I did some (a lot of!) brake work lately, starting with replacement of a rusted-out brake line. The car had been not been run for probably seven months and wasn't used much in the two years before that either. I replaced the wheel cylinders and calipers because I had trouble with all the bleeder screws. Then I bled the system, using new DOT4 fluid, as advised by a friend who seems to be a good amateur mechanic.

All seemed fine until I had to brake very hard about a week ago. Some time later when applying the brakes normally, I noticed the pedal was firm at first, then sort of "dropped out"---went soft. Then that happened some more times. Now the pedal is very soft and bottoms out basically every time. I don't seem to be losing fluid (none on the ground, level is not dropping). I think I have a bad master cylinder. Does it sound as if that is likely?

So I bought a new master cylinder and was ready to do the R&R, but I notice the cap of the new part says "WARNING: USE ONLY DOT 3 BRAKE FLUID".

Now I am worried that I shouldn't have used DOT4. I have most of a quart of DOT4 that I was planning to use with the new master cylinder.

Putting aside the performance issues such as boiling point, would it be a mistake to use the DOT4? Or should I ignore the warning and stay with DOT4?

I mean would I be risking damage to any system components?

I guess since I am replacing the MC, now would be the perfect time to go back to DOT3 if appropriate.

What should I do?

DOT3, DOT4 and DOT5.1 are all interchangeable. Any application that calls for DOT3 can also use DOT4, and most manufactures have switched to DOT4 since about 2006.

The best explanation I can think of for you is that your brake system was operating at higher pressures than before you did all the work, and the higher pressures caused the master cylinder failure.

This is unbiased information form a ASE Cert. Parts Specialist, NAPA Customer Care Associate and Die-hard Bonneville Enthusiast.

Pepin,

Im not an expert by any means but here are my thoughts.
Are you sure you bled the entire system. Since air is compressible, it can cause soft breaks. It just sounds like too much of a coincidence that the master cylinder "failed" after pretty much redoing the entire system. But then again the master cylinder could have failed due to old age, I mean if your lines were that rusted to require replacing your master cylinder was probably on the way out too.

Like what rocketstar said, I very much doubt the fluid had anything to do with the problem.

Brake fluid is hydrophillic (water loving), so water in the atmosphere will be attracted into an unsealed brake fluid. The longer it is open the more water it attracts. The water then causes rusting, pitting, etc. So once, I open a bottle and use what I need I throw the rest way.

The same principle applies when the brake fluid is in your system. Although at much slower rate. This is one reason why manufacturers recommend to flush brake fluid periodically.

Thanks for the replies.

Burns, I didn't make it clear, but I was driving the system with the new parts and the new DOT4 for something like a month, satisfactorily and without any change in the pedal. I probably drove only a few hundred miles, but could air bubbles hide out somewhere without any observed softness, and then cause softness later? My simple guess is no, but I am not even close to being a brake expert, so can somebody please comment about whether air could cause softness that only shows up later?

Burns,

I also had a leaking fuel line. The fuel was gasohol, and I had been smelling gas for awhile. I replaced very rusty sections in all three fuel lines and just the one brake line, all near each other, behind the right rear wheel. I think the rusting was mostly from salt on the outside, but also from water on the inside. The old lines looked horrible, and chunks of rust were falling off in my hands when I touched them.

Also replaced both trans cooling lines, as one of them was dripping at a rubber/metal joint.

A couple years before all these hydraulic repairs, I changed the brake fluid several times by sucking it out of the reservoir with a turkey baster, replacing it with new DOT3, so I don't think the brake fluid was terribly full of water when the line finally sprung a leak.

The brake line leak showed up when I hit the brakes hard once, like this internal MC leak that I suspect I have now.

I do think I bled the brake lines pretty well, but it is possible I ran it out of fluid when I had the leak, and didn't get the MC bled right. But as I say, I was satisfied with the braking action for a month after bleeding before the pedal went soft.

Maybe I am underestimating the rate of water uptake, but I wouldn't expect the leftover DOT4 would take up too much water in a month, since I have it capped tightly. Somebody please yell at me if I am all wrong about that.

RocketStar,

Thanks. Then I guess I'll suppose that the maker of the new MC reservoir is too cheap to update the reservoir caps they provide, so that they haven't gotten rid of the outdated warning to use only DOT3.

Yes, I think maybe the MC suffered its highest pressure in a long time when I hit the brakes so hard about a week ago. I think the MC that is on the car now is the original.

The GM service manual lists a symptom "brake pedal travel gradually increases". They don't waste any words on exactly what that means. I guess it might mean that the pedal is softer today than it was yesterday, which is the way it has happened with my car. They list the most common cause of that symptom as brake line leak and (internal) master cylinder leak.