinse thoroughly in alcohol, push the piston back and forth a few times, rinse again, then shake it out and dry it. To center the piston with the block on the car, you will need to relieve the pressure on the good side by opening wheel cylinder bleeders.
Just my 5 cents worth.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
) or that it travels to the end of the chamber and for lack of good pressure on the defective side, fails to return to center. This can be visually checked by looking into the switch hole, or mechanically checked by passing a straightened paper clip into one of the front line seats; it should pass straight through without hitting the piston. For the rear, which does not pass through the block, see if the paper clip goes more than half the width of the block. If it is stuck and will not return to center but is not leaking, remove the block and replace it or if you are lucky, a soak in an alcohol bath and careful depression of the piston through the seat on the end may be enough to put it back into place. Do this with the switch out, then visually confirm that the piston is centered. You can also use compressed air if the block is removed from the car. Rinse thoroughly in alcohol, push the piston back and forth a few times, rinse again, then shake it out and dry it. To center the piston with the block on the car, you will need to relieve the pressure on the good side by opening wheel cylinder bleeders.
