|The MGA With An Attitude
NON-RETURN VALVE in the Master Cylinder - HT-116
At 12:37 PM 2/5/2009 -0600, Bill Young wrote:
>>"Is it just a choice of terms or is there a fundamental difference between a non-return valve and a residual pressure valve"?
It is a huge difference. I hope you like long an detailed explanations as I anticipate some of your questions, and maybe this can reduce the number of email exchanges.
In the MGA (and most vintage cars) the "non-return" valve operates similar to a check valve in that it allows free flow in the forward direction and restricts flow in the return direction. It is not a check valve because it will never completely stop the return flow. It is not a residual pressure valve because it will not retain residual pressure indefinitely. The return flow function of the "non-return" valve is essentially a small open orifice. It simply slows the return flow. As the return flow is finished the "residual pressure" drops to zero, because the restrictor orifice is always open to allow free return of fluid to the reservoir. It only slows the return flow so it takes a few seconds to fully return, thereby allowing you time to do successive pumps of the pedal to pump up the brakes in event of a low pedal condition.
So, within the first few seconds after you release the brake pedal there will be a small amount of "residual pressure" in the hydraulic line This small return pressure exists specifically because of the springs on the brake shoes pushing back on the fluid. This is after all the only force that makes the brake shoes release and the fluid return. The returning fluid flows slowly through the "non-return" valve reverse flow orifice because the orifice is a flow restrictor and the pressure is low (being a function of the return springs on the brake shoes). Once the brake shoes have returned to their mechanical rest stops, the shoes stop moving, the fluid stops flowing, and the "residual pressure" drops to zero, because the orifice in the "non-return" valve is always open to allow fluid to return to the reservoir.
The term "non-return" valve is a bit of a misnomer, as it should more appropriately be called "slow-return" valve. But in the short time it takes to lift your foot and give the pedal a second pump, it certainly does prevent fluid from returning. More correctly, it prevents "much" of the fluid from returning as it does allow a little fluid to return in that short period of time before you hit the pedal again. But keeping most of the fluid in the line for the short time allows you to "pump up" the brakes (while the function serves very much like a check valve).
The important point in response to your question is that a few seconds after you release the brake pedal the line pressure goes back to zero, and there is no residual pressure. The residual pressure you might measure during that short time is low and fleeting.
So far my comments have all been about springs and brake shoes of drum brakes. Disc brakes in fact act in a similar manner in retracting the brake pads, even though there are no "springs" involved. They can do this by virtue of elasticity of the "O-ring" seals in the caliper cylinders and friction between the seals and the pistons. When the pistons are pushed forward they slide through the O-ring seals, but friction pushes on the seals to deform the seals like a rubber spring. When the hydraulic pressure is released the o-rings return to normal rest position, pulling the piston back a tiny amount in the process. The brake pads can then "float" on a thin cushion of air drawn in between the rotor and pads, so the pads do not drag and wear when brakes are not in use. The process of the O-rings drawing the piston back forces a small amount of fluid to return through the lines, very similar to the springs on drum brake shoes. This fluid return is subject to the same rules (physical laws) as for the drum brakes.
When disc brakes are new there is a substantial amount of piston and fluid return, enough so you notice some pedal travel on first application of the brakes. Over time and use and heat cycling the O-rings in the calipers go hard and inflexible, essentially changing from soft rubber to hard plastic. When this happens the pistons no longer return, and the pedal has shorter travel, and the pads drag constantly on the rotors causing heat and wear and bad fuel economy. The cure is to rebuild the caliper cylinders to install new O-rings.
>>"As I understand both keep a few psi of pressure in the lines after the brake is released,"
Uh, no. The non-return valve only holds a bit of residual pressure for the time that the return fluid is flowing, but that small return pressure drops to zero as soon as the slave cylinders have returned to normal rest position. There is no residual pressure that would hold the brake shoes or pads forward. This is why the brake shoes must have adjusters to bring the shoes up close to the drums, so there will not be intolerably long pedal travel.
>>"and any pressure in the lines would both enable you to pump up the brakes"
>>"as well as keep the springs from collapsing the shoes completely as well".
No. The non-return valve does not do that.
>>"The disc brake cars should have a lower tension spring with less line pressure than the drum brake cars".
Simple disc brakes have no springs at all, not anywhere between the pads and the master cylinder reservoir. The "return spring" effect only comes from flexibility of the piston seals in the calipers.
>>"In the US disc applications are usually around 2 psi and drum brake systems about 10 psi according to the information about the Wilwood valves sold by Speedway".
I just checked the Wildwood web page here: http://www.wilwood.com/Products/006-MasterCylinders/010-RPV/index.asp
and that's not what it says. It says nothing about standard or stock applications on production cars. These are strictly special application after market parts. I see two different applications here, disc and drum brakes, as the functions are entirely different.
The drum brake valves are proposed to hold the shoes essentially in light contact with the drums at all times. That will achieve the stated purpose of minimizing pedal travel, but it will also make the shoes drag on the drums creating increased wear and heat. That might be desirable for some forms of short term competition use, but no one in their right mind would want the brakes dragging continuously during a long road trip. The way drum brake return springs work there is a hair line difference between shoes dragging or returning completely, and nothing in between. Different forms of variable friction in the system will keep moving that transition point so there is no way to pick a fixed residual pressure that will reliably hold the shoes near the drums without dragging. The intent of the residual pressure valve then is to keep a little more residual pressure in the lines so the shoes always drag a little rather than always retracting fully. I am absolutely sure that 10 psi residual pressure will keep the shoes in contact with the drums at all times (and dragging). The drum brakes would require constant maintenance and frequent change of friction linings.
The 2 psi valves are intended to prevent fluid run-down in race vehicles cars where the master cylinder is located lower than the brake calipers. This might be useful (maybe) in certain competition applications. I do not know of any production car that would have the master cylinder lower than the brake calipers. Furthermore, the application to a race car is also suspect. Function of the 2 psi RPV is predicated on the fact that the returning caliper pistons will generate somewhat more than 2 psi return pressure (otherwise the pads would always drag). Suppose that the return flow pressure from the caliper seals and pistons is maybe 5 psi (minimum) to make the fluid return through the RPV with 2 psi back pressure. Once the pistons return far enough so the pads are no longer dragging, and the O-rings (piston seals) are approaching their normal rest position, the residual pressure can drop to 2 psi and the return flow will stop, retaining that 2 psi residual pressure. So far it's a good story, but what would happen if you left the RPV out of the system?
Recall that the caliper piston seals have enough friction with the piston to pull the piston back and generate at least 5 psi return pressure for a short distance of piston travel. It follows then that it should require at least 5 psi pressure differential in the reverse direction to upset the piston and allow more return flow so the piston would withdraw further. For this to happen as a matter of gravity acting on the fluid, the master cylinder would have to be about 10 feet lower than the caliper to produce the 5 psi pressure head (or partial vacuum at the caliper). So that's not an issue here.
Then we get to the idea of vibration acting on the caliper to make the piston move. If the piston is say just 2" diameter, the face area of the piston is 3.14 sq-in, and the force needed to move the piston through the seal is 5 psi x 3.14-sq-in = 15.7 pounds. If the piston weighs about 4-ounces (1/4-pound), then the acceleration required to produce that much force on the piston is 15.7 / 0.25 = 62.8 G's. If this is due to vibration then the amplitude (peak to peak distance) of that vibration has to exceed the total elastic travel distance of the piston seal before the piston slips through the seal. This is not what we normally think of as vibration, but more like getting hit with a hammer. Also that force is applied in a horizontal direction, the direction of travel of the slave piston, not the vertical direction of the suspension travel. The only application I can imagine that could do that is an off road vehicle running rough ground at high speed, constantly hitting ruts and rocks the would displace the front wheels in a lateral direction (the direction of steering). And on that note, I can hardly imagine such a vehicle having a master cylinder mounted lower than the brake calipers.
The more common application of a low mounted master cylinder would be a minimally low slung race car running on generally smooth pavement at very high speed. The master cylinder(s) is likely to be mounted directly in front of the driver's heels a few inches above the road surface. Indy racers and F1 cars generate about 5 G's going fast around corners, and they may be subject to some lateral shock loads and fairly high amplitude vibration, but it's still hard to imagine the slave piston shaking loose from the friction of the seal. That might require some sort of odd fluid dynamics action that would cancel out the friction at the piston seal.
When disc brakes were first applied to MGA Twin Cam in 1958, and later to the MGA 1600 in 1959, there was a little mechanical friction device inside of the caliper cylinder that was intended to prevent the piston from long travel return during moderately long periods of non-use, like a minute or so flying down the Mulsanne straight at top speed at Le Mans, or an hour of driving on a straight road without using the brakes. By end of MGA production in 1962 the factory realized this device was not needed, and the MGB to follow didn't have it. People rebuilding MGA brakes commonly leave these parts out of the caliper assembly to no ill affect (and have been doing so for decades). To my knowledge, probably no car with disc brakes built since the mid 60's has such a device. By comparison, if some production cars did once have a residual pressure valve, the manufacturers would have eliminated it decades ago as an unnecessary part and a cost reduction.
>>I know that if you are converting a drum brake master cylinder to work with discs for an American car you need to remove the check valves or you will get excessive drag on the front discs.
I'm no expert on the MGA, so asking you your opinion on this".
That's an interesting "info" statement on that page, but I don't believe it. Either the brake shoes drag or they return to normal rest stops, and nothing in between. The brake shoe springs are fairly high force but relatively low spring rate compared to long travel (lots of stretch for preload). There is very little difference in spring force from the point where the shoes touch the drum to the point where the shoes return to the rest stops, so it would be nigh impossible to select a fixed residual pressure that would catch the shoes near the drums without actually touching the drums.
I can think of a spring mechanism that might allow this to work, but it would be nothing like normal long travel brake shoe springs. The shoe return springs would have to have a very high spring rate so the return force drops off quickly while the shoes return only a short distance (similar to the piston seals in the disc brake calipers).
In any case these Residual Pressure Valves have absolutely nothing in common with the master cylinder non-return valve. They serve an entirely different and unrelated function.
Incidentally, this RPV is nothing more than a common pressure relief check valve (free flow in one direction) often used in hydraulic machine control circuits. At first glance it struck me as very familiar, as I have recently been looking at in-line 2-psi pressure relief valves (with no return flow), like this:
This is a function used in medium pressure fuel injection systems common in modern cars to maintain constant pressure on the fuel manifold for the fuel injectors (around 40 psi). The fuel pump is higher pressure and higher flow, and the excess (unused) fuel goes past this relief valve to return to the fuel tank.
I want to use one of these relief valves in similar manner at 2-psi on the MGA carburetors, and allow excess fuel flow to return to the fuel tank. In the process I would wrap the return fuel line several times around each carburetor float chamber using the return fuel flow to cool the carburetors, and using the fuel tank as a heat sink and radiator. I have been dieing to try this on the MGA to eliminate vapor lock in hot weather, just haven't gotten around to it yet.