Dual-pot master cylinder, false sense of security?

[mrriggs]

The dual-pot master cylinder is an ingenious device. The secondary master is hydraulically driven from the primary so that pressure remains equal despite different flow requirements from mismatched front and rear brakes. If the secondary fails then the piston will bottom out in the bore which allows the primary to still build pressure. If the primary fails then it will bottom out on the secondary piston so the secondary will still build pressure. It all sounds great, in theory.

Is it just my experience or does the theory not hold up to actual practice? Thinking back to all the times I've had brake failures, in every case the pedal went to the floor and I had no brakes. Not once did only half of the system stop working and the brake light in the dash come on.

Has anyone had a text book failure where one half of the system goes out, the dummy light comes on, and you are able to make it home safely on the remaining half that is still functioning?

[mcfairmont]

I have always found the same results. Realisticaly, if a dual chamber master cylinder worked as many describe, how come when you bleed the brakes, either front or rear, the brake pedal will go all the way to the floor when you open one bleeder? Shouldn`t the second chamber "kick in" to prevent that from happening? I work at a Toyota dealer, and we ocassionally get PUs and 4Runners come into the shop with a rusted thru brake line, and even with 1 chamber of the master cylinder full of fluid, the brake pedal goes all the way to the floor. Same with supposedly self adjusting drum brakes. Not uncommon for a new car come in for its first oil change, and the park brake lever is hign, and you can adjust the rear brake starwheel 15 or 20 notches before you get a good pedal and park brake lever. Works in theory, but not so much in real life.

[mcfairmont]

I have also found the same thing with engine clutch fans. They are supposed to have no or very low drag when the engine is cold, but as the engine gets hotter, should get tighter, and increase the speed of the fan blades. However, I have always found the fan blades much stiffer to rotate when the engine is cold, usually accompanied with much fan noise, but when you shut down a hot engine, the fan blades will continue to spin for a while after the crankshaft stops, and the fan is very easy to rotate when hot. Kinda backwards from what is supposed to happen.

[83GTJIM]

Funny. I have thought of this same thing recently. Back in 83 I was 16 and working at Boy Scout camp outside of Moberly, MO. We had recently purchased a 1968 Mustang and I got to take it with me for the summer as we spent half the summer in one camp, broke it down and them moved and had a second session of summer camp in Sunrise Beach, MO. There was a section of road leading to the camp that was gravel before getting on blacktop. I was driving with a couple of other camp counselors in the car and as we approached a T intersection I went to step on the brake and it went straight to the floor. As a new driver with less than 6 months of experience I will never forget that feeling. I told my passengers that we had no brakes but they thought I was kidding. I looked ahead trying to see if any cars were coming from either direction on the intersecting black top road. This was a very rural area and we were lucky, nobody coming. With my inexperience all I could think to do was to shift it to neutral (C4 auto). As we got up closer to the T intersection I could see there was a small gravel road on the other side. We shot across there, me honking the horn all the way and going across faster than I would have ever dreamed of going. We finally slowed down after the EMERGENCY BRAKE thought popped into my mind. It mostly worked and we came to a stop. I thought it was a catastrophic failure and was afraid to call my dad. I didn't know much about cars yet and I thought it was going to cost too much to be worth fixing. Car got towed in and there was a hole punctured in the steel brake line under the drivers side, probably from a piece of gravel. They spliced in a repair and no big deal. Later in life I have thought about that day. Why didn't I think to use the emergency/parking brake earlier? What if there was another vehicle crossing that intersection? AND WHY DIDN'T THE DUAL BOWL MASTER CYLINDER DO ITS JOB? The red "BRAKES" warning light did come on in the dash so the plunger in the proportioning valve did trip.

[brianj]

I have had this happen, and it did work, BUT you had to pump the brakes just as if it was airbound. Maybe taking up the slack in the rear shoes?

[Walking-Tall]

Has anyone had a text book failure where one half of the system goes out, the dummy light comes on, and you are able to make it home safely on the remaining half that is still functioning?

Nope.

[Walking-Tall]

I have also found the same thing with engine clutch fans. They are supposed to have no or very low drag when the engine is cold, but as the engine gets hotter, should get tighter, and increase the speed of the fan blades. However, I have always found the fan blades much stiffer to rotate when the engine is cold, usually accompanied with much fan noise, but when you shut down a hot engine, the fan blades will continue to spin for a while after the crankshaft stops, and the fan is very easy to rotate when hot. Kinda backwards from what is supposed to happen.

Such as the delusion of multi-viscosity lubricants. As I learned in mechanical engineering schooling, and as my father asked me many years before that: Have you ever seen any lubricant that gets thicker when it gets hotter? There isn't any such animal. Maybe the early clutch fans with a bimetallic coil (like a carburetor's automatic/electric choke) in the front will function as purported from heat from the radiator, but the newer ones with some kind of lubricant in them to solely control them, forget it (I say the best fix for a clutch fan is a mechanical or electric fan to replace it). For instance, most erroneously think something like 10W30 is at a "10" viscosity when cold and a "30" viscosity when warmed up... the opposite is true... well, it approaches to be true. A 10W30 oil is to be no thicker than "30" when cold and no thinner than "10" when hot. Hence me never bothering to bother with multi-viscosity lubricants. As always and forever, a good heavy duty straight 30 weight takes a pounding and gets the job done during the warmer months, and a straight 20 weight gets the job done during the winter.

[erratic50]

I had one text book "brake" light came on failure in my Mustang with a blown from line, but no brakes.

on my 74 Galaxie the master cyl failed. Behaved exactly the same way.

last fall I had a brake caliper hang up slightly. It built enormous heat and boiled the brake fluid in the master cyl - again NO brakes! Once it cooled off they worked ok. Replacing the caliper solved that issue 100%. Scary stuff though!

[Haystack]

I blew a line on my 86. Didnt realize anything was wrong till I hit a redight in a 60mpg zone. Coasted through the light and into a gas station parking lot. Turned around and drove the car home. Didn't think to hit the pedal mounted ebrake. I was able to slow the car down with no fluid in the front reservoir, bit it didn't seem to matter much. No way was I able to lock them up like when the brakes normally work.

[gr79]

i guess one could practice in a large empty parking lot or road to try and stop without using brakes.
Downshift? Cut off engine (and power steering)?
At least check e brake condition.

Or
Year 2011, a local moron:
https://www.youtube.com/watch?v=_hFW3Pp3sJ4

[BMW Rider]

Such as the delusion of multi-viscosity lubricants. As I learned in mechanical engineering schooling, and as my father asked me many years before that: Have you ever seen any lubricant that gets thicker when it gets hotter? There isn't any such animal. Maybe the early clutch fans with a bimetallic coil (like a carburetor's automatic/electric choke) in the front will function as purported from heat from the radiator, but the newer ones with some kind of lubricant in them to solely control them, forget it (I say the best fix for a clutch fan is a mechanical or electric fan to replace it). For instance, most erroneously think something like 10W30 is at a "10" viscosity when cold and a "30" viscosity when warmed up... the opposite is true... well, it approaches to be true. A 10W30 oil is to be no thicker than "30" when cold and no thinner than "10" when hot. Hence me never bothering to bother with multi-viscosity lubricants. As always and forever, a good heavy duty straight 30 weight takes a pounding and gets the job done during the warmer months, and a straight 20 weight gets the job done during the winter.

You totally misunderstand how oil viscosity ratings are determined. Oil viscosity rating is determined by flow rates of the oil at a given temperature. For the primary viscosity rating (the second number), the oil flow rate is measured at 100 degrees Celsius (212 F). It will be thinner at that temperature than cold, but it is how fast it flows at that temperature that determines the viscosity. The W rating is determined by the rate of flow at a low temperature. The actual test temperature varies depending on the viscosity rating given as determined by SAE. So, an oil rated as 10w30 will flow at the same rate as a plain 30 weight oil at 100 C. At the specified cold temperature, it will flow like a straight 10 weight oil would at that temperature. Thus the 10w30 oil will have better cold flow rates than a straight 30 weight oil and better high temperature viscosity than a straight 10 weight oil. This allows better oil flow at cold start up and better engine protection at operating temperature.

[Walking-Tall]

You totally misunderstand how oil viscosity ratings are determined. Oil viscosity rating is determined by flow rates of the oil at a given temperature. For the primary viscosity rating (the second number), the oil flow rate is measured at 100 degrees Celsius (212 F). It will be thinner at that temperature than cold, but it is how fast it flows at that temperature that determines the viscosity. The W rating is determined by the rate of flow at a low temperature. The actual test temperature varies depending on the viscosity rating given as determined by SAE. So, an oil rated as 10w30 will flow at the same rate as a plain 30 weight oil at 100 C. At the specified cold temperature, it will flow like a straight 10 weight oil would at that temperature. Thus the 10w30 oil will have better cold flow rates than a straight 30 weight oil and better high temperature viscosity than a straight 10 weight oil. This allows better oil flow at cold start up and better engine protection at operating temperature.

Not in the least. I said that most believe the opposite of what you just said and what I said. The merit of importance, function, and marketing and cost of multi-grade lubricants, you have now elaborated, for which I sum up as being largely unimportant... bells and whistles, razzle dazzle, etc. A total misunderstanding would at least require disagreement. I don't believe we have. Oil's behaviour in a controlled laboratory at whatever temperature means little to nothing in the real world inside a running engine's crankcase, with never the two of any of them heating the oil the same. Hence my summation of un-importance. Flow rates determined by viscosity = oil's thickness, period, and increased temperature thins lubricants, which equals a lower viscosity rating. Potato, potawto.

[xctasy]

https://en.wikipedia.org/wiki/Talk%3..._coupling_unit.

Could somebody change the article accordingly (my english is too bad for that) because it seems that the fable about the miraculous change of the thickness of the silicon oil in the visco coupling seems to disperse quickly? I found the same wrong explanation of the visco coupling principle in the polish Wikipedia (apparently translated from the english "original")

Its the ASTM/APi standard that does it.

Common misconception is that fluids become thicker with heat. My Nissan AWD uses a fluid that runs a viscous coupling. Even very smart people get the basic fundamentals wrong, and then it becomes a sh!+ fight of claim and counter claim.

Popular Mechanics Sep 1979





Search "popular mechanics Power transfer through the silicone fluid"

Page 95




The ASTM and API tests are benchamarked against temperature and oil weight type, just like Iso Octane blends for 87, 89, 91, 93 or 105 RON tests in a combustion engine, or the MON tests.

The difference is that the dilatant fluid the article mentions works based on shear stress propogation, not heating.
Water supports very little shear stress, maple syrup, lots. The ASTM and API tests are Index Tests. The don't create better understanding, just better ways to measure flow rate under different compative tests.


In a Sybolt tube at the various temperture in an oil filled Tamson bath, you define the oil consitancy with temperature as per the API ratings.


You can argue the toss aboutwhat it means. I use the index narrative, and are very carful not to go beyond the index tests to create laymans use generalisations as specfics.

[bob85gt]

dual pot master cylinders do work. I have lost the front brake and the car will come to a slow stop using the rears. I have also lost the rear and the fronts work fine. the dumbest thing ford did ( brake wise) was in my 93 Taurus they went to one pot on top of the dual pistons. my rear line sprung a leak and all was fine till the pot ran dry and air got in the front brakes. surprise!!!