Brake Problem, Confused

I posted this thread in the ford probe forum, but thought i might have a better chance here.

Ok, my fiancee was driving the car the other day and the brakes failed on her and she got into a collision.

What is weird is that the brakes were acting like lines cut or no fluid, no brake response, no fade just simply not there at all, she pushed the pedal down with ZERO resistance and nothing, then BAM. Crashed, minor front end damage but still runs.

Heres what i dont understand, after this she was able to drive it and the brakes were working fine, just as always, until the drive home at a stoplight it happened again, just nothing, ran right thru the light.

When she brought it home i checked it out but i wasnt able to repeat the problem, brakes work fine up and down the street, admitedly i havent drove it much to try and recreate the issue but still.

im no mechanic (electrician) but im pretty decent with a wrench but i cant figure this one out.

Master cyl appears good, pads & rotors are decent, fluid level is high with no leaks, im baffled.

Im wondering if this could be the power booster, or the master cyl being bad but im just not able to tell, i thought i would try and gather some input before i go replacing parts.

Any input on how i would check this would be greatly appreciated, or any past experience at all. Again what baffles me is how one minute they work fine, and then all of a suddun...nothing, no pedal resistance at all, and then bam right back to normal. in my experience when brakes go out there is some warning not weirdo symptoms like this.

Car is 1996, 4cyl 2.0, don't know if its abs or not.

i just wanted some input before i go and start replacing parts at random.

Thanks in advance, sorry for the long read.

Hello

Welcome to AFs!

Now....if this was my 17 old daughter I would ask her if she was texting, downloading a ringtone, or changing the station on the radio. Would I get the truth? I doubt it.

Your fiancee is an adult and we need to believe what she says.

Brakes in my opinion aren't something that work and then fail and then work again. It wouldn't be the brake booster because the brake pedal would be very hard to push. If you want to try this, without the engine running push the pedal a few times until there isn't any vacuum left.

Brake calipers, cylinders, and master cylinders seals either work or start failing (leaking) and you begin loosing brake fluid. The brakes will either be mushy or not work at all when the fluid is gone.

I would drive the car until the problem happens to you. Be ready with the E-brake (parking brake).

Would it be possible to switch cars for a while? My wife and I have done this when there is a problem that I need to figure out or she doesn't want to deal with.

Regards

Dan

What you describe are classic symptoms of a bad master cylinder. The internal seals fail and let fluid past instead of compressing it in the lines. You won't see any leaks, and the braking will come and go. Often times you'll have to "pump" it several times and one of those times you'll get a seal and it will provide braking.

There are two circuits in the master cylinder. One controls the front brakes and the other controls the rear. They never fail at the same time. What may seem like no braking is just a funny sinking feeling when the pedal drops toward the floor. If you keep pressing, you'll eventually get braking on the other circuit, but of course at a highly reduced effectiveness.

Get a new master cylinder, hers is "bypassing" internally. There is no leak, it just spits the fluid back into the reservoir.

This is a hard problem to replicate when you run into it. I've sat there at the shop with my foot on and off the pedal 20 or more times, engine on, engine off. Sometimes you have to "tickle" it, playing with the brake pedal to get it to do it.

You never lose the entire brake system, but it seems that way since you expect some resistance at the pedal. In reality, it'll go to the stop point, which often is not enough to stop the car safely.

Bob

Why does the master cylinder work some times and not others? I've NEVER experienced in all of my years on working on "USED UP" vehicles.
Dan

Master cylinders have O-ring seals in them. As they wear against the iron or aluminum surface of the bore, they wear out. Depending on how the pressure is applied, they will "catch" and seal, or sometimes they will not seal and fall toward the floor.

Ever have one of those old-school bike pumps? As the seal wears out, sometimes it catches and pushes pressure, and sometimes you almost fall on your face because it doesn't catch pressure.

Why does the master cylinder work some times and not others? I've NEVER experienced in all of my years on working on "USED UP" vehicles.
Dan
In 26 years of working on cars, I have come across this ONCE.

My 88 Bonneville did this a total of 3 times over a period of 1 week, (once for my wife, twice for me) where the pedal went right to the floor, with nothing happening.

All 3 times a quick pump of the brakes got the brakes back.

Since I am lazy, I did nothing about it, other than check the fluid and lines for external leaks. Oddly enough, that same master cyl worked for 3 more years, problem-free.

BTW FireGuy, tell your fiancee to practice using the parking brake while moving. That is one function of the parking brake...... a last-ditch stopping method when everything else fails.

Master cylinders have O-ring seals in them. As they wear against the iron or aluminum surface of the bore, they wear out. Depending on how the pressure is applied, they will "catch" and seal, or sometimes they will not seal and fall toward the floor.

Ever have one of those old-school bike pumps? As the seal wears out, sometimes it catches and pushes pressure, and sometimes you almost fall on your face because it doesn't catch pressure.

curtis73

Thank you for the excellent explanation! Yes I've experienced that with a pump. Even the "old" guys can learn something once and a while.:iceslolan

Dan

I guess I've just had a larger number of junk cars :) Its a pretty common thing for me. I had it happen in a 78 chevy pickup with a pickup camper on it towing an 18' boat on the downhill side of a bridge with a toll booth at the end.

Fortunately it was the rears that went out, so I still had front truck brakes and trailer brakes, but that was a white knuckle time :)

My 96 Impala SS master cylinder is starting to go. I notice that sometimes it travels more than it should, but then a quick re-pump will bring it back to normal. I'll probably let it go until it really fails, because I'm too cheap to spend money right now.

I guess I've just had a larger number of junk cars :) Its a pretty common thing for me. I had it happen in a 78 chevy pickup with a pickup camper on it towing an 18' boat on the downhill side of a bridge with a toll booth at the end.

Fortunately it was the rears that went out, so I still had front truck brakes and trailer brakes, but that was a white knuckle time :)

My 96 Impala SS master cylinder is starting to go. I notice that sometimes it travels more than it should, but then a quick re-pump will bring it back to normal. I'll probably let it go until it really fails, because I'm too cheap to spend money right now.

Curtis73

Well I preaty much in the same boat.....I've never owned (until reciently) a "used" car..........they've all been used up.

Dan

My 96 Impala SS master cylinder is starting to go. I notice that sometimes it travels more than it should, but then a quick re-pump will bring it back to normal.
Master cylinders are a bit funny. A few of mine have done this, but not all.
I have vehicles that are 25, 32 and 41 years old, all with the original master cylinders, that still work perfectly.

My 85 Dodge One Ton which is 23 years old and the MC is working great!:iceslolan:popcorn:

Master cylinders have O-ring seals in them. As they wear against the iron or aluminum surface of the bore, they wear out. Depending on how the pressure is applied, they will "catch" and seal, or sometimes they will not seal and fall toward the floor.

Ever have one of those old-school bike pumps? As the seal wears out, sometimes it catches and pushes pressure, and sometimes you almost fall on your face because it doesn't catch pressure.

The master cylinders I've been in all had cup seals rather than o-rings. Very similar to the bike pump Curtis is talking about above.
When they start to wear you can usually rest your foot on the pedal and have it sink slowly to the floor. But a sharp hit will seat the seals hard against the bore and make them work properly.

Often when the seals are going, there are traces of brake fluid leaking out of the cylinder and down towards the brake pedal. If brake fluid leaks inside your car it can eat carpets, matts, paint etc.

I've seen it happen with freshly rebuilt masters a few times. That was enough to make me buy new ones from there on, also did the same for customer's cars whenever they could be talked into it.

I did brakes daily for about 7 years and probably saw 50 cases total.

Bob