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1972 Corvette. I’ve read some
who have said that the brass proportion block isn’t actually a valve, just a brake light switch. Regardless, if it is a valve that can close off either front or rear brakes, would an accurate test be to gravity bleed the brakes? It seems to me that if fluid flows from both the front and rear calipers, that means that it is centered and would not be an issue in any braking problems. Does that make sense? Thanks
It's a pressure differential switch which detects an imbalance between the front and rear circuits. It does not close off either. It is not the brake light switch, but it will trip the dash tell tale if it detects an imbalance..
So I am having an issue with braking, and what I’m hearing you say is that the switch won’t actually affect braking if it was somehow defective, but it will tell you via the brake warning light if you have an issue with brake system balance. My issue is a nice firm pedal after bleeding with all new calipers, pads and hoses, but the brakes just don’t stop as they should. The pedal stays firm and does not get soft. I did not replace the master cylinder but did bench bleed it I gravity bled the entire system.
Let me throw this out there. Looking at a diagram of how a MC works, there are three seals inside. One in front the pushes the fluid to that circuit, one in the middle and one in the back that pushes to the other circuit. It looks to me like if for some reason the center seal failed then it would allow fluid back up into the master cylinder. The result would be a firm pedal because the one circuit is pressurized, but the other wouldn’t be working because it’s leaking back up into the reservoir because the middle seal kept it pressurized but now isn’t. If that were the case, the MC could be the culprit???
It is a 'distribution' block which sends EQUAL brake pressure to front and to rear brake calipers. The internal valve is as mentioned above...a differential pressure valve which stays centralized UNLESS one portion of the braking system has a leak of some kind. That differential valve will shift to block of the 'leaking' half of the braking system so that the other half remains functional.
The switch in the proportioning valve is one of the three possible grounds of the "brake" light switch in your dash (not the brake lights at the stern of your car that other drivers see). The other two grounds are in your ignition switch (crank position, to test the bulb), and your emergency brake lever.
Here's what my 79 shop manual says about the terminology.
Brake proportioning block
