Proportioning Valve & Dual Master Cylinder

[Doug B]

Is it really necessary to have both on RQB versions of the Avanti ?

[Gunslinger]

Yes, it is. Proportioning valves are designed to send most of the hydraulic brake pressure to the front brakes which do most of the stopping. If you simply run lines from the larger master cylinder chamber to the front and separate lines from the other chamber to the rear brakes, the equalized pressure will lock up the rear brakes prematurely. In other words, you'll end up with an unbalanced brake system.

The proportioning valve is there to maintain the appropriate front/rear pressure to keep the entire brake system safely working together.

[Doug B]

Thanks Gunslinger. You have briefly covered the many pages I have read on the subject lately.

My confusion seems to come from having had master cylinders with primary and secondary pistons. As I recall, one served the front brakes and the other, the rear. The proportioning was provided by the larger and smaller diameters of the pistons.

One of my references shows '66 and '68 Chevs having 1.125 in, and 1.0 in. pistons to serve discs and drums. This will do the same as a prop. valve. Yes ?

My next question is Avanti-specific. Does the larger or smaller brake-fluid reservoir serve the front (disc) brakes and why ?

[warren55]

As for the larger reservoir size, disc brakes require more pressure and fluid than drum brakes. When it comes to proportioning, drum brakes will lock up at at lower pressure than disc brakes, so at max braking without a proportioning valve your rear brakes would lock up first. Not good, if you've ever been there.

Here's a good article that can explain it better than I can:http://stoptech.com/technical-support/technical-white-papers/proportioning-valves

[Doug B]

Thanks Warren. My problem seems to have been caused by this quote from a Chilton Chevrolet manual which states "A dual reservoir master

cylinder is used on all models since 1967. The front portion of the master cylinder supplies hydraulic pressure for the front wheels, and the rear

portion supplies the rear wheels."

I have been searching for a printed reference which states the opposite, since all such master cylinders I have seen, have the smaller reservoir

at the front.

Knowing that the front (disc) brakes require much more fluid than the rear (drum) brakes, makes it obvious that the larger reservoir must feed the

disc brakes no matter whether the larger reservoir is at the front or rear.

All further references to this subject should refer to the larger and smaller reservoirs rather than to the front and rear. Agree?

[warren55]

Yes I do!

[silverstude]

On ANY Master cylinder, you assign the port with the longest stroke to the FRONT brakes, whether disk or drum.

[Doug B]

And how does one determine this ?

[silverstude]

Look at the profile of the MC. It's easy to see where the longest stroke is assigned. Some front ( Ford), some rear (GM)