Technical 4 Wheel Disc Brake question.

Ok i have some questions for all you brake experts. I am running a under floor manual brake and hydraulic Wilwood clutch pedal ***embly and 4 wheel disc brakes on my 32 project (yea I know its not very traditional, but its what I have and no Im not changing it.) the front are the standard GM disc brake kit like the ones from many of the catalog suppliers, and on the 9" rear it has a SVO Ford style disc brake set up. The M/C is a GM style M/C where both reservoirs are the same size, and I believe it has a 1" bore. its from a under floor Speedway type ( not really sure where it came from) power ***ist pedal set up. The reason I am not running the booster is because there isn't enough clearance with my clutch M/C. and tubular style k member .
So my questions I have are, since the reservoirs are the same size does it matter which outlet I plumb for the front or the rear? The next question is do these M/C's already have built in residual valves or do I need to plumb some into the system and if so what psi is recommended. I have a manual proportioning valve already plumbed for the rear, all lines are 3/16" stainless, and braided flex lines going to the fronts and one to the rear T. I have not called any tech lines because I am not 100% certain which companies have supplied these parts because I didn't buy them, they came with it when I bought it and it was a stalled project since 2007 which had since p***ed through a few hands before I got it. So I figured I would check here before I call Summit, Jegs, or Speedway trying to figure all this out. Thanks for help in advance.

 

Some info on residual valve, if yours has one or not.
https://www.jalopyjournal.com/forum/threads/master-cylinder-residual-valve.433296/
Sounds as if most disk brake applications did not have residual valves, so since your mounting below the floor the 2# inline residuals should work.

 

So looking at these diagrams I am a little confused. My understanding most underfloor master cylinders are mounted in an opposite direction than a firewall mounted one. So my question is if both reservoirs are are the same and I’m running a proportioning valve to the rear does it matter which line goes into which port on the M/C or are they metered differently.

 

Your guide lurks beneath the hood of a GM. (Maybe '65 or newer Vette.)

 

I know what your meaning, but that doesn’t really answer my question completely. I realize that on most GMs or maybe all that the front brakes are plumbed our the port closest to the firewall, and rears towards the front on disc drum applications, but in that instance the reservoirs are two different sizes. My question is if they are the same size what’s the difference, Unless they are metered differently inside the M/C. Or maybe I’m over thinking it

 

Mine are set up like the top illustration. Has worked great for me.

 

You should plumb the front half of the master to the front brakes and the rear to the rears. There's a reason for this; in the event of a master cylinder seal failure, you're less likely to lose full pressure on the front half and as the front brakes do the majority of braking, you'll have a better chance of stopping.

One other thing should be noted; if the rear calipers are floating types, you'll need a short flex hose at the caliper also to allow for free caliper movement/pad wear.

 

So other than a possibility of failure there is no other reason? Not being a smart ***, just curious.

 

You might try calling ECI. IIRC it is Ralph, he will answer your question, even if you didn't buy your stuff from them. But you should figure out exactly which calipers you have. The rears are probably like Explorer. The fronts could be Metric or earlier.

 

In most cases, no. There's a few systems out there where it does make a difference though. Jaguar for one uses a two-piece piston in their masters with a spring-loaded gap between them. When applying the brakes, the rears get pressure first (enough to lightly apply the rear brakes) before the rear master piston contacts the front piston to apply the fronts. This function on most cars is done in the factory 'proportioning' valve with a pressure delay valve. These do need to be plumbed with the delay going to the front brakes whichever 'end' of the master you use. This is a small safety feature.

But it makes sense to plumb the master front/front and rear/rear for safety. Think about it; if the rear seal fails, it vents pressure directly to atmosphere, no rear brakes but you still have the fronts. If the front seal fails, it vents to the rear seal. You have a loss of pressure, but will still have some available.

Of course, if both seals fail you're SOL....

 

Yes the fronts are the metrics, the rear from what I was told are like the ones on SVO mustangs, and I’m sure others.

 

I get the safety factor, I talked to ECI and CPP with a GM Vette style MC where the reservoirs are the same the rear outlet closest to the flange goes to the front and the front to the rears, and the explanation I got was because that’s the way the factory did it. Other than that no real explaination. Now I get it with the M/C with 2 different size reservoirs, but still no real explaination as to why one vs. the other when they are the same size. So I guess my questions is answered kinda hahah!

 

I am impressed by the reason, of chain-of-failure, so that front calipers are energized even if the first seal fails.
As to your reservoir dimens, all it may mean is you have similar reserve fluid for each axle. Consider; Lg piston, 2, 4, 6? piston variations, req. more reserve.

 

Now that makes more sense than because that’s how the factory did it. But if both reservoirs are the same size volume wise then technically it wouldn’t matter which port is used if I’m thinking correct. It’s nothing more than my curiosity at this point. Well that and it would make the way I wanna run the lines look better, but once the body is on your never gonna see it!

 

If you are certain the master cylinder is 50/50 in operation, the fronts can be plumbed to either the primary (closest to the mounting flange ) or secondary port. This is important because reservoir size may look the same but actual fluid volume output ( pri/sec piston strokes) may not be the same.

A few cars came from the factory with the rears plumbed to the primary; some late '60s/early '70s GM intermediates, and on some Fords I tested over the years. The only masters I know of that have to have the fronts plumbed to the primary circuit are those with tip-style screw-in proportioning valves (A front system failure allows the primary piston to "tip" the prop valve, byp***ing any proportioning and providing full master cylinder pressure to the rears)
I don't remember the reason most vertical split masters are plumbed primary front, but I will bring this subject up the next time I get I get together with some retired brake engineers I worked with and hopefully have an answer.

One thing to note is your pedal ratio. Typical power ***ist ratios are around 3:1, while manuals are +/- 6:1. You may have to alter your pedal ***embly.

 

Steve, I don't know of any dual masters that do not have two separate pistons. That's the whole purpose of a dual master; two independent isolated hydraulic systems. If a seal fails, that system is down, period.
The front "hold off" or "metering" function is achieved with a +/-100 psi spring-loaded external valve.
There is no "atmosphere" in the brake system, because it's sealed. The piston simply moves through the fluid with a failed seal.

 

Now we are getting somewhere, the pedal set up I am using is a manual brake ratio. Thanks for making some sense of this for me.

 

Bob, you misunderstood me. Yes, all dual masters have two pistons but most are machined onto a single common shaft. Press the brake pedal, and both pistons move the same distance in the bore at the same time.

Jaguar masters are different in that the front/rear pistons are two separate pieces with a spring-loaded gap between them; no actual mechanical connection. Press the pedal, and the 'rear' piston moves first, the front doesn't move until the rear has moved enough to close the gap and contact the front. This action supplies the 'hold off' function, but internally rather than a separate external valve. Because of this, Jaguar also didn't use an external pressure-differential brake warning light typically found on most dual-systems because it would 'blink' each time you applied the brakes. Their 'brake warning light' was nothing more than a 'low fluid' light (which is another whole story...).

As to the 'venting to atmosphere', if the seal closest to the mounting flange fails, doesn't it vent to atmosphere? I know I've pulled bad ones off that were leaking fluid out of the bore, I've even seen one that leaked enough that the vacuum on the power booster 'vacuumed' all of the fluid out of the rear reservoir.

 

The primary piston has two cups; the forward primary cup creates the pressure by closing off the comp port, while the rear secondary cup seals off the rear master bore and just "moves" fluid. A rear secondary cup failure will allow fluid loss at the mounting flange, but no pressure loss....well, until the primary fluid reservoir is empty.

 

As I understand it the front brakes always apply before the rear. This is built into the master cylinder piston. In my 35 Ford I plumbed it that way using the port farthest from mounting flange as the front. I've never seen any modern car master plumbed differently and wouldn't reccomend it if the master wasn't specifically designed to work that way.

 

There is nothing in the master that dictates which apply first, in fact with disc in the front and drums in the rear the metering valve delays the front brake application to allow the rear to overcome the spring pressure then the proportioning valve controls front-rear bias from there.


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Fwiw,
True, but your diagram does not show this, - if you look closely.

 

This may help.

 

BJR. Thanks. It doesn't get any simpler than that. But, I'm positive that someone will disagree.

 

Great diagram, thanks for sharing this.

 

I disagree with the long stroke/ short stroke diagram, unless they are different diameters both pistons move the same distance.


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Correct, the diagram doesn’t really show the direction of the master cylinder mounting that’s why I was asking.

 

Just my observation....I was bench bleeding my M/C last week that has the same size bowls and I noticed the bubbles started and stopped at different times between the two bowls. My ***umption was the “front” (the bowl that was closest to my booster) supplied fluid first as designed from the factory. Now reading this thread and seeing the diagrams it would appear my ***umption was correct.

Couldn’t the stroke distance and start/stop duration be caused the tension(s) of the springs?

To the OP of this thread: SSBC supplied the all aluminum GM style M/C and I would suggest replacing yours if it’s sat for 7 years...mine had rotted the seal and I could see fluid on the booster.

Ben


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No the springs are to help piston return, if the area for hydraulic fluid to act on is the same size on both pistons they have to move the same distance, provided there is no air anywhere. They supply fluid at the same time, the instant the primary seal on the primary piston covers the vent port, which is almost instantly, there is pressure built which starts to move the secondary piston, there is no discernible difference in which applies first.

In fact the bottom diagram posted the larger reservoir would be for disc the smaller would be for drums.

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When I plumbed up my setup I wasn't sure which way was correct. My thinking was that because I was using the GM metric front calipers I should do it the way Chevy did it. This is how I connected the brake lines using a Corvette style MC, four wheel disc brakes (GM metric front calipers and Ford calipers on the rear) and MC under the floor. This is from the '69 Chevrolet ch***is service manual.