What is the hydraulic unit below the Brake Master Cylinder?

Asked by Dec 03, 2010 at 06:56 PM about the 1977 Toyota FJ40

Question type: Maintenance & Repair

I have a 77 FJ40 that has a tendency to lock up the rear wheels before the front on heavy
braking. This is not a good feeling especially on snow and ice. The rear brake line goes through
a unit below the master cylinder that I think would be a pressure regulator for the rear brakes.
Is that what it is and how do you test it's function?
Sliding around in Clororado.

2 Answers

It sounds like you are describing the brake proportioning valve. I attached a picture of one. If you have rear drums and front disc brakes you probably also have a metering valve. They may also be integrated into one unit. Here is a link that explains what they are and do. http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake2.htm I don't believe they are adjustable. It sounds like it might be corroded inside. I'm not aware of any method of testing one but you might try googling it. They should be cheap enough to replace and given what they need to do it would be cheap insurance.

1 out of 1 people think this is helpful.

Thanks for your reply. I believe that you are right that it is a proportioning valve. Mine looks different from yours (smaller and below the M/C) but must have the same function. Since this FJ40 has been modified with lift and bigger tires I assume that even if the valve were replaced, the problem could persist. My thought is to replace it with an adjustable proportioning valve (Volvo used them for years on their "split" brake system) and play around with the adjustment till I get acceptable results. Has anyone created a suitable pressure gauge set up that I could record my before and after pressure readings? How much PSI do you think the M/C creates? In the hundreds??? Let me know what you think.

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