I just replaced the rear pads on the '94 960. Everything seems fine, but I'm wondering about something in some recent posts:

Someone described the procedure for retracting the piston in the calipers when replacing the pad, and suggested opening the bleed screw while simultaneously pinching the brake supply hose to ensure that no fluid gets forced back toward the master cylinder. I'm wondering what is different about ABS brakes so that you wouldn't want to allow fluid to be forced back into the master cylinder.

In my case, I opened the bleed screw and retracted the piston (without pinching the brake hose). Some fluid came out the tube I had attached to the bleed screw nipple, but I'm sure some of the fluid went back up the brake hose, too. Did I potentially cause any damage to the ABS hardware with this procedure? I'm not getting any codes or warning lights, so maybe everything's OK.

Thanks for any input.

R. W. Reagan

As Chris notes, forcing old fluid up into the ABS unit is not good because it carries crud with it. Crud settles into the calipers because they are the low point in the system. Forcing it back into the ABS is the same as adding dirt to the expensive ABS.

But I doubt you did harm: the bleed screw relieved most of the pressure and allowed fluid to depart. Next time, wrap some vice grips in tape and squeeze the line enough to just clamp it shut. And flush your fluid every two years.

To bleed or flush, get a pressure bleeder from IPD. If you use the pedal technique, make sure you do not press more than two inches down (less than full pedal travel) so the m/c seal does not travel into potential worn or corroded territory inside the m/c. Mark where the pedal goes at full travel when the system is working and then don't go beyond that when bleeding or flushing.

It is doubtful that you did harm to it.

That procedure also hinges on how clean your brake fluid is. If you change it (flush system) regularly, you can generally just replace the pads without bleeding the system. The idea is obviously to keep the really bad junk out of the ABS modulator. If you opened the bleed screw, most of the fluid probably came out of there, since it would be the path of least resistance. I doubt you did any harm to it. If you're nervous, though, bleed the system with a large amount of fresh fluid. The fluid will cycle through the ABS modulator and into the calipers, giving you a fresh starting point.
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chris herbst, near chicago

Having a car where i have done nowt (nothing) with the breaks since i got it 6 months ago would i be wise to assume crud in calipers, if so would the wise man go buy himself a presure bleader and blead the brakes copiously (after all fluid is cheap...) to flush any crud out of the system. and in fact before changin pads flush the system via the nipple on a 'just in case' method to flush any crap out of the caliper there by removing the need to clamp the lines and possibly damage them..


just the musings of a probably derranged DIY mechanic...

That's pretty much what I do. If I've got clean fluid that's been flushed well, I don't worry about pushing much dirt back into the ABS modulator.

I like to do a flush and brake pads at the same time. Flush the system. Then open the bleeder when I retract the caliper. THen once it's all back together, bleed it the rest of the way. Voila--takes only a few minutes and the system is guaranteed clean.

Personally, if I'm only doing brake pads (not flushing), I usually just open the bleeder, but I don't clamp the brake lines.
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chris herbst, near chicago

think im gona do that anyway, dont know how long its been in there...gona go buy me a prsure flusher...sounds like a good way to go

That's good to hear. So the ABS hardware will allow the fluid to flow through without risk of bursting seals or something internally?

By the way, what's the preferred procedure for exchanging the brake fluid? I replaced fluid in a high mileage Mazda by pumping the pedal and alternately opening and closing the bleed screw as the pedal was depressed. After a few weeks, I had to replace the master cylinder. It seems that the pedal stroke to the floor during the bleeding process allowed the piston in the master cylinder to travel past portions of the cylinder wall that it normally didn't get to. With the age and mileage, the unused portion of the cylinder wall apparently had corroded enough that it damaged the seal on the piston, leading to master cylinder failure.

Is this a concern on the 960? If so, can I replace fluid with a gravity feed instead of pumping the brake pedal?

Thanks again for input and responses.

Put a small wood block under the brake pedal to limit the pedal travel.

DanR 94 964 215k miles
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DanR

Two instant-deaths for master cylinders:

1. Major pedal-stomp
2. Bleeding and depressing the pedal too far (farther than normal hydraulic pressure would allow with a closed system)


One of the most common repairs when someone buys a used car, is to have the master cylinder replaced. This is because a lot of idiots who test drive used cars think that by ramming the pedal to the floor to see "if them brakes are good", they are diagnosing a functional brake system. In another few days, weeks, or whatever... bang--new master cylinder time.

The ABS modulator will let the fluid travel through it. You shouldn't have a problem with the ABS seals. (One thing I would recommend is activating the ABS a couple times a month during wet weather or on gravel or sand or something. It keeps the internals working and the ABS servo motor operational).

The pressure brake bleeders are nice.
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chris herbst, near chicago

Based on questions raised by my former recommendation to acvitate ABS periodically, I am retracting that recommendation in it's entirety.

Such driving could be misinterpreted as a recommendation to exceed the limits of 'normal' driving habits. Altering your own driving habits for diagnostic purposes (or any other implied reason), should be strictly avoided.
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chris herbst, near chicago

I do not mean to cause trouble but, I guess I will anyway. In your first paragraph you say do not stomp on the brakes to test them. Then in the second paragraph you say to "activate" the ABS system once a month. Don't you have to stomp on the brakes to "activate" the ABS system. Again, I don't want to piss anyone off, just making a comment.

In the end, it seems to me that if you plan on bleeding your brakes you should purchase a pressure bleeder or you are asking for trouble. I learned something new, that is why I am here. I would suggest that the FAQ's be updated with a strong recommendation to use a pressure bleeder.

Since I stipulated "wet conditions" quite clearly in my first post, and it still raised questions, I immediately smell an open door to liability.

A few years ago, some moron who couldn't find the business end of his wrench (and perhaps his own ass), destroyed his oil pan by jacking the car up by the drain plug. This is because he grossly misunderstood, and misinterpreted very EXPLICIT instructions. He happened to be a starving litigator, or ex-law student, or something. And he tried to peg a bunch of people here for "making him wreck his car".

Now, when I hear a question like "you said to do [this or that]", coupled with possible consequences of said action, the hair on the back of my neck stands up.

In other words, just WHAT IF you 'tested' your ABS by hammering your leg down on the pedal, thereby MAYBE unknowingly destroying an aged geezer of a master cylinder. Or maybe it was already wheezy and old. But the signs showed up in a day or so of your 'test'. Then you, or someone, piles up because the brakes lose much or most of their function.

Then: "you said that I should...", "You told me to...", and "Now look what happened because of what you told me to do."

Or whatever. It could go any number of ways, but I think that in this case, I will retract my statement based on liability, rather than serviceability. It is similar to someone coming into your building during a big snowfall, and saying upon entering, "It's really, REALLY, REALLY slippery on your front stairs!"

Then when they leave, there is a little 'accident'. Whether real or not, they warned you. Telling them to "watch their step" isn't good enough; they'll just say "I did watch my step." And you're still liable. But if you ran out and salted and shoveled the stairs when they told you, that is different. Then if they fell on their ass, you at least made reasonable efforts.

Call me paranoid; I'll call myself careful. And I assure you, I have more experience in matters of liability than I am disclosing here.

In the interest of safety, I recommend not driving your car in any way other than you normally would drive in the course of daily travel. Anything outside of that would be at your own risk. That includes driving in such a way as to activate ABS brake systems outside of what could be considered "normal" driving habits for you or anyone.

I assure you that my statements are not personal, in any way, shape, or form.
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chris herbst, near chicago

Hey, sorry about the whole thing. I don't want to make enemies and am certainly not looking for fuel to sue someone.

I got one of the small pressure bleeders like the ones IPD sells a couple years ago. I DON'T KNOW HOW I EVER LIVED WITHOUT ONE! When I think of all the hours spent coaxing one of my kids or the wife to help and the time pushing on the pedal, I appreciate the bleeder even more.

I like the small ones because the amount of fluid needed to make it functional is less and the clean up after is easier. Just fill the bleeder with new fresh fluid, connect to an air pressure source and the master's reservoir and go to each wheel in turn until fresh fluid comes through. What used to be a several hour project is now accomplished in about 1/2 hour.

I have an air compresser but a spare tire would work just as well as an air pressure source.
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'96 965 with 16' wheels at 99K. Had '85 745 Turbo Diesel for 200K.

It is an interesting delema you have brought up. I would not think that draining your brakes by gravity is a good way to flush the fluid out of your brake system. I would think some old fluid would stay in the passages of the master cylinder.

As far as Volvo's are concerned, I am sure there is a risk in damaging the master cylinder on any high mileage car that has not been regularly flushed of fluid. However, I do not know how you would change pads without bleeding the brakes a little bit after installing the new pads. Thus, whenever new pads are installed the pedal would travel throughout its range of motion. Knowing how quickly Volvos go through pads, I would think that would prevent the bottom of the master cylinder from prematurely corroding.

Another issue that comes to mind is your reasoning as to why your master cylinder failed. Maybe some of the crud that got pushed into the master damaged some of the seals? That is another reason to open the bleed valve on the slave cylinder when pushing on it. On the other hand, I agree with the other poster that I fear pinching the brake line because it may crack.

I see no problem with what you did. If you want to get rid of the dark
brake fluid the proper proceedure is to bleed the brake line. I for one
would never clamp a brake line. You could wind up weakening or rupturing
the neoprean lines.
Ralph...

RW - that was my suggestion a few months ago - about opening the mcylinder cap and pushing the fluid back through. I think the only harm here would be pushing dirty fluid back through the system. I've done it both ways (1) cracking the bleeder screw or (2) opening the mcylinder cap. I only do the latter on brake system I'd recently bled.

As they say in Ga.... 'you've done good'.