Full Version: brake failure
As recounted in an earlier thread, I had the flexible part of the front left brake line fail while I was driving. This resulted in a complete failure of the brake system. I was lucky that I was in the middle of nowhere. As I think back on this, should such a failure result in the loss of only half the brake system?
I have wondered about this myself. In theory, you have 2 circuits. If one fails, the other should work. But that does not seem to be how it goes. The piston will push the brake fluid to the path of least resistance - and that is out the burst brake line.
Zach
Zach
My experience with a broken flexible brake line was the same as yours - total failure. This was 20 years ago in a '69 Valiant. Luckily, I was going about 5mph in a parking lot.
It's just like opening one of the bleed screws, the pedal goes to the floor.
you will loose all the fluid, thus all the brakes.
True dual circuit systems have dual reserviours seperate from each other....two individual little tanks for the fluid.
Once really well thought out caliper (that happens to fit) is the Volvo p1800 caliper...
It has dual circuits on each caliper...quite cool really.
BTW, not something I read on the Internet...I happen to own a few Volvos..
Rich
True dual circuit systems have dual reserviours seperate from each other....two individual little tanks for the fluid.
Once really well thought out caliper (that happens to fit) is the Volvo p1800 caliper...
It has dual circuits on each caliper...quite cool really.
BTW, not something I read on the Internet...I happen to own a few Volvos..
Rich
If you care to look inside of your brake fluid reservoir you will see a dividing wall inside that gives you, in essence, separate reservoirs for front and rear circuits.
QUOTE(Demick @ Aug 26 2008, 07:43 AM) 
My experience with a broken flexible brake line was the same as yours - total failure.
Same here, happened to me as well ...
Andy
QUOTE
If you care to look inside of your brake fluid reservoir you will see a dividing wall inside that gives you, in essence, separate reservoirs for front and rear circuits.
The 914 master cylinder is really smart that way. Two reservoirs in one.This is a great thread. If the dual system does not save you from brake line failure, what does is save you from?
QUOTE(McMark @ Aug 26 2008, 10:49 AM)
If the dual system does not save you from brake line failure, what does is save you from?
Wouldn't you need a master cylinder that has two separate chambers for a true dual circuit to work ???
Andy
QUOTE(SirAndy @ Aug 26 2008, 02:06 PM)
QUOTE(McMark @ Aug 26 2008, 10:49 AM)
If the dual system does not save you from brake line failure, what does is save you from?
Wouldn't you need a master cylinder that has two separate chambers for a true dual circuit to work ???
AndyExactly. If there is only one piston pushing the fluid, it is not a true dual circuit system.
If the system were a true dual system, the pedal would not go to the floor in a failure. Braking would be reduced as you would have 2 instead of 4 brakes - but you would still have brakes.
Let me ask first, how many had front brake lines fail?
Each half of the reservoir feeds into the MC separately, and the MC has a divided piston. So the MC should act as a true dual system. However since the front brakes do most of the work, you might not notice that the rear brakes are still working. Also the seal between front and rear circuits of the MC may fail in older MC's and prevent the dual design from working.
You DO have a FUNCTIONAL emergency brake don't you?
Each half of the reservoir feeds into the MC separately, and the MC has a divided piston. So the MC should act as a true dual system. However since the front brakes do most of the work, you might not notice that the rear brakes are still working. Also the seal between front and rear circuits of the MC may fail in older MC's and prevent the dual design from working.
You DO have a FUNCTIONAL emergency brake don't you?
From the Factory Manuals:
TANDEM BRAKE MASTER CYLINDER
The tandem brake master cylinder corresponds in its basic design to two normal brake master cylinders connected in series, with one piston supplying the pressure for the front brake circuit and the other the pressure for the rear brake circuit. Since no initial pressure is permitted in the line system of disc brakes, the brake system has no floor valves. Throttle holes at the two connecting points for the front and rear wheel brake circuit permit bleeding of the system without the use of a bleeding device by manipulating the brake pedal in quick succession.
OPERATION OF TANDEM BRAKE MASTER CYLINDER
The two pistons divide the tandem brake master cylinder into two pressure stages, one stage serving the front wheel brake circuit and the other the rear wheel brake circuit. If one brake circuit fails as the result of a leak, the vehicle can still be braked with the other brake circuit.
If, for example, the front brake circuit leaks, the two pistons of the tandem master cylinder as well as the brake fluid between the pistons are initially moved forward until the piston for the front wheel brake circuit rests against the bottom of the housing. The pressure built-up for the rear wheel brake circuit can now begin.
When the rear brake circuit leaks, the piston for the rear wheel brake circuit is first moved up to the stop against the stopping sleeve. The brake is not actuated,since the brake fluid escapes through the leak. Continued stepping down on the brake pedal will then move the piston of the front brake circuit forward under the influence of the pressure exerted by the abutting rear piston. The pressure for the front wheel brake will then be built up and the vehicle will be braked.
The loss of a brake circuit is characterized by an essentially longer free travel of the brake pedal. In addition, the normally obtained deceleration of the vehicle is more or less reduced, depending on which brake circuit is out of action.
When the brake pedal is depressed, the piston for the rear wheel brake circuit is pushed forward by the piston rod. As soon as the primary sleeve covers the compensating hole, pressure will be built up in the thereby closed pressure chamber (rear wheel brake circuit), which will then move the piston for the front wheel brake circuit forward. When the compensating hole of the front pressure chamber is also covered by the primary sleeve and the pressure chamber for the front wheel brake circuit is thereby closed, a uniform pressure will be built up in both pressure chambers. The brake fluid displaced in the respective pressure chamber will then flow to the pistons of the calipers via the pertinent lines.
QUOTE
TANDEM BRAKE MASTER CYLINDER
The tandem brake master cylinder corresponds in its basic design to two normal brake master cylinders connected in series, with one piston supplying the pressure for the front brake circuit and the other the pressure for the rear brake circuit. Since no initial pressure is permitted in the line system of disc brakes, the brake system has no floor valves. Throttle holes at the two connecting points for the front and rear wheel brake circuit permit bleeding of the system without the use of a bleeding device by manipulating the brake pedal in quick succession.
OPERATION OF TANDEM BRAKE MASTER CYLINDER
The two pistons divide the tandem brake master cylinder into two pressure stages, one stage serving the front wheel brake circuit and the other the rear wheel brake circuit. If one brake circuit fails as the result of a leak, the vehicle can still be braked with the other brake circuit.
If, for example, the front brake circuit leaks, the two pistons of the tandem master cylinder as well as the brake fluid between the pistons are initially moved forward until the piston for the front wheel brake circuit rests against the bottom of the housing. The pressure built-up for the rear wheel brake circuit can now begin.
When the rear brake circuit leaks, the piston for the rear wheel brake circuit is first moved up to the stop against the stopping sleeve. The brake is not actuated,since the brake fluid escapes through the leak. Continued stepping down on the brake pedal will then move the piston of the front brake circuit forward under the influence of the pressure exerted by the abutting rear piston. The pressure for the front wheel brake will then be built up and the vehicle will be braked.
The loss of a brake circuit is characterized by an essentially longer free travel of the brake pedal. In addition, the normally obtained deceleration of the vehicle is more or less reduced, depending on which brake circuit is out of action.
When the brake pedal is depressed, the piston for the rear wheel brake circuit is pushed forward by the piston rod. As soon as the primary sleeve covers the compensating hole, pressure will be built up in the thereby closed pressure chamber (rear wheel brake circuit), which will then move the piston for the front wheel brake circuit forward. When the compensating hole of the front pressure chamber is also covered by the primary sleeve and the pressure chamber for the front wheel brake circuit is thereby closed, a uniform pressure will be built up in both pressure chambers. The brake fluid displaced in the respective pressure chamber will then flow to the pistons of the calipers via the pertinent lines.
QUOTE(davep @ Aug 26 2008, 11:19 AM)
However since the front brakes do most of the work, you might not notice that the rear brakes are still working.
I had the left *rear* blow out on me and the brake pedal immediately went to the floor and there was absolutely no brake at all left.
That was in the 914, with the stock 17mm master cylinder.
So, if the 914 system is in fact a true dual circuit, something is not working right ...
AndyPS: Also, i vaguely remember from drivers school that a true dual system is supposed to be diagonal and not front/rear.
Left front and right rear and right front and left rear.
EDIT: And i *DID* do the pedal pumping, but it didn't help at all. Luckily, back then i still had the handbrake installed ...
If you didn't want to read that, it mentions that in the event of failure, the pedal will go to the floor, but you should then be able to lift and repress the pedal and be able to stop the car.
It is most definitely a dual circuit system. Check out pieces #12 & #21. Front brakes piston, rear brakes piston respectively.
QUOTE(McMark @ Aug 26 2008, 11:28 AM)
If you didn't want to read that, it mentions that in the event of failure, the pedal will go to the floor, but you should then be able to lift and repress the pedal and be able to stop the car.
See edit to my post. I'm not that stupid (i think)
First thing i tried was pumping the pedal. But i got nothing out of that ...
Andy
Why exactly can you build no pressure when bleeding the brakes (pretty much the same thing as a broken brake line)?
Perhaps you are building a very small amount of pressure in the opposite circuit, providing a very small amount of braking force? However, once the pedal is on the floor, you are no longer maintaining any pressure.
Perhaps you are building a very small amount of pressure in the opposite circuit, providing a very small amount of braking force? However, once the pedal is on the floor, you are no longer maintaining any pressure.
I would imagine diagonal fail brakes would give unsafe braking conditions...
QUOTE(McMark @ Aug 26 2008, 02:34 PM)
I would imagine diagonal fail brakes would give unsafe braking conditions...
It is the standard on cars these days. Every new car I have bought had the brakes set this way.
Zach
in 1967 it became a federal standard for all new u.s. sold cars to move from single to duel resvoirs and duel master cylinders. they were directed thru a "balance valve" that a mechanical peice which was internally pushed over to the low pressure side if the pressure balance was not normal (equalized). that not only grounded the "brake light" on the dash but sealed off fluid loss to the leaking system. they also are balance front/rear. if you look at a drum/drum or disc./disc. you'll notice the researves are equal. a disc/drum has unequal reseaves (disc. requires more fluid displacement).
if your brakes failed with just front or rear the system wasn't operating on the other to start with. interesting how the manuel says to pump to get the secondary to operate. the system was designed for just that reson... to stop you if a line, caliper, ect... blew out.
mike
if your brakes failed with just front or rear the system wasn't operating on the other to start with. interesting how the manuel says to pump to get the secondary to operate. the system was designed for just that reson... to stop you if a line, caliper, ect... blew out.
mike
The volvo setup has a split master and it uses the front for one piston in each caliper and the rear for the other piston...its really quite cool.
I have also lost brake lines and pumped with no success...and not just in a 914.
I think this is why racers use two seperate masters..
Rich
I have also lost brake lines and pumped with no success...and not just in a 914.
I think this is why racers use two seperate masters..
Rich
Interesting discussion. Just to be clear, I, like others here, had absolutely no brakes and I tried pumping. And I know all four brakes were working fine before the left front failure.
You guys are starting to scare me now. When doing my first restoration I noticed that these cars don't have the brake balance valve that American cars have in the event of a brake line failure. I wonder if I can put one in????
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