Residual valve v. s. a Adjustable Proportioning valve

DalicarliaEric

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Hi,
My first questions on this excellent forum. I plan to switch from drums (front) to discs (a Wilwood kit) on my 1966 Chrysler 300. My intention is to keep my Single M / C.

To get a faster response and that the rear brakes are engaged before the front brakes, I intend to install a 10 lb residual valve on the rear brake line. I see that some mount an adjustable proportioning valveon the rear brake line. See the attached diagram.

This valve can, as far as I understand, only decrease the pressure. Maybe a stupid question. Is this adjustable valve necessary? I have both the adjustable proportioning valve and the residua valve in my garage, so it is no problem to mount both.

Thanks in advance for advice.

adjustable valve.jpg
 
Cribbed from Summit:

"A Brake Proportioning Valve is commonly found on vehicles with front disc and rear drum brakes. It is a safety valve that restricts flow to the rear brakes during a “panic stop.” This prevents the rear wheels from locking up before the front wheels."

Add it so you can tune-in your brake system. Worst case if you install it is you'll run it fully open. Much better than the worst case if you don't install it.
 
As I understand it, the residual pressure valve would be for the rear brakes and the proportioning valve would be for the "pressure balance" (as disc brakes take more pressure to operate)? The residual pressure valve keeps a minimum pressure in the rear drum hydraulics for better initial "apply" response and is not needed for the front side of things on a disc brake/drum brake set-up.

I'd recommend getting a '67 power brake disc/drum master cylinder, repro plumbing for a '67, might wire in the pressure-loss sensor to the existing parking brake "BRAKE" light at the parking brake wiring, and call it done.

What does Wilwood recommend for a power disc/drum brake booster?

OR, if using the single master cyl is important, then mimic the '66 Chrysler power dise/drum set-up in all aspects (other than what Wilwood supplies).

Just some thoughts,
CBODY67
 
Thank you for your wise answers. OK, I understand that the adjustable proportioning valve is useful just in case of a panic stop. Not so much with normal braking. I will of course install the adjustable proportioning valve.

I do not see what Wilwood's recommendation is for a power disc / drum brake booster, but here is a link to Wilwood's Front Brake Kit Part No .: 140-15196-R for more information. Wilwood Disc Brakes - 1966 Chrysler 300 Base (Drum Brake) - Front Brake Kit No.: 140-15196-R
 
Once the proportionong valve is adjusted for the particular application and such, it works in ALL stops, not just a panic stop.

The issue with the power brake booster is that all factory installations had a dual-diaphram booster, which is longer than the normal power brake vacuum booster. With two diaphrams for the intake manifold vacuum to apply boost with, as disc brakes require additional brake fluid line pressure to stop the car, the pedal pressure input can still meet the USA FMVSS pedal pressure regulations (max pedal pressure to stop the car at a particular decel rate). The dual-diaphram booster was also used on power disc/drum B-body cars, too. Has nothing to do with the weight of the car, just that disc brakes require more line pressure to stop the car than drum brakes (as you now have) do. Some 3000lb and under cars are light enough that a power assist is not really needed for their disc brake set-ups (although some have had a power booster on them).

I suggest you contact Wilwood Customer Assistance and get their orientation on their booster recommendations. BEFORE you got involved in a swap that you suddenly find that you need additional parts to make it work. Just to be safe.

On the "slab" C-bodies, using a larger diamter booster is not a bolt-in situation. After the re-design for the Fuselage cars, then a larger diamter single-diaphram booster was used for all power brake systems. The issue is that a certain amount of diaphram area is needed to generate the greater boost needed for the use of disc brakes with similar pedal pressures (which the driver sees and feels).

In prior threads, some have stated that they got by just fine with their normal power drum brake booster. But I also suspect that IF Chrysler had deemed that to have been "good enough", they would have done similar. Better to be "safe" rather than "otherwise", to me. Which, to me, that means that for best results, an OEM design-components should be replicated when possible OR if they can be improved upon by selected aftermarket items OR, as one upgrade website promotes, using OEM replacement items from local auto aupply sources (which you might not have ready access to, respectfully given your location).

One "generational" issue I have is that I remember when Wilwood started selling front disc brake kits for drag race cars, along with a related master cylinder. Given that their focus was drag racing, "weight" was an issue in their designs (meaning physical "light weight"), meaning that they were weight-saving over similar OEM designs, notably so, by comparison. Looking at their current kit offerings, I can't help wondering about the robustness of their items when compared to OEM designs. Not that the Wilwood items are bad, just that they seem a bit "light" to me. No more, no less.

One "plus" for the Wilwood items could be their demonstrated longevity in the business and good availability (I suspect) of replacement parts when needed. Given the suspected future availability of drum brake system parts.

Pluses and otherwise, to me. Maybe I'm just getting a bit more conservative in my advancing age?

Enjoy!
CBODY67
 
Thank you CBODY67 for your description of the problems that can occur when converting from drums to discs. I will keep your advice in mind as I continue to work on the brakes.
 
I would definitely not keep the single pot MC when converting to front discs. I would get a 2-circuit disc/drum MC. Buy one for a 1973 Chrysler. Most disc/drum MC's come with a residual valve installed in the outlet for the rear brakes, or at least provision to install a residual pressure valve in the outlet. (Cardone MC's do not have provision for a presidual pressure valve in the outlet, so you'd need to plumb-in an external one.)
 
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