C3 brake distribution block/switch

[Jeffrey S.]

Hello All-
Has anyone had any experience disassembling/repairing the brass distribution block/switch on a '69 (power brakes if it matters). The warning light is always on on mine (the brakes are working perfectly so I know it is not a sign that there is unequal pressure). I know that there are vendors who can rebuild these but I always prefer to do it myself if possible. The switch portion looks like it has a hex fitting so it appears that this can be removed. Also, can anyone tell me exactly how this switch functions? Thanks as always for your help and expetise.
Jeff Salz

[Joe L.]

Re: C3 brake distribution block/switch

Jeff----

The brake warning light serves a dual function. It lets you know if the parking brake is applied and it also lets you know if there is pressure imbalance between the front and rear brake hydraulic systems, possibly indicating a failure of one or the other. The first thing that you need to do is to check to see if the parking brake warning function is the culprit here. A bad switch or serious mis-adjustment could be the problem. The switch is located under the parking brake console. It's GM-discontinued, but I think that it's reproduced if you need one.

The brake distribution block and pressure differential warning switch is a rather simple device. Within it is a piston which is normally "centered". Either end of the piston is exposed to the hydraulic pressure in each of the braking systems and equal pressure causes the piston to remain centered. If a pressure imbalance occurs, then the piston moves off-center, in the direction of the lower pressure. The center of the piston has a "groove" in it which locates the end of a toggle-like switch. If the piston moves in either direction, then the toggle switch deflects. This causes the brake warning light to illuminate.

You can remove the switch from the brake distribution block by turning out the hex fitting. However, if there exists a problem with the assembly, it usually does involve the switch component. I know of no way to repair this switch and I know of no source for replacements of the switch, itself. So, if the switch is bad, which is usually the problem, I don't think that you'll be able to repair or replace it. Commercial rebuilders may have identified a source for the switch, but I don't think that a retail source is available. However, you may be able to salvage one of these switches from some other non-Corvette application and use it to repair your block. GM NEVER offered these switches as a seperate part; they were available only as part of the block assembly.

Other than the switch, I don't think that there is usually any other problem with the block assembly. For PRODUCTION and early SERVICE blocks, I don't know of a way to remove the piston, clean the innerds, and replace any o-rings. There may be a way, but I think that it would involve drilling out or otherwise removing one of the tubing seats. Later SERVICE blocks have a brass hex fitting on one end so, presumably, these would be easier to disassemble, even though GM never intended for them to be disassembled and SERVICED.

[Jeffrey S.]

Re: C3 brake distribution block/switch

Joe-
Thank you so much for your detailed response. I think I will try to take out the switch from the block and test it for continuity open and closed. It might also be possible that the piston is stuck on one side of its travel and perhaps I could free it up manually. Anyway your help is much appreciated.
Jeff

[Clem Z.]

try just bleeding the front brakes and see what

happens and if that does not work try just bleeding the back brakes. this sometimes will help center the switch.

[John H.]

Re: try just bleeding the front brakes and see wha

The device that appears to be a switch on the top center of the block is actually just an electrically-insulated fitting with a solid rod on the end; the "spool" inside the block is held centered by springs, and it has a machined groove in its center where the rod protrudes from the fitting with the wire connector on it. When the spool is centered, the end of the rod isn't touching anything; when there's a pressure loss in one system sufficient to move the spool off its centered position, one side of the "groove" touches the rod on the fitting, grounding the warning light circuit, and the light comes on. With no pressure in the brake system, if you show continuity from the connector pin to the block, the "spool" is stuck.

[Jeffrey S.]

Re: try just bleeding the front brakes and see wha

Clem and John:
Thanks for your input. I will try to bleed the front brakes and the rear brakes as you suggest. I will also try to open one of the fittings (either rear or front) and apply brake pressure while this is open and try to re-center the "spool" as John refered to. Also, if the "spool" was stuck one way or the other, wouldn't that also cause the malfunction? The switch may be operating properly but the contact may be jammed to one side and not returning to the center on its own.
Again, thanks for your help.
Jeff

[Mike McKown]

Re: try just bleeding the front brakes and see wha

Jeff:

I think what you will find is the spool is stuck to close off either the front or rear brake system. If you use an ohmeter to check the switch, you will find it will be grounded if your brake light stays on. I don't believe your switch has any moving parts. Bleed the brakes like Clem suggested and use PLENTY of pedal pressure.

A defective master cylinder will also cause your brake light to stay on. If you think your brakes are working perfectly, take your car to an open space and lock the brakes up if you can. If you don't get all fours, the problem is probably in the end that didn't lock.

[Jeffrey S.]

Re: try just bleeding the front brakes and see wha

Mike:
Thanks again for the advise. I suspect you are right that the spool is stuck to one side. I seem to remember a few years ago I had problems with the rear brakes and had lost them due to air pumpimg (this has been corrected) and perhaps that caused the spool to move and get stuck in that direction. Your suggestion to lock up the brakes will have to wait until next spring as the car is put away for the long Michigan winter. I will try bleeding the brakes after the holidays.
Jeff