Probably one of the most talked about subjects on the HAMB is brakes. "Why don't my brakes work". "How do I know what master cylinder to use". "What is this funny valve do?" So I thought being tech week and all I would throw together a little write up on some brake science and general information. Sound good? Well I'm doing it anyways!!! First we will start with the master cylinder. At the beginning of hydraulic brakes we had a single pot master cylinder, one hydraulic system in order to operate the brakes. The beauty of hydraulics is the ability to multiply force, we will get to that part in a while. Above is a diagram of a simple master cylinder, what happens when the brakes are applied is the primary piston is pushed forward by the pedal pushrod, as soon as the primary cup seal covers the vent port, about 0.030", there is pressure being created in the system. As seen below. Hopefully our vehicle comes to a stop at this point, and we are able to release the brakes. Upon brake release the return spring pushes the piston back. as the piston is pushed backwards a low pressure is created in front of the piston causing the cup seal to collapse letting fluid behind the piston, being supplied from the reservoir through the replenishing port, to travel either around the piston or through bleed holes in the piston to in front of the piston allowing it to return quickly. Any fluid still returning from calipers or wheel cylinders can now return to the reservoir through the vent port. Now this is all fine and dandy until we spring a leak in the hydraulic system somewhere. One of Pascals laws says the pressure in a closed system is equal and undiminished in all directions. In other words spring a leak and you now have no brakes. Not good.... So around 1967 tandem master cylinders became mandatory as a safety precaution. A tandem master cylinder is two separate hydraulic systems in one, for our use in HAMB land they are split front/rear, the front is a separate system than the rear. As we can see in the above picture there are now two of everything, the primary side being the closest to the firewall and the secondary side being closest to the front of the vehicle, regardless of how the brakes are plumbed. When the brakes are applied the primary piston is moved ahead by the brake pedal pushrod and once the primary cup seal covers the vent port pressure is being created ahead of the primary piston. It is that pressure that hydraulically applies the secondary piston, once the secondary piston covers it's vent port then pressure builds in the primary system. Remember its about 0.030" of piston movement to cover the vent port, in other words pressure is built in both systems virtually instantly. Now the beauty of a tandem master cylinder is if one system springs a leak the other will still operate. If the primary system springs a leak then there can be no pressure built in the primary system so the primary piston now contacts the secondary piston mechanically so it can still apply it's brakes. Like so: There will be increased pedal travel but we will have half the brakes still operate. If there is a leak in the secondary side then the primary will still work and still apply the secondary hydraulically and the secondary will bottom out against the end of the bore. Again, increased pedal travel but still half the vehicle brakes operating. Like so: Brake release is the same as a single pot master except both are doing it at the same time. Much more to come.......
Ok, now that we have covered master cylinder operation we will move down stream. Depending on whether its drum/drum or disc/drum there can be different valve combinations. There may be residual valves, typically used with drum brakes but also with discs if the master is lower than the caliper/wheel cylinders. A residual valve is essentially a one way check valve, when the master is the lowest point the residual valve keeps a very small amount of pressure in the brake lines to prevent the fluid from running back to the master cylinder, typically 2lbs with discs and 10lbs with drums. The next use for residual valves is with drum brakes no matter the master cylinder location. we need them with drum brakes as upon brake release the drum return springs cause the wheel cylinders to be retracted quickly and will actually cause a low pressure in front of the cup seals causing them to collapse and draw air into the hydraulic system. Not good. The residual valve keeps 10 psi of pressure trapped in the brake lines to keep the cup seals in contact with the wheel cylinder to prevent air entry. 10psi WILL NOT, in anyway, have anything to do with speeding up drum application, the return springs are way stiffer than that. The residual valves are usually located in the master cylinder outlets. Some where along the way, someone said "hey, there's another way to prevent air entry in wheel cylinders." Thus cup expanders were born. The cup expanders keep the lips of the seal against the wheel cylinder to prevent air entry. They can either be a separate piece clipped to the spring or the spring can be wound into a cone shape on the ends. Next will likely be a pressure differential valve, its job is to alert the driver of a difference in pressure between the primary and secondary systems. When there is a difference in pressure the piston shuttles over an contacts the switch contacts, completing the circuit, turning the light in the dash on. Despite the popular belief I have not seen one that blocks fluid flow, nor have I been able to find actual, published, documentation that any of them do, I'm not adding that to create a debate, strictly for information. Their purpose is NOT to shut off the leaky circuit for safety, that's why we have a tandem master cylinder. Next in line if we have a disc/drum system is a metering valve, the metering valve is in the front circuit to delay disc brake application to allow the pressure in the system to overcome the return springs on the drums to allow them to start to apply at the same time as the discs to prevent a nose dive situation. Somewhere around 2-20 psi they are blocking pressure from getting to the calipers until the pressure gets to somewhere around 70-300 psi. The operating pressures depend on vehicle weight etc. Now we will likely have a proportioning valve in the rear brakes. It's purpose is to slow the pressure rise to the rear brakes to prevent the drums from locking up due to the self-energization of drum brakes. Simply put is the drums do not rely strictly on wheel cylinder force to apply the brakes but rather are somewhat self applying, whereas discs rely strictly on clamping force to apply. In the above chart we can see the metering valve action delaying the front brake application then both system pressures rising together until the split point or the point the proportioning valve starts to slow pressure rise to the rear brakes to provide balanced braking to prevent the rear from locking up first. To reduce manufacturing costs and less leak points most manufacturers combined the pressure differential, metering and proportioning valves into one piece called a combination valve. The above picture is the brake system with the various valves at rest, no brake application. Now we see initial brake application, the metering valve is closed, pressure is rising equally in the rest of the system. The metering valve opens and the pressure is the same in the entire braking system. As the pressure rises past the split point the pressure going to the rear brakes rises slower than the rest of the system. The split point is dependent on vehicle weight etc. as well. Hopefully understanding what the various valves do we can see why mixing and matching parts in a vehicle that has completely different dynamics than the vehicle they were designed for can result in a "why doesn't my car stop" situation. Much more to come still..........
And space for even more brakes!! Ok now onto the math and science part of our brake systems. I am not going to get into caliper and wheel cylinder operation as I think they are pretty straight forward, instead I am going to focus on the relationship between input piston (master cylinder) size, (foot) force, system pressure, and output piston (caliper/wheel cylinder) force. I mentioned above that Pascals law states "pressure is equal and undiminished in all directions in a closed system". That means that if one brake line is a foot long and another is ten feet long, the pressure is the same at all parts. A brake system is a hydrostatic system, meaning virtually no flow, the fluid acts as a solid to transmit force, remember fluid cannot be compressed, the reason its so important to get ALL the air out of the system. In a typical brake application there is less than a teaspoon (5ml) of fluid "flow", only force transferred. here comes the math!!! (sorry) An easy way to remember the needed formulas needed is called a tradespersons triangle. F = Force P = Pressure A = Area Now to get the needed formula you simply cover the variable you want to solve for, for example if I want to find the force I would cover the F and be left with P and A or F =P x A. If I need to figure out pressure I cover the P and am left with the F and the A or P = F/A therefore the last one would be A = F/P. Simple right? The triangle works for Ohm's law as well but I'll leave that to @Crazy Steve. Lets look at a simple brake system... Now to apply our formula we need to figure out the area of the master cylinder bore and the caliper piston. Think back to junior high and A =Pir² Therefore the master cylinder has a bore of 7/8 so the radius is half of that, or 0.4375". A = 3.14 (0.4375x0.4375) = 0.6010"² I rounded for easy math but you get the idea the more decimal places you use the more accurate it will be. The caliper is 6.7737"². With the 80 lbs of input force on the master cylinder area of 0.6010"² we can figure out the system pressure. Using the triangle above our formula is P = F/A or P = 80/0.6010 so P = 133.1115 PSI. Now lets see what happens when we push harder on the pedal, lets use 100lbs P = 100/0.6010 or 166.3894 PSI, we can conclude that pushing harder creates more pressure DUH!! Now lets change the master cylinder to one with a bore of 1" or 0.7850"² now we get 101.9108 PSI with 80 lbs of input force. So now we can conclude that increasing the input piston size will lower system pressure therefore requiring more input force, It is also safe to say that decreasing the input piston size will increase system pressure without increasing input force perfect right? The reduced input piston requires increased travel to displace the fluid needed to accommodate the caliper/wheel cylinder travel as seen below: So using the original 7/8" master cylinder with the 80lbs of input force we created 133.1115 PSI, lets see how that translates to what happens at the wheels. In the front we have a caliper piston area of 6.7737"², using pour triangle we know our formula is F = P x A or F = 133.1115 x 6.7737 = 901.6574 lbs of force. Quite a return on our initial 80 lb input. From this it's safe to say that increasing output piston size will increase the output force as well as increasing pressure will also increase the output force. We can even add more pistons! Forgive the metric units we are a metric country up here but the principle is the same. Lastly the rear output force would be F = P x A or F = 133.1115 x 0.7854 so F = 104.5458 lbs. The same applies to the wheel cylinder in that a larger diameter will provide more output force. How the hell can we have a balanced braking system when the front output force is almost nine times as much as the rear? Well to simplify it disc brakes relay strictly on out put force to clamp the pads against the rotor to stop the wheel (the brakes stop the wheel not the car) whereas drum brakes are self energizing and actually generate their own braking force through self energization. Above is a Leading-Trailing drum brake assembly, the Leading (front) shoe is self energized while the Trailing (rear) shoe is relying strictly on wheel cylinder out put force. Above is a dual-servo drum brake assembly, servo acting means one shoe acts on another, dual-servo simply means it happens in both directions. Notice the primary (front) shoe is shorter and generally softer or "grabbier" friction material than the longer and thicker Secondary (rear) shoe. The self energization action is drum brakes is the reason for less output force in the rear, this self energization can also generate tremendous braking torque however the disadvantage when compared to disc brakes is they can be less predictable and prone to brake fade due to increased swept area and being enclosed. this concludes my brakes 101. Any questions? Test next Tuesday!!!
As a professional mechanic (years) I had to 'recap' lessons to young 'techs' after they returned from school. This supersedes most of my prepared 'sermons', Bravo to Bruce for a fine collection of total brakedom. Thanks! I can now pass this on to some who 'missed it' along the way...
This thread is a condensed version of my brake classes for our first year apprentices. Sent from my iPhone using H.A.M.B.
Well done! Reading this takes me back to Technical College when we learned about this stuff. Twenty eight years ago, wow I feel old! I love to tell people that I have a college degree in this stuff when they ask: Where did you learn that?” Sent from my iPhone using H.A.M.B.
Cool tech......and yes in Cali you had to be a state certified for brakes......have my grandpa's license around somewhere.
I ran out of ambition today to add to it, it’s supposed to rain all day tomorrow, more to come Sent from my iPhone using H.A.M.B.
Updated!! I could go on for days so I had to stop somewhere.... If someone has brake related questions I can try to add it to this thread. Thanks for reading and hopefully it helps at least one person.
Great Tech! Can you comment on the importance of appropriate brake line sizing-to-fluid volume for front calipers to rear drums? Sent from my iPhone using The H.A.M.B. mobile app
Thanks, really the line size isn’t very important, remember once all the air is out of the system it’s pretty much a static system. The fluid is just there to transfer force. The only fluid displaced is the fluid required to fill the space behind the caliper piston and/or the wheel cylinder pistons when they move out to apply the brakes. It doesn’t know what size the line is. Most factory stuff is 3/16” or 1/4”. If I’m plumbing one from scratch I typically use 3/16” Sent from my iPhone using H.A.M.B.
Two comments. Most proportioning valves should more correctly be called pressure limiting valves, generally preventing fluid pressure from rising above a pre set point. If in doubt, check with a pressure gauge. Distribution blocks with a shuttle valve brake fail switch will often shut off the failed side. Working in a brake shop it was not unusual to get a call from a customer that had triggered the valve while bleeding the system. Common on European and Asian systems.
I won’t speak for Asian or European stuff, not my wheelhouse, but I’ve yet to find by published service information that references blocking fluid when shuttled over, again why have a tandem master then, I’m not saying they don’t exist, I have yet to find anything that says they do. Proportioning valve is the industry standard term and again I haven’t come across one that only allows pressure rise to a preset point, the pressure rises at a slower rate after the split point. Also I have checked them with a gauge, we have gauges mounted permanently on a couple of out trucks to watch the valves in action so to speak.
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