Soft brake pedal in hot weather

I’ve got a 95 Civic. In hot weather the brake pedal becomes soft and gives way when I’m stepping on it in traffic. Pumping it helps, but it doesn’t solve the problem.

It had the same symptom last summer but with cooler weather the problem goes away!

Any ideas? thanks

Anytime you have a pedal that is soft I would say it is a master cylinder failing. But it should happen all the time.

The interesting thing is that it happens when it is hot. What does the fluid look like? Is it very dark in color? It could be that the fluid has a high water content which might explain the soft pedal when it gets hot.

If the brake fluid has water in it there are going to be other problems as well. There will be components failing from rust.

I would have someone check the brakes and the condition of the brake fluid. It would be helpful for the person checking it to have it on a hot day so he can experience the soft pedal.

I just checked the brake fluid in the small holding tank (with the dipstick) and it looks OK. I compared it to the consistency of some brake fluid you buy from the auto parts store. I’d expected it to be a little thicker (like trans or motor oil), but it wasn’t.

Yes, the problem appears only when there’s hot weather (over 80F). This evening it’s about 65F outside. I started the car, let it run a bit, drove it around for a while (1 mile), and the brake seems OK. In the same conditions of use the problem appears when it gets hot.

Thanks for the suggestion though. I thought maybe there’s air in the system, but wouldn’t air expand when it’s hot and make the brakes feel harder?

Oh, the brake fluid looks dark in the little tank, but when I take the little dipstick out it looks very light. I mean, I don’t know what’s further below in the tank and the system, but the little dipstick (2-3 inch) shows a very light blond fluid on it and it looks very thin. (but so does the fluid in the can from the store).

The line is between the min - max levels in the tank, maybe I should top it off?

[b]This sounds like a defective brake master cylinder.

The master cylinder is located under the hood with all that engine heat. And since the master cylinder a made from casted almag, it has a tendency to abosorb heat.

The hotter the days, less heat is able to escape from under the hood. This excess heat heats up the master cylinder. As the master cylinder heats up, it causes the internal bore to expand slightly. If the cup seal inside the bore is marginal in wear, as the bore expands from the heat the seal starts to allow the brake fluid to by-pass it. And this is what causes the brake pedal to become soft.

Replace the master cylinder and it should fix the problem.


Thank you. Great answer. I thought that if the master cyl. had a problem it’d be there all the time. The hot weather was the puzzle, that’s why I thought maybe air was in the system or something similar.

I’m very handy with tools and fixing stuff, though I don’t have a mechanic’s garage. Is it relatively simple to replace the m.c. ?

[b]Replacing the MC is pretty straight foward.

First loosen and retighten the line fittings to the master cylinder.

Remove the two nuts that hold the master cylinder to the vacuum brake booster, and remove the line fittings.

Remove the master cylinder.

When you get the new replacement master cylinder, there should be instructions with it explaining how to bench bleed the new master cylinder. DO NOT SKIP THIS STEP.

After bench bleeding the new master cylinder, install it onto the brake booster and connect the line fittings. Top off the master cylinder with brake fluid.

Have someone pump the brake pedal, and then hold the pedal down. Now crack open both line fittings at the same time to bleed out as much air as possible. Retighten the fittings before the brake pedal is allowed to come back up. Repeat this a couple of times.

Now bleed the brake system at each wheel following the proper sequence for your vehicle.

Check for a solid brake pedal.


Thanks again for the good advice.

I have one last question. Is it best to press the brake pedal with engine running or not? I guess the same should apply when bleeding the brakes. Right?

[i’d imagine with engine on since all cars today have power brakes]

Some manual I have (Haynes? Chilton?) says that a scanner tool is absolutely required for bleeding ABS. Is that wrong, or are we assuming that OP’s car does not have ABS, or what? (That warning does have me scared to try bleeding my ABS cars, should it ever be needed.)

What does the scanner do? It’s a simple procedure of getting the air out of the system. There’s nothing electronic about it.

The important thing is to follow the right procedure. In my Civic w/ABS, it’s the rear right first, then front left, etc…

I’ve researched a couple how-to sites and noone mentions anything about a scanner.

Tester’s thought on the master is probably it. But air in the line can cause the same type of symptoms. If there is a little air in the line, it will expand with increased temperature. However the pedal would be softer not harder. The expanded “bubble” would just make more compressible space before the hydraulics work. Regardless, that kind of problem will be solved as well after a MC change and complete bleed.

You also need to make sure the booster pod is matched to the master cylinder. It usually is but there is a special gauge for measuring this, but I don’t know where to get one.

Well, I could not find that warning in the Haynes book where I thought I had seen it. Maybe I dreamed it. If I see it again, I’ll post here.

Thanks for straightening me out. And sorry for dragging this off topic.