Brake failure?

I have a 90 Buick Century with an integrated master cylinder/proprotioning valve assembly. All 4 brake lines go to the side of the master cylinder. The system is split diagonally. Just
replaced the master cylinder and observed the following. If I run either of the front caliper pistons all the way back on one side of the car and step on the brakes, the petal goes to the floor while the piston is being pumped back out to clamp tight to the rotor. Well this seems like the master cylinder does not have a backup(secondary) feature. I would have thought that this was a defective section in the master cylinder, but it works exactly the same after I have run either passinger or driver front piston back into the caliper. I also ran the caliper piston back in to the caliper, put the wheel on and drove down the driveway at about 10 mph. Upon applying the brakes the car did stop slowly, with petal to the floor. I am not sure how well it would have been at 50mph if a brake line blew out? Anyone with experience in this area have thoughts as to this issue? thanks
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That's the way it works - if you push a caliper all the way in, you'll experience a "no pedal" initially upon applying the brake. That should go away quickly as the calipers push back out and the system stabilizes.
Brake lines don't typically "blow out", though it is common enough in corrosive environments like the northeast, for them to rust through and develop a leak. What you'll experience under these conditions is a loss of pedal that will typically pump back up, and then bleed off again due to the leak. You'll have brake enough to stop the car, even under highway conditions. I've had a line rot off completely, such that it was wide open and when the brake was applied, it just blasted the fluid out through the ruptured line. Actually - had this happen more than once on different cars/trucks. You always have the ability to pump them back up to a decent pedal and stop - although you'll feel the pedal bleeding off through the leak, and you'll end up on the floor until you pump it up again. At some point you're going to run out of brake fluid and there will be no more pumping the pedal back up.
All of the above is exactly what you should expect. The secondary or backup capability of a master cylinder is not meant to compensate for a ruptured brake line - nothing can do that. As for the pedal having to come back after pushing in the calipers - that too is expected. You have to get the pads back out and under pressure before you're going to feel any pedal. You have to remember that when you apply the brake you are creating pressure, so you'll only benefit from what you create. If the closed loop system of a brake system requires a given fluid flow and pressure before the pedal becomes firm again, then one or two initial pumps may not be enough to push the caliper back out to where they need to be.
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-Mike-
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I had a brake line failure due to rust on my 93 Buick this summer. My wife was driving it. She went three blocks from her work and got stopped at a 7-11. I found the where the leak was on the front of the car. I wanted to get it home for repairs and found I had to go get a plug to put in the master cylinder to stop the fluid going to the front otherwise all the fluid would drain out of the master. I did that and managed to get the car home with only rear brakes.
I remember years ago they started a dual recover for the master cylinder so if you lost front or rear brakes you still could stop. But with this car the fluid will drain down quickly and you have nothing. Poor design but its the only think on this car I have found I don't like.

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Yeah - that's pretty much the way it works with all master cylinders. You really can't design around it except to do as GM does and actuate the brakes diagonally - though, a rotted through line is still going to pour out the brake fluid. The dual master cylinder is not intended - nor can it, prevent losing the fluid when a line rots through. About the only thing you could have done differently is to simply crimp the offending line over and drive it home. You'd probably still lose some fluid, but you could probably get it quite a ways before you had any real problem.
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If I remember right I had a 1963 Olds Cutlass F85 (loved that car). One day a front brake line broke. I lost all the fluid in the front part of the master but still was able to stop and even drive home with only the rear brakes working.

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It's not? Unless they've come up with a new design that's exactly what it's for.
And you aren't going to pump up any brake enough to stop you if there is a leak in the line. If it's a small leak you might get a momentary grab out of the brakes on that side. If there's a leak, the design of the master cylinder will allow full operation of the brakes hooked to the side that does not have a leak upon the first press of the pedal. (Then you don't waste fluid and possibly paint on the vehicle by pumping the brakes and spraying fluid all over.)
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You most certainly will pump up a set of brakes with a ruptured line. This is not a guess, but a well tested statement. They won't stay up because of the loss of pressure through the rupture, but they most certainly will pump up if the leak is small. If the leak is big enough you won't hold the pedal very long at all but if it's a small leak the pedal can hold for quite a while. Push enough fluid out and you'll lose all pedal. Don't worry - you'll most certainly be pushing fluid out through the leak. You will only have full pedal up to the point that you push out most of the fluid. You'll still have brakes even with the pedal on the floor, but not full braking power.
Do a test - cut or disconnect a brake line. Pump the pedal in the garage to simulate driving conditions. Pay attention to how long it takes before you lose all pedal. Don't believe for a moment that the dual cylinder is designed to prevent you from harming you paint by spraying fluid all over. It attempts to balance, but a rupture will push fluid out - guaranteed. You will find that with intermittent application (such as is common in normal driving), that you can indeed pump the pedal back up for a bit, but after a point the fluid is low enough that you can't do that any longer. You'll successfully evacuate the master cylinder quite nicely, to a point that you lose all pedal with one leaking line. Like I said - and I said this throughout the discussion you snipped the above quote from - you'll still have some amount of brake, but it will not be a complete brake.
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No need - it's the nature of usenet.

I agree with the principle of conserving fluid - that is in part what I was trying to get across as well. I think our thoughts are more in line with each other's than disparate.
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Did you bench bleed the new MC before installation? This step is very important on many vehicles, these days. It sounds like you may still have air in the system.
Dave
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snipped-for-privacy@yahoo.com wrote:

Sounds normal to me.
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wrote:

Please explain. The fact that the secondary circuit isn't working doesn't seem normal to me.
Dave
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Hairy wrote:

You are correct. I mis-read the orignal post. The master cyl could be bad. I wonder if he bled the system/mc diagonally?
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