Sudden Brake Failure then Recovery

[BartusCompater]

A couple of weeks ago I was on the freeway going about 60-65mph when I went to stop and the brake pedal went down with little resistance and no noticeable stopping power. I had to quickly downshift in order to slow myself down and pull off of the freeway. After I got off the freeway and took it to a safe parking spot I did a quick look around for some of the usual suspects: low brake fluid in the reservoir (it was still full) and leaking/broken brake lines, and couldn't find anything.

I then decided to fire it back up and the brakes were working fine. I drove around the block and did some quick stops, pumped the pedal, went around corners and everything seemed great. So I got back on the freeway, keeping my distance still and ready for the brakes to go out... by the time I was back up to speed (60-65mph) the same issue occurred and the brakes seemed to fail entirely. Once I slowed the car down a bit and pumped the pedal the brakes then recovered.

Ultimately I decided to just park the car and not to drive it until I figure things out. I'm just not sure where to start looking. The brake lines don't seem to have a leak anywhere. I do know however that my reservoir is pretty worn out (maybe moisture somehow got in the lines, would this cause an intermittent issue?), and my rear brake pads haven't been changed out in a long time. Wondering if this is possible a master cylinder issue though, and since they aren't too expensive I was thinking that I might just replace it.

If this doesn't sound at all like a master cylinder though I'll save myself the trouble. Does anyone have a hunch as to what might cause this kind of intermittent brake failure?

[bobplyler]

Since you were on the highway, it might be a sticking caliper. That could heat up the brakes pretty well. Maybe you have some water in the calipers? When was the last time you bled the brakes?

[BartusCompater]

I bled the front brakes just a few months ago after replacing a burst brake hose. I haven't touched the rears since I got the car almost 4 years ago, so I should probably do that regardless. Would a sticking caliper cause the pedal to go to the floor though?

[bobplyler]

I one time had some brake pads that faded to almost uselessness when they got hot. This happened just about 1 month after I got them, and I was driving down a mountain in north Georgia. I immediately put on some different pads. A sticking caliper could also get very hot. If you have some decent pads, that shouldn't be a problem.

If you try and diagnose this, then check the temps of the brakes after it happens. They are normally hot, but they shouldn't be HOT-HOT.

[vandor]

Had something similar happen on a Volvo, but the failures were 2 months apart, and the brakes always fully recovered.
It was a failing master cylinder.

It could also be what was mentioned above, that there is a sticky caliper that heats up the fluid, the fluid boils, you loose brakes, but when you stop and the brakes cool down the brakes come back. To check for this you'd want to feel the wheels and see if any of them are hot.

[boogiedude]

Sounds like failing seals in the MC. If the fluid in the line was heating up and boiling, he would get a very firm pedal wouldn't he? Also, fading pads wouldn't affect how easily the pedal moves down. It would just mean that no matter how hard you stomp on it, the brakes just don't grab, as opposed to the pedal moving with less effort

[Zippy]

My experience tells me it is a master cylinder but I am not sure. I would drive it around to warm things up a bit then stop. With a slight but firm continuous pressure apply the brakes and wait to see if the pedal moves to the floor. Sometimes when I have had a master cylinder fail it has acted inconsistent at first, only to fail completely later. Good luck!

[Zulu]

I hade the same symptoms on my car a few months ago. Replaced the master cylinder and now all is well.

[vandor]

>If the fluid in the line was heating up and boiling, he would get a very firm pedal wouldn't he?

No.
When fluid boils it becomes gas.
Fluid= not compressible
Gas = easily compressible

[TulsaSpider]

M/C are cheap! good peace of mind!

[BartusCompater]

Took it out for an extensive test today. The brakes work perfectly except for when I get it up to highway speeds. Then they suddenly soften a great deal and harden back up after slowing down and pumping them a little bit. No apparent brake fluid loss.

Should I order a master cylinder?

[BartusCompater]

So yesterday I replaced my master cylinder and fluid reservoir and then flushed the entire system with a power brake bleeder from the rental store where I work (highly recommended by the way). I am now able to stop on the freeway again, which is very nice.

I realized as I started working on everything that I may have had water in the brake lines. My old reservoir was very worn out and, as I noticed yesterday, leaking. I've been missing some of the rubber molding that keeps water out from under the hood as well... hence I believe water was dripping down into the reservoir. The master cylinder needed replacing anyway as it just looked old and really beat up.

To my fright something else popped up after the repair however... a horrible grinding noise coming from the rear end that seemed to be in sync with the turning of the wheels. I was afraid jacking up the car by the pumpkin on the rear axle had damaged something. Turns out after following all of the proper procedures for replacing a master cylinder and bleeding all of the brakes I forgot the important step of tightening my lug nuts. Problem solved. Back on the road. WHOO

[dmwhiteoak]

Damn, that was a close one. 