Posted by Jesse Ybarra
on 10/5/2009, 20:05:45, in reply to "Re: Dual Master Cylinder Conversion"
64.12.117.15
In an attempt to prevent a lot of confusion about why my personal needs of my M-679, require me to not use a typical proportion valve, I left out that detail, because I have different needs than other FCs. I have built the M-679 to have available, a large amount of braking capacity that is totally unnecesary for the average FC truck.
Jeff, thank you, it is good you asked, because it needs to be well understood, that this system, is not for everyone.
I will now try to clarify my design, I am not an automotive engineer, to try to engineer or recommend for anyone else, a better way to stop. I am only doing what works for me.
Here is how I arrived at my design:
I believe that when doing a panic stop at freeway speeds, the standard 79-85 350/350, 3/4 ton Chevy pick up tires, have about 60% front weight / 40% rear wieght, with discs in front and drums rear, the front brakes clamp on the rotors and the rears expand shoes on drums, they were made with a factory engineered, non adjustable braking system, including self adjusting rear drums, these 4,000+lb trucks require a proportioning valve to delay the rear brake pressure, to prevent excessive skidding.
An average stock FC-150,170 3,250-3,750lb, has 80% front / 20% rear weight, with new, adjusted brakes they can panic stop, skid the rears, at boulevard speeds, today's freeways, were not around when these were made, actually very few paved roads kept them safe, with 11" drums, for the most part, I think the OEM brakes work OK and are safe, in most stopping, for a truck of it's age, except on a paved steep, oily downhill or ice, it is not unusual to be in a spin during a panic stop.
My M-679, has unusual, to an FC, stopping needs, Original wieght on an original M-679 Ambulance, with the Cerlist, tiny T-90/D-18, was 4750lb empty, The M-679 has the added weight of a larger heavier than a pick up, rear body with rear doors and framing.
Additionally I have the added weight of:
a full diamond plate floor,
heavy gauge upright columns,
full complete rollcage,
additional heavier springs,
14 bolt disc brake rear,
10 bolt disc brake front,
16.5x10 Alcoas,
very heavy Goodyear MTR 37" E rated 10 ply tires, the replacement body panels that were hand formed are 20 or 18 gauge,
the frame has additional reinforcement,
all of the inside and underside of the bed, has all been done with bed liner coating, plus undercoating on the frame and underbody.
At this point without the engine /trans, it is heavier in the rear than the front.
The new built 434" SBC engine was moved back, mounted behind the front axle, with the use of the heavy duty rebuilt 6.2 H1 mechanical 700r/np208, making the vehicle weight almost 50/50.
Around 6,000lb.
I left out the explanation of the proportioning valve, because I don't use a typical one as found on a GM, it has no adjustment, this is why I chose the system, that I am running, by having 2 separate Wilwood master cylinders, that can be individually adjusted, to make exact amount of adjustment between front / rear.
Because I chose the 1 ton larger size of the rotors, calipers and lines, I feel comfort knowing, that I can totally fine tune my braking needs, while driving, by use of pressure gauges and adjusting the sequence of master cylinder shafts, Additionally, I have an inline Wilwood pressure adjustment valve, that is adjusted while driving, to adjust and create a balanced bias between the front and the back calipers. This pressure valve can be increased or increased, by manually adjusting a knob, for example while carrying a heavy load or trailer. Instead of having a preset non adjustable proportion valve, I am able to increase or decrease pressures mechanically as needed. New medium duty trucks have a rear frame to axle mounted lever that changes the pressure requirement, as the bed goes down from additional weight, a signal is sent and the anti-lock brake computer sends a signal to give more equal pressure.
The need for more stopping is that the tall 37" tires are turned with 4.10 gears, with a 4 speed overdrive, is very different from a conventional pickup for example that would have discs in front drums in the rear. Imagine, at freeway speeds it will be almost coasting when the gas pedal comes up on this application, I need all 4 brakes to be working equally at the right sequence, normally on a typical vehicle, an engine slows down the truck, then the fronts, then the rear are used for braking. With this high gearing I immediately need healthy brakes.
This is my best explanation of a factory GM 74-86, (this is not related to the newer ABS) proportion valve:
Proportioning valves are used with a single casting master cylinder or with a dual single master cylinder, that is one housing one shaft pushing two different pistons at the same time, typically these housings have dual outlets one 3/16" for the rear and one 1/4" for the front, these typically go into the inlet side of the proportion valve, from the name proportion valve, it would seem by definition that the proportioning valve actually has an ability to adequately be able to proportion or equalize the pressure, instead what the proportioning valve does is divides the pressures to create a delay making the front brakes have pressure first then the rears, discs in front to have maximum pressure, then the drums catch up as the vehicle is already slowing down preventing the rear brakes from locking up.
I tried to be specific enough to explain what I did, but it is for my application, as Craig originaly said, there are other master cylinders and sizes, avaliable.
I think Craig's and my enthusiasm, here is that both the Wilwood racing products or Tilton machine people really do have a good master cylinder product line, these are new and very available, thru Speedway, Summit, Jeggs and most Dirt track racing shops. These are new master cylinders, (not 50 year old rebuilts) that can be easily mounted as a single or dual application, that can be mounted under the stock FC dash, with stock FC brackets, that fits neatly without sticking out past the dash, without cutting anything.
The only draw back is the fact that you can run a remote resevoir, rather than having all that fun, to pull out the little hose thru the dash plug to check the fluid by filling it, until it spills.
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