Nathan,

I won't be towing anything or hauling anything heavier than a few groceries.

 

It is not a "metering" valve, and it is already in an OEM Master Cylinder for drum brakes. It is a residual valve and it keeps the fluid in a drum brake wheel cylinder from draining back to the M/C due to the brake return spring whose purpose is to retract the brake shoes. If there were no rewidual valve, the brake pedal would be low and the brakes would have to be pumped up.

The idea with the use of dual brake cylinders is simple: that of mechanical advantage. First of all, the residual valve is simply a one way valve that keeps the drum brakes from returning fluid. It has a slightly delayed onset so some fluid is returned. Since a rear drum brake is self energizing, it needs very little line pressure to lock up.

The disc brakes typically have a 2 1/4" piston. Since a disc piston does not retract, it needs very little fluid to be energized. This allows the use of a very small M/C piston for high mechanical advantage. To get 1200 PSI from 70 lb of foot pressure takes 17:1 MA. The pedal has about 6:1 and the rest is the ratio of the piston sizes. For GM single piston calipers, we typically use a 3/4" piston size (3:1) giving us a total of 18:1 MA. Not only can we lock up the brakes with medium foot pressure, we can modulate the braking much better because we have a pedal with 5-6" of movement and a wide range of foot pressure prior to lockup. Stock OEM disk/drum M/Cs have typical front rear piston sizes of 15/16 and 1 3/8 inch and have a built in residual valve. Because the front piston is not small enough, there is not enough MA and a booster is needed. This excessive pressure on the rear brakes would make them lock up if their OEM metering valve were not used, as the larger piston is still not large enough to lower the line pressure.

Using two aftermarket M/Cs of correct sizes would eliminate the need for any valve front to rear, but since front rear balance has a lot to do with the type of vehicle, its front rear weight balance, tire sizes, etc., it is a good idea to put in a proportioning valve which can be adjusted.

There is a great article on brakes written by brake expert Dean Oshiro at www.hotrodheaven.com/tech/brakes which has practical examples of commercial products.

I personally hate brake boosters because they are a patchwork approach, not an engineered solution. If I were to be offroad and lost engine vacuum, I would want my brakes to work. I also like brakes which do not require antilock braking systems when on ice to keep from locking up due to inadequate pedal feel........

Regards,

Alanco

 

Alanco, there is a small error in your explanation of a res valve actually. It has little to due with springs and more to do with master cylinder height location in relationship to height location of the wheel cylinders and calipers. The reason resi valves are rated in PSI is to ALLOW the fluid to drain back from the drums into the master or the brakes would stay locked ( applied) and that is not a good thing.Same thing with disc brakes, they need to backflush a bit to relieve pressure or stay locked. It is the reason for an air space in the master cyl. What the resi valve does is allow any fluid under the return force of the drum brake springs to return into the m/C reservoir until that force is at or less than X lbs PSI at keep additional fluid from draining back. When the wheel cyl is on a higher plane than the master, fluid will flow down due due gravity just like the old Model A'a. The res is not needed if the wheel cylinders or calipers is lower than the master cylinder. You may note: under floor M/C needs resi, firewall mount M/C NEEDS no resi valve. This info can be gleened from most aftermarket brake sys manufacturers and Motor's ( and Chiltons) Car Manuals. Never heard any different in all my years as a B-F/E tech. Anyway the rest of your explaining is great stuff and the numbers are valuable. Jon

 

You are correct because I did not really explain the ratings on the residual valve. Most are about 10psi. When I said there is a delayed onset of the valve I did not explain that the delay is the pressure bleed down to 10psi to allow for the shoes to retract. Even though theoretically a M/C that is higher than the wheel cylinder would not need a residual valve, the residual valve will still keep more pressure in the line to the rear than would gravity with only a 2foot head in a truck and less in a car. In any case, OEM M/Cs have a built in valve on the dual disk/drum types.

The other part of this is that the OEM proportioning valves are designed to drop the application of line pressure to the rear brakes. Using a proportioning valve not designed for the application will not work correctly in most cases. Using an adjustable proportioning valve will allow tailoring of front rear balance to your application.

When using two aftermarket single master cylinders for front/rear, one would install a residual pressure valve in the line to the rear, and a proportioning valve in the line to the rear. This allows you to use the amount of rear brakes that you want in combination with the front.

The other part found in OEM dual master systems is a pressure switch that is installed between the front line and the rear line, so if either the front or rear has no pressure, a switch is closed that lights up a brake warning light.

Thanks for your discussion....

Alanco

 

A metering valve is not a residual valve. The OEM combo valves are made of 3 parts. Metering valve, pressure differential switch and a proportioning valve. To explain in better here is an excerpt from How Stuff Works.


"The metering valve section of the combination valve is required on cars that have disc brakes on the front wheels and drum brakes on the rear wheels. If you have read How Disc Brakes Works and How Drum Brakes Work, you know that the disc brake pad is normally in contact with the disc, while the drum brake shoes are normally pulled away from the drum. Because of this, the disc brakes are in a position to engage before the drum brakes when you push the brake pedal down.

The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in.

Having the rear brakes engage before the front brakes provides a lot more stability during braking. Applying the rear brakes first helps keep the car in a straight line, much like the rudder helps a plane fly in straight line."

 

Okay, I have a stock 87 Chrysler OEM combination valve installed exactly the way it was in the donor car with the same stock brakes front and rear but my front brakes are engaging before the rear. What would cause that? According to tests I've made from suggestions made here previously, the combination valve is good. What else might cause this?

 

Vern,
I was able to get a chance to take my truck out on a dirt road yesterday. My fronts do lock up first when I hammer on the brakes. I tried it on pavement also but it was hard to tell, I had my boy watching but I'm not sure he understood what I was trying to do.
Like I said, overall my brakes feel good. I have good pedal feel and the trucks tops straight and true. I won't be changing anything on my truck.

Bobby

 

Bobby,

Glad to hear that. Maybe what I need is a Heinz 57 assortment of various manufacturers' brake parts for my truck like you have instead of everything from the same donor.

According to the "experts" the rear brakes should be engaging first but as long as I can stop quickly and safely I could not care less on which end the brakes engage first. I'm pretty easy on brakes anyway but I do live in a mountainous area so really good brakes is a necessity...

I'll eventally get this figured out...

 

And finally, I usually do not add a "brake" warning light meaning I do not put a pressure differential switch in. With brakes with a good feel, it is easy to tell when the fronts are missing, and easy enough to tell when just the rears are missing. The pedal pressure will be a lot less with one side out. One thing you taught me was that the OEM valve is a triple combo unit, which I have not been using. I did know that when putting in rear disc brakes, we had to take that unit out and use a proportioning valve that was adjustable.


Regards,

Alanco

 

Vern,

I think you're right. I know I need to readjust my rears now that I have a few miles on the truck, its on my to do list for "someday"
Just think, we have another 4 months or so to discuss, argue, debate, theorize, etc this issue while you while away the winter hours.

(sorry to see the speaker grill get up over your limit, there will be others. I'm in no big hurry for one either...someday)

Bobby

 

The first thing to check is the rear brake adjustment. If you are using the triple combo switch/valve, it has to be working correctly as well. What can happen is that the pressure differential part of it has tripped and is not centered. If the switch is closed, it must be recentered.

Regards,

Alanco

 

The rear brakes are adjusted tight and the combo valve is centered. What's next?

 

If we were factory mechanics we would replace the M/C. However, I would rig a pressure gauge to the rear lines on the rear axle, maybe easiest on where the brake hose fits. See what the line pressure is and how fast it comes on. Could be: metering valve, bad brake hose, kinked or dented rear brake line somewhere, Stuck wheel cylinder on one wheel (due to rust).

Regards,

Alanco

 

51 Dueller ,Alanco, good responses. Now, if we can get most of the thread to somehow get read and absorbed it would solve alot of issues for most. It would be good if it came up on a search ( if folks did that) ...................Jon

 

Vern,
You took everything from the donor vehicle and plumbed it the same way. The only thing that could possibly be different between the 2 vehicles is the piston diameter of the rear wheel cylinders. Any chance of doing a bit of research to see what the difference might be? I'm guessing that our trucks probably ran 1" wheel cylinders in the front and 7/8' in the rear.
You could probably go to the Autozone, Napa, or Checker Auto, Bendiz brakes, etc to find what diameter your donor car had.

Bobby