Disc brake without a power booster

I have no idea what you are refering to, so maybe you can enlighten me so I can see what stuff you think I should edit.
Why not bring this up on that thread, instead of this one?

 

I have manual disc brakes on my wife's '68 Camaro. As long as you use the bigger bore master cylinder with at least 11" rotors and calipers they work great.

 

It just fits with your position.
Physics are defied and manufacturer instructions be damned as well.

I'm not spoon feeding it to you either

 

I currently drive a 65 mustang upgraded to discs in the front no booster it's absolutely fine to drive.

 

V8 Bob, I'm not disputing what the residual valves are for at all. You are correct in what they do, when they should be used, etc. What is wrong is the following statement that you said is correct. No way will strong springs and rapidly returning shoes cause a vacuum inside the cylinder that will suck air past the cups. I'm pretty sure the same statement is what 31 Vicky was questioning.

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

 

well guys i aint about to jump in this shit but i do have a question.sorta, ive been build cars and bikes for 40 yrs.so i do know just alittle,well enough to get me into trouble anyway how can air be sucked in and no fluid ever leak out and please dont sday vacuum because it just dont make any sense.gotta have a place for air to get in and if it does it can also leak out.cant it?

 

I'm sure you're gonna get an answer outlaw.

 

Mm?, do I make popcorn now and chance it will get cold. Or?, Ah F*** it, I'll just make another bag if need be

 

I said that (not Bob) and already admitted I probably said it wrong …. WTF im sorry already.

Ok like I said earlier
Look at a common cup from a wheel cylinder. They are obviously designed to keep fluid in … they seal better the more presser there is.

We have all seen badly pitted cylinders right?
Do you think that water somehow circulated its way in there in the fluid? .
Not likely it got past the cups … because they only work one way .. they don’t keep stuff out.

Just like any kinda rubber seal …. What if you put a SBC rear main seal in backwards it would do a great job keeping oil from going in but would leak oil out like crazy.

 

I have a 28 A Sedan with disc and no power. Takes a little more foot power but stops great as compared to the drums before.

 

Ok, already got the popcorn fart's. So I'll take time out to say, Brake Fluid is a magnet for H2O. It don't need a bad cup to start collecting water. It does just fine all on it's own to start missing things up. That's wht you should flush your brake system annually. Ok, stomach is a lil flatter now. Back to popcorn.

 

Follow the bouncing ball BOB

The fluid most certainly can and does flow back, the valve controls the fluid and specificly controls the back flow rate by maintaining residual pressure on the line. With out the valve the fluid could leave the wheel cylinder with enough momentum to create a void and allow the air to fill that void past the cups before the fluid "bounced" back. More of a case of the fluid moving further (not faster) than the cups. ( we can talk about*Sir Newton someplace else)

So the residual valve controls the fluid. That's all it does.
The laws of physics control the air .

These cups are under pressure to return via spring pressure. If the springs returned faster than the fluid could move back - the system would be pressurized, (see ebb speeds post here) the cups would be plowing thru or shearing the fluid on their way back to home position

OK like I said , basically correct , just a few facts mixed in that are incorrect.

Then we get the BOB statement100% correct, (its not)sorry 31Vicky you are wrong ( no I'm not) exchange.
Now either he missed what was being said, don't understand what what said, or in such a hurry to express his pseudo omniscient expertise and say I'm wrong, complain how he's waisting his time explaining this and yada yada yada.. I say pseudo because any real expert would have seen and known this. Especially after I said perhaps he missed what I meant and to re read it - nope more BS and a complaint of waisting his time.

What's the purpose of the residual pressure valve ? To provide residual pressure
Why do you need them on all drum brakes? To keep air from entering the system.
How does it do that ? Physics and fluid dynamics.
Some master cylinders have these built in already.

 

I have been an instructor over 23 years now and I love this question because what I have learned is most people already have their minds made up before they finish reading someones answer.I sure don't have all the answers but I would think that brakes have evolved thro time and so has the systems.lets just start with a master cylinder and drum brakes as long as your tee fittings are about center then your front brakes will apply first then your back.Now disc front drum back they came with a proportioning valve more pressure to the front less to the back.now disc drum no proportioning valve you will need a 2lb residual valve for the disc and a 10lb for the drum for 2 reasons to hold a little pressure on the brakes and if the cylinder is lower than the calipers it want drain back.As for the air in the system brake fluid is heavier than air so even if it don't leak fluid air can still get in.I would start at the cylinder check the cap look at the rubber gasket you can tell if that is it.Next check the wheel cylinders cup could be leaking and not showing because of the dust boots.Tighten all the lines.Heat sometimes will cause this.That is my two cents spend it on someone checking them.

 

Exactly, that is the basis for a lot of disagreements both here and elsewhere.

 

False, on a 2 circuit master the piston is divided into 2 pistons, a primary and secondary with a spring between them and they're gaped and can slide into one another. The primary piston, the rear one has one piston cup seal, the secondary has 2 piston cup seals. In what ever case, example GM vs FoMoCo, there systems are reversed from each other. Meaning the front line on Gm is front braking and Ford is rear braking. Now the primary (rear piston now) has to first over come any slack. It could be warped rotor's up front or lack of back shoe adjustment at the rear. Until that primary circuit over comes all the slack of pad or shoe spacing, the primary piston will not begin pressurize the secondary piston. If no pressure ever develops on the primary, the gap between the 2 piston's close's causing the 2 pistons to meet mechanically (piston to piston) then allowing secondary piston to apply the brakes only on that half of the circuit.

 

Now we have some meat to go with the pop corn !

 

Yum Yum, bacon fat instead of butter

 

The secondary piston in a dual master cylinder is a floater. Because of that, pressure doesn't build in one one half of the system before the other. Any return spring pressures are too low to be significant.

 

Veach did not define a single or dual master, but simply stated the fronts would apply first with drum brakes, which isn't true, with either a single or dual master, and a tight well bled and maintained system.


With a dual (tandem) master, the secondary pressure rises within a very small fraction of a second of the primary. There is no "slack", as a solid column of fluid is between the pistons. But you're right that the pri. or sec. system pressures may be delayed due to bad adjustment, severely worn pads and/or shoes, before they equalize.

There is no hard rule on what master cylinder circuit goes to the fronts or rears, at Ford, GM, Chrysler, etc. They all have used the primary for fronts and rears, depending on year, application, brake layout, system design, etc. I've had police versions of a popular civilian car line with opposite master cylinder circuits. The main thing to remember is the larger visable chamber on the older masters always goes to the fronts, regardless if it's primary or secondary

 

That large chamber is only there because of government regulations along with D.O.T standards. They system works just the same with small or large chamber. It's designed large because there are people that do not check there fluilds. So, as the caliper displaces the fluid from the top and front brakes are worn, there is still fluid to spare.

Ok, listen, all I'm stating here is safety. Not being a know it all. But there's many people doing these jobs on just a general understanding but don't have all the fact's to understand how it all really works.

 

You lost me with the comment about post #17 and the 180. Please explain.

The larger chamber (when obvious) is always for the front disc brakes, because as the pads wear, the caliper pistons move out, requiring more fluid to fill the caliper. With drum brakes, the fluid level does not normally drop, (except for leaks!) because the return springs always bring the shoes back to their anchors and refilling the fluid reservoir.


Many drum/drum masters have chambers that may look the same, but internally the volume output is not 50/50, because the pistons travel slightly different distances, and is why the outlet port sizes are different, so the lines cannot be switched. Plumbing the fronts to the smaller rear-intended section of the master can result in a displacement issue.
Using common late' 60s-'80s disc/drum masters on drum/drum and disc/drum systems makes correct plumbing easy because of the obvious larger front chamber.

 

7/8 bore wilwood with gm metric caliper on my front discs and 10 in rear drums with proportion valve and residual on the front, will throw you through the windshield and very good pedal feel...

 

Guys, there are many misconceptions about brakes - it comes up on every one of these discussions I've ever read. Guys will argue till the ends of the earth. The deal is simple - are we here to learn or convince everyone how right EACH of us are???

Ebbsspeed, I'm afraid you are wrong here - Bob has it right. Before you guys circle the wagons and throw me under the physics bus would you humor me for just a minute and let me defend my statement??

I would like to quote from HPBooks "Brake Handbook" Written by Fred Puhn Registered Professional Engineer. Page 47 Second to last paragraph, "Residual-Pressure Valves - A cup-type seal has one problem when used with a drum-brake wheel cylinder. As the seal is retracted, it's lip is relaxed and AIR can be introduced in the system. On drum-type brakes, this motion of the cup-type seals in wheel cylinders will draw in air each time the brakes are released.
There are two devices that prevent this. One, called a cup expander, is a thin metal cup installed between the seal and the return spring. This device exerts pressure on the lip and prevents air from passing the seal when the brakes are released.
The second is a simple spring loaded valve in the master cylinder. This valve, the residual-pressure valve, keeps slight fluid pressure on the system at all times - even when the brakes are released, a spring closes the valve when pressure drops to a preset value - usually 6-25 psi. This pressure is enough to keep the cup type seals in wheel cylinders against their cylinder walls, but not enough to overcome return spring force. Excess residual pressure would cause brake shoe drag."

Now ASSUMING I am understanding your position I think the above is in contrast to what you are stating. If I am not understanding what you are saying and the above does not apply then I think it's fair to state that more than one of us are not interpreting what is being said correctly. Much like the statement Bob made about the position of the fitting having no bearing on front/rear pressure - Bob was trying to make a simple point, but his words quickly twisted to become much more broad than what he was trying for (at least that's my impression). I am certain he was not suggesting his comment as a blanket statement - instead a simple example - basic hydraulics.

There are many incorrect statements on this thread - usually followed by "it worked for me" type logic. I know it's Hot Rodding, but something as critical as brakes should be put up to a little more scrutiny than "it worked for me"

Ebbsspeed - please don't take any of this as a personal attack - something it's not meant to be. I'm just a third party with no dog in this fight.

I'm no expert, but I did stay at a Holiday Inn last night.

 

I totally understand what residual valves do, and why they are used. I take no offense. Now please reciprocate and humor me for a minute. The only thing I am saying is not correct is the following statement:

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

I agree with your quotes from the "Brake Handbook", and the fact that, lacking cup expanders or residual pressure, air can and will get into the system due to relaxed cup lips. Now carefully read the above statement again. Under the conditions described in that statement, the cups are not relaxed. If the springs are pulling the shoes together faster than the fluid can move, the result in the brake cylinder is pressure. Anyone who understands hydraulics should come to the correct conclusion that one of two things will happen in the described scenario.

1. The fluid, not being able to move faster than the springs are retracting, will resist the spring's retraction.

2. If the springs do in fact retract faster than the fluid in the cylinder can move, there will be a leak of fluid from somewhere in the cylinder.

Simple illustration, we'll put it in freeze frame. Let's apply the brakes, and then, while they're applied, pinch the brake line to one wheel cylinder so that the fluid cannot move back into the brake line and master cylinder. Now we've got an exaggerated example of fluid that moves slowly, or cannot move. Let's also assume that the brake shoe return springs are HUGE, and CAN retract the shoes, even though there is fluid in the cylinder. What's going to happen? Air is going to get sucked into the cylinder? I think not.

 

Holy crap !!!!! Now just when all the HAMB experts had be doubting my memory and high school auto shop teacher it turns out ((((((after 2 fuckin book quotes!)))))) the internet know it alls don’t know SHIT!

 

And how many time do I have to say it.
I didn’t state this accurately GTF over it already.

 

Ebbsspeed, Fair enough. I see the point YOU were/are making - humor me a bit more as I won't try to pretend I remember everything everyone has written (and admittedly I am a bit too lazy to re-read everything. I was getting the impression that the popular belief here was that the ingress of air past the wheel cylinder seals wasn't possible - which as we clearly agree is incorrect and in fact quite possible. So now the question is HOW/WHEN that can happen. I think we're on the same page - although the typed words are getting several of us mixed up. The statement above may have been better stated as, "The master cylinder pistons can move the system fluid faster than the wheel cylinder piston seals can retract and THAT can induce air into the system."

Are we closer???

 

I realize that, and originally pointed out that it wasn't correct. The someone said that it was, and all hell broke loose.

I'm over it.

 

Yes, agreed. Hope this one is over and done.