Disc brake without a power booster

I did not have any special valves in my P&J under the floor pedal assembly. I used only the Camaro parts that were originally on the firewall.

 

(Iv asked this before an no one spoke up)
I was taught that the residual valves purpose is to keep air from being sucked past the wheel cylinder cups …. Not to keep fluid (that cant flow back anyway) from flowing back.

What say the brake heros?

 

Well if the system is well bled & not leaking , it can't suck air and have air displace the fluid unless the fluid leaves. If you keep the fluid from leaving, that will stop the air. Air is fighting to get in anywhere at 14ish psi.

Same principals involed as putting your finger over a straw

 

Probably the bore size. The catalogs show a different pt no. for power brakes and manual brakes.

 

Brake shoe return springs can compress the cylinders faster than the fluid can move = sucking air past the cups.
That small amount of line presser (from the valve) keeps air out but cant overcome the springs and make the shoes drag.

 

x2 Proper MC bore diameter is critical to avoiding a hard or soft pedal

 

The fluid would have to move faster than the springs and create a void or space for the air to get in . No ?
There needs space ( no fluid) for the air to go

 

Agreed, but could note suorce a Mopar 1" MS with the same mounting bolt pattern. Did find a 1.125" that is available soon, but that's too big for sure.

Yea I bled and bled the system. Everything is new, even sprung for stainless brake lines.

 

Spongy is not the same as too much travel. You would need to be way off for cylinder size or pedal ratio to cause a spongy feeling pedal.

 

Ok
This is why I brought it up …. If I was told wrong what is the function of the residual valve?

 

I think you were told basically correct
Just a few facts in the mix are incorrect.

 

Well it was like 40 years ago

 

Basically, a residual check valve is there to keep a small amount of pressure on the caliper or wheel cylinder it's servicing. With wheel cylinders in a drum brake, it keeps the shoe's springs from completely compressing the fluid back out of the cylinder. Most drum brake situations use a 10lbs residual check. You can always tell when you need one 'cause you have to give the pedal a quick "pump" before you have brakes. Most calipers need about 2lbs to keep the pads right up next to the disc. Both situations also act as a sort of one way check valve to keep the fluid in the lines in situations where the master cylinder is mounted below the floor boards.

 

take a look a mpbrakes.com (Master Power Brakes) for tech info on proper parts to use

 

Residual pressure valves are used more in disc brakes with underfloor pedals. Disc brakes don't really transfer fluid when braking and the fluid must stay in the lines under pressure just enough to keep the pads in contact with the rotor. It is less than a teaspoon full of fluid being transferred.
Drum brakes have the wheel cylinder that expands and contracts to apply the brakes, it takes lots of fluid to make that happen.
In either case no air can enter, like being sucked past the cups.
I have never seen a residual valve on an early drum brake system, we add them nowadays but the factories never provided any that i am aware of. You just did a double pump on the pedal and you were there - of course they didn't drive in 4 lanes of traffic at 60mph either.

 

It's doable...I run '78 front disc on front of my 72 F100, no booster, and this setup has been working great with the original drum/drum master cyl.

4TTRUK

 

My '73 Dodge Dart Sport came rolled off the assembly line with non-power disc brakes. Stopped just fine for me.

 

There it is in a nut shell. Never go over a 1" bore master first and foremost when going manual. The pedal leverage is pobably not going to be an issue since the current power master is or should be a larger bore. I don't know about FORD product's but look at the back of the master where push rod goes. On some manual vs power, a longer push rod is needed on a manual because it has a deep pocket for the rod to go in.

Edit note, I see that this is a project that has not had braking system completed before hand. But the basic words I've said still apply for general referance.

 

Many light to mid size disc/drum cars in the '60s up into the '80s came with manual brakes. Generally a 1" master and 6-7:1 pedal ratio is used on manual systems.

You are 100% correct.

Sorry, but you're wrong. A partial vacuum can develope at the wheel cylinder cups during fast brake release, caused by the very strong shoe return springs.

Tommy you're correct that some power vs manual masters use a different bore size, but many are the same. The main difference is the push rod counter bore depth in the primary piston. All power masters that I have or know of have deeper bores than the manual master. This is important when using a non-retained pedal push rod in a custom non-stock system.

Modern production disc brakes never used residuals, because all masters were firewall mounted in the '50s before discs became available. The 2 lb valve is strickly aftermarket, and should only be used with discs and floor mounted masters.

Sadly, you're not going to get 100% correct info from MPB, or several other big suppliers. Considering hydraulic brakes have been around for over 80 years, it's a shame so many people still do not fully understand how they work, especially those that sell brake componets.

Partial vacuum allows air to enter past the cups into wheel cylinders.
All drum brakes have used residuals, up until the early '70s, either internally in the master cylinder (sometimes called check valves) or externally in line.

 

No I am not.
Perhaps you missed the meaning or context of my post and what I was replying to.

 

Wow alot of brake info - great tuff - thanks all for helping

 

31Vicky, you are correct, and V8 Bob might be missing the point.

If the following were true:

"Brake shoe return springs can compress the cylinders faster than the fluid can move = sucking air past the cups"

then the fluid would be forced past the cups and not only leak, but forcefully squirt, out of the wheel cylinders. If the cylinders were being compressed faster than the fluid could move there would be no other place for the fluid to go than out into the atmosphere. Compressing the cylinders would result in pressure, not an air-sucking vacuum. If it did, then someone would be breaking the laws of physics, and before you know it we'd have fire and brimstone coming down from the sky! Rivers and seas boiling! Forty years of darkness! Earthquakes, volcanoes! The dead rising from the grave! Human sacrifice! Dogs and cats, living together! Mass hysteria!

 

Look at the rubber in a wheel cylinder.
Its only designed to keep fluid in and really only when there is fluid presser on it.
Just like any common seal …. Installed backward they leak.

Like I said im just quoting what I was told in shop class from years ago about them sucking air ….. I don’t necessarily remember all the details perfectly.
Im sure a Google search or two could find the correct / accurate answer.

 

Well Ebb, I've come to realize that the laws of physics and manufacturer explicit instructions do not apply to some places , Grainger, northern Indiana is apparently one of them. It took a lot of convincing Lol

 

Not sure about every make or model, but on most the Chevy's I've owned the difference between a power/master and a manual master was no difference. Chevy simply uses two different holes in the brake pedal arm to change the ratio, but the same master.
On my Austin gasser I started with a power booster, but ran into trouble with a bigger cam that wouldn't develope enough vacuum to keep it. I removed the master and reworked the mounting to use the same master as a manual master cylinder. Then I reworked the brake pedal to give more leverage and a better ratio. My disc/drum (with a power master converted to manual) works great to stop the car.

 

I am probably wasting my time with some of you, but I'll post this anyway. I learned, among many other facts, the following first hand over 40 years ago, at the beginning a long carrier with a major U.S. brake supplier, very well known here, in Granger, and throughout the world.

The following taken out of a Bendix brake system service manual, circa 1974:

Residual Pressure Check Valves

"The function of residual pressure check valves is to maintain a slight pressure of 6-25 PSI in the hydraulic system.
…..the most important purpose is to prevent air from entering the hydraulic system past the lips of the wheel cylinder cups when the brake pedal is released and fluid returns from the wheel cylinder to the master cylinder.

The check valves are used only in the outlets connected to drum brakes because disc brakes do not require residual hydraulic pressure.

In recent models, some auto manufactures have eliminated residual pressure check valves,
1-because of disc brakes, and
2-cup expanders in drum brake wheel cylinders. The expanders help to prevent air from entering the lip of the cups."

Production modern disc brake vehicles all had the master cylinders firewall mounted, but with non-stock disc systems with floor mounted masters, an aftermarket 2 lb residual may be needed.

The above speaks for itself, but am curious, 31Vicky, what manufacturer explicit instructions do not apply

 

The above speaks for itself, but am curious, 31Vicky, what manufacturer explicit instructions do not apply



"Manufacture instructions" refers to your remarks on a different brake thread , not on this thread

You might want to go back and re read this thread, you may even want to edit some stuff.

 

Just did this on my car, you probably want a MC with a 7/8" bore. Over an inch and they get pretty stiff.

 

I don't really want to jump into the middle of a pissing match, but the one hot rodding reality that the above statement doesn't take into account of is when the master cylinder is mounted lower than the calipers or wheel cylinders. I have many times set brakes up where the customer demanded a clean firewall and wanted the master below the floorboards, (my preference is for firewall mount) and I duly strung the lines and plumbed the whole deal only to find in practice the "one pump" syndrome I discribed a bit earlier. Even on disc brakes, when the master is below the calipers, the fluid in a car that sits for a bit has a tendancy to want find the lowest point of the plumbing when it's not held in the caliper by say, a residual check valve. Into the master cylinder that fluid goes. Jump in the car after it sits a few days, pump the brakes once, ad suddenly you have "pedal" again. This can be easliy solved with the appropriate residual check valve. I had never heard of the air rushing past the wheel cylinders and such, but could easily see how it could happen.

Just my .02 and a ton of experience...

 

I addressed the issue using 2 lb residuals with floor mounted masters and discs, and have always stated that residuals should be used with all drum brakes, old or new, and reguardless of master cylinder location. Discs with high (firewall) mount masters do not need and should not have residuals.
I agree a low mount master running to discs without a residual could result in a displacement issue, something I never encountered, because production disc vehicles never had below floor masters.