Technical The real purpose of the residual valve in drum brakes?

I have been chatting on another site regarding the purpose of the residual valve for drum brakes. All over the internet are descriptions that say "residual valve is used to hold pressure against the springs in the drum. This allows the drum brake to engage more quickly and reduces pedal travel."

I've always believed that the purpose is to keep constant pressure on the cup seals so that air cannot enter the wheel cylinder when the brakes retract, and also any tiny leaks would result in fluid seeping out rather than letting air into the system.

The pedal travel thing seems like absolute bullshit to me. I would argue that 10 psi of residual pressure in the wheel cylinder (area in square inches usually less than 1) will in no way keep those damn springs from pushing the cups/pistons back until the shoes hit their stops. If that 10psi could overcome the spring tension, then the shoes would not retract, and they would be in constant contact with the drum. Isn't the only thing that will affect pedal travel (in a good working system) the adjustment of the shoes?

Any links to good descriptions with references to sound mathematics and physics would be appreciated.

 

I dont' know for sure, but I thought it was to keep to much fluid from draining back into the master cylinder, situations where the drum cylinders were located higher than master cylinders mounted under the floor.

Norb

 

I was wondering about the same issue... why would pedal travel increase when the residual valves kept the shoes tight to the drums all the time? I always have to readjust the shoes from time to time to reduce pedal travel...

 

Drum brakes,if you don't have Residual,an run a master with out it with Drums,your pedal will need more movement/feel like its not working at first part of push.
If you use a drum master cyl with residual on a car with all disk,they will drag an over heat !
It's fairly simple. If ya don't over think it !

 

In a sealed system the fluid could not possibly drain back into the master cylinder unless air was entering at some point. The OP makes perfect sense to me.

 

I'm pretty sure you're right.

 

But, DOES the residual valve pressure hold the shoes tight to the drums? I don't believe it does. A 1 inch diameter wheel cylinder has an area of 0.7854 square inches, which means the 10psi of residual pressure is providing a "push" of 7.854 pounds against the shoe. Those springs fight back with one heck of a lot more than that. If they didn't then we wouldn't need all those fancy brake tools to stretch those springs over the retaining pin. That's why shoe adjustment is directly related to pedal travel. The springs return the shoes all the way to their stops, and if the shoes aren't adjusted correctly it takes a lot of pedal travel to push them out far enough to contact the drum.

 

Ebbspeed, you are correct, the brakes have to be adjusted to work properly, that is what keeps the shoes on the drums. Dana has it right, the 10 lbs is to keep the wheel cyl cups expanded so no air can get past them. This was for cars with pedals on the floor, hanging pedal masters create the same effect through gravity, the height difference between the master and the wheels creates around 2 lbs pressure, enough to keep the cups expanded, but 10 lbs is better. This and only this, is the residual valve's job.

 

I checked a couple of my old auto textbooks and they said the purpose of the residual check valves was to keep the cups seated in the wheel cylinder so they don't leak or let air in.

 

My old Audels Truck and Tractor Guide states, the residual valve's purpose is to keep hydraulic preload, preventing low brake pedal. No mention of preloading the seals to prevent leakage....hmm.

 

That easily could be one and the same, though not specifically expressed. i.e. air leaking into the wheel cylinder could/would lead to increased pedal travel while the air was being compressed before brake shoe movement commenced.

As several posts above point out, the shoe return springs are too strong to be stopped from brake shoe retraction by a mere relatively few pounds of residual fluid pressure. Brake shoe adjustment (and/or air in the brake system) are the only things I can think of that account for increased pedal travel in an otherwise correctly operating system.

Further, while I didn’t question the story about fluid bleed back when M/C height was lower than wheel cylinders, I now question that theory.

On the other hand, the difference in the recommended residual valve design pressure of 2 psi disc, 10 psi drum, is based on the typically significant difference in seal design between disc caliper piston seals vs those found in drum brake wheel cylinders.

Ray

 

Your right ... designed to keep the cup seal sealed - no air in and no fluid out .... the steel expander cups in some assemblies are designed for the same purpose .... and yes the adjuster mechanism is designed to keep the shoes adjusted for the correct pedal travel .... at least that's what they taught us in trade school !!!

 

Engineered Components has a good page: See brake facts...
Engineered Components, Inc. - Your One Stop Brake Shop! (ecihotrodbrakes.com)

Its important to keep the seals inflated. These age, especially with heat, if the master is below the floor a bit of residual pressure is needed.

 

I had a 1959 Ford F250 that I converted to a dual master cylinder. I would get air in one of the front wheel cylinders and the truck would pull to the side when braking. The master cylinder was on the firewall and used the stock ford pedal and linkage.

A quick bleed would fix it - - - until the next time. Put a 10 PSI residual valve in the front brake circuit and never had the problem again.

Did not think I needed an adjustable proportioning valve on the rear brake circuit - until a deer jumped in front of me one night. Stopped in time, but, the truck was about 40 degrees sideways.

Wisdom is gained by making making lots of mistakes...

Russ

 

And surviving!

 

I'm thinking that If I go out and dig out some of my old auto mechanics text books it will say pretty well what Ebbspeed said in post 1. The primary purpose is to maintain a bit of pressure on the cups so they hold a seal against the cylinder walls.
The residual valve was then called the check valve though.




I know that on my 48 with the 68 C-10 master cylinder under the floor and no residual valves the brakes would draw air in past the cups and have to be bled if I let the truck sit for three or four weeks. As long as I was driving it a few times a week or every day there was never an issue but every time it sat for a while because I was driving a "work car" I'd have to bleed the brakes. It's 17 degrees out right now and I don't feel like getting my feet wet and cold going out in the snow to the garage but I'll try to dig one of the older text books out tomorrow or look through my brake class papers from Napa Eis brake classes.

 

I am in the process of building a 53 Dodge P.U. I have G.M. intermediate calipers in front 9in back and a 79 Chev P.U. manual brake mc. When I was hunting a mc I called both Speedway and Wilwood's tech lines. Both recommended 1" mc with a 10# residual to the back. I questioned both about the residual with firewall mounted pedal and mc. Both stated that even though not absolutely necessary but is highly recommended. Reason being that I would have a better pedal and pedal feel. Not being a brake tech myself and also not being a high dollar item I would try it. I haven't drove it on the street yet but in the shop it is one of the best feeling pedals I've had. It was also one of the easiest completely new systems to bleed I have ever had.

 

Early Corvette disk brakes used "lip" seals more like the cup seal in drum brake wheel cylinders.
Reportedly they are prone to air entry and other maladies on cars not used regularly

https://image.mamotorworksmedia.com...iclefiles/TechTipFiles/C2-C3BrakeCalipers.pdf

 

Residual valves are not back check devices that hold 100% pressure when the pedal is released. That's why they come in different percentages to allow for some let off, so shoes or pads don't drag until they burn up. You do not need a leak or air to enter the system to allow brake fluid to return to the master. The pressure applied to the brakes when stopping, is the same pressure that will push the fluid back into the master when the pedal is released. A residual valve just doesn't allow it to completely return, and keeps some residual pressure of a set amount to stay in the lines after the valve.

 

They are strictly to keep air from entering past wheel cylinder cup seals upon brake release. When the return springs snap the pistons back into the cylinder it creates a low pressure which can collapse the seal and pull air in, later wheel cylinders use cup expanders which eliminates the need for residual valves. When the master is lower than the wheel brakes they prevent fluid from running back to the master.

But all this has already been said several times.


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When you stick a drinking straw in your drink and hold a finger over the end and remove the straw it stays full until you remove your finger. Same principal. Brake fluid can not drain back out of the lines without air entering somewhere. End of story.

 

They are to help the wheel cylinders seal. Some cars after 1975 have springs in the cups and don't need the valve, Chrysler products and possibly others.

 

This sounds like about the best explanation I’ve ever heard. Maybe it really depends on the type and condition of your wheel cylinder seals wether or not you need a residual pressure valve.

 

Without meaning to sound like an arrogant ass, it’s because it is the correct explanation.

Seal condition doesn’t matter it’s the design of the wheel cylinder, some have a cup expander wound into the end of the spring, some cup expanders are metal disks that clip to the end of the spring. They prevent the seal from belabor to leave the wall of the cylinder upon release.


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The answers are all over the board I will add one more as was told to me by a rebuilder that did power brake units for our auto parts store. A residual check valve is used in disk brakes to keep slight pressure on pads since they always have contact on the rotors and have no springs to pull the pads back. He also said drum brake MC's do not have a residual check valve. This is what I have been told and used this practice in all of my builds over the last 50 plus years. Just something to think about.

Pat

 

The lathe cut seal is what retracts the pads on disc brakes, the pads are not always contacting the rotors just really close. If they were always in contact there would be a lot of wear and heat....I already answered the drum brake thing. Find a new source, he’s wrong. Nothing modern uses any sort of residual valve, disc or drum


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""I've always believed that the purpose is to keep constant pressure on the cup seals so that air cannot enter the wheel cylinder when the brakes retract, and also any tiny leaks would result in fluid seeping out rather than letting air into the system."" OP

A residual valve allows the brakes to be pumped "up" when there is a loss of brake fluid in the system. Since the valve maintains full lines [i.e. above no air in], any fluid that leaks out on brake application is back filled when primary cup returns to home through the compensating port. Thats it.

 

When you step on the brakes the caliper, or wheel cylinder expands. Release the brake pedal and they try toreturn to the static position, which pushes fluid back into the reservoir. No air required to enter the system.

 

Not at all what I said

 

Interesting reading.
I'm at that point in my project, gathering parts to build the brake system. After reading several articles on the subject, and getting contradictory info about residual valves, I finally called and talked to a long-time builder that I trust. His advice was to stop reading shit on the internet (his first piece of advice EVERY time I ask him about anything, LOL) and, with a firewall-mounted Disc/drum system, to use a GM combination valve for a similar weight car- it will have the correct residual and proportioning valves built in, to put a metering valve between the M/C and the front brake tee, and then you're done.
That's the way I'm going, parts are on order; now I'm looking at all the metal work to do....