Brake Proportioning Valve (for 1968)

[cadmanof50s]

Under what conditions was the brake proprtioning valve (i.e. front to back line, under the driver side frame) used? Was it on all models with disc brakes?
The assembly manual implies that it was only used on 350s...AIM L48 A1 and A7. Why only the 350?


Let me ask this differently, should my 1968, with a 327, and disc brakes have a proportioning valve?


Vic

[JohnZ]

Under what conditions was the brake proprtioning valve (i.e. front to back line, under the driver side frame) used? Was it on all models with disc brakes?
The assembly manual implies that it was only used on 350s...AIM L48 A1 and A7. Why only the 350?


Let me ask this differently, should my 1968, with a 327, and disc brakes have a proportioning valve?


Vic

See this:

http://www.camaros.org/suspen.shtml#valve

[rskev]

Vic, Not sure on what or when they used this. but, one thing for sure is that the use of this regulator valve was inconsistent. My 68' rs 327 with a/c does not have one of these, or show evidence that one ever existed. Hope this helps!

[cadmanof50s]

Thanks John and Kevin.

From all that I have read and been told, there seems to be no rhyme or reason for the usage of this valve.
On that note, I have purchased a new set of "split" front to back brake lines and I will be installing this valve in my car.

Will this make a difference under normal driving  conditions?

When I drove the car this summer, (without  this proportioning  valve), I noticed that the front brakes grabbed sooner than the back. (Having said that..I would like to note that single piston "aftermarket" calipers and discs were installed,  and,  I was not sure about the condition of the "hold-off" valve (which my car does have).

I will be replacing/rebuilding all brake components, including front calipers and rotors.  Every component will be replaced with new/or rebuilt "correct" parts. I'm hoping to see  better driveability  (or should I say..brakeability).

Thanks again,

Vic

[JohnZ]

Thanks John and Kevin.

From all that I have read and been told, there seems to be no rhyme or reason for the usage of this valve.
On that note, I have purchased a new set of "split" front to back brake lines and I will be installing this valve in my car.

Will this make a difference under normal driving  conditions?

When I drove the car this summer, (without  this proportioning  valve), I noticed that the front brakes grabbed sooner than the back. (Having said that..I would like to note that single piston "aftermarket" calipers and discs were installed,  and,  I was not sure about the condition of the "hold-off" valve (which my car does have).

I will be replacing/rebuilding all brake components, including front calipers and rotors.  Every component will be replaced with new/or rebuilt "correct" parts. I'm hoping to see  better driveability  (or should I say..brakeability).

Thanks again,

Vic

The function of the subframe-mounted proportioning valve (avoiding rear brake lockup) only becomes important under panic-braking conditions; it doesn't come into play until hydraulic pressure exceeds about 400 psi, and isn't fully effective until around 1000 psi.

The feeling of front brakes "grabbing first" is usually caused by one of three things:

1. The hold-off/metering valve isn't working as designed (it denies pressure to the front calipers until system pressure of 30-40 psi is reached, to allow the rear drum shoes to expand to the drums before the front calipers start to clamp the rotors).

2. Adjuster starwheels on the rear shoes are frozen, so the automatic-adjust feature doesn't work when you back up and apply the brakes.

3. Incorrectly assembled master cylinder, missing the RPV (residual pressure valve) under the brass tube seat in the rear outlet port; the RPV maintains 8-16 psi in the rear drum system to overcome the shoe pull-back springs and allows the shoes to stay out close to the drums for instant response to fluid pressure.

[cadmanof50s]

Thanks for the info John.

No problem checking the first two items.
Any more info on the RPV valve?

How do I check to see if it is there, etc.

Vic

[Steve68]

No problem checking the first two items.
Any more info on the RPV valve?

How do I check to see if it is there, etc.

Vic

You may find this helpfull regarding item #3.
http://www.camaros.org/forum/index.php?topic=2938.msg18849#msg18849

Steve

[cadmanof50s]

Very helpful, Steve. Thanks!

Checked my MC and indeed the rear section has the RPV (as described).

Sent the whole unit out to get checked (and rebuilt if necessary). Told the shop to make sure that they replace the RPV.

Vic