Need help diagnosing brake issue

[tomservo92]

When I first start my car in the morning, the brake pedal is very stiff. Once started, the pedal acts normally and there is no degradation in brake performance. When I turn it off, the pedal acts normally for awhile but will stiffen up again over time (I've seen it do it in less than 2 hours). I replaced the check valve since it was easy and inexpensive but no luck. SInce the check valve is new and functioning properly, I don't think the hose is the issue. IMO that leaves the brake booster itself. Am I missing something else?

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[Mike Pfeifer]

Sounds about right to me, assuming no one is pressing the pedal after the engine is turned off.

 

[tomservo92]

Sounds about right to me, assuming no one is pressing the pedal after the engine is turned off.

There better not be since I'm the only one that drives it!

I've done a fair amount of wrenching on my cars but I have never had to replace a brake booster. How hard of a job is it?

 

[sk47]

Hello; I will follow this thread in hopes of learning what is happening. After reading your description I am not sure what you mean by "stiffen up".
My first guess being you are feeling that the "power" portion of the power brakes is being lost. In other words, feels more like manual brakes as the pedal gets hard to push to get a breaking effect. Like the vacuum boost is gone or lessened.

As to replacing the booster it has been decades since i did such so might be different on a newer car. Using a stock type booster ought to be Fairly straight forward. Undo the vacuum hose and used to be two to four nuts. Use to be adjusting the pushrod was the technical part.

Some will depend on if you plan to reuse the master cylinder or buy a booster with a new master cylinder on it. Would not surprise me to find now days it is all one part.

Then there are the sensors and wirings. New cars have antilock brake systems and computer feeds. This is where my old knowledge likely will not suffice. Hope the system will not have to be reprogramed.

good luck

 

[tomservo92]

Hello; I will follow this thread in hopes of learning what is happening. After reading your description I am not sure what you mean by "stiffen up".
My first guess being you are feeling that the "power" portion of the power brakes is being lost. In other words, feels more like manual brakes as the pedal gets hard to push to get a breaking effect. Like the vacuum boost is gone or lessened.

As to replacing the booster it has been decades since i did such so might be different on a newer car. Using a stock type booster ought to be Fairly straight forward. Undo the vacuum hose and used to be two to four nuts. Use to be adjusting the pushrod was the technical part.

Some will depend on if you plan to reuse the master cylinder or buy a booster with a new master cylinder on it. Would not surprise me to find now days it is all one part.

Then there are the sensors and wirings. New cars have antilock brake systems and computer feeds. This is where my old knowledge likely will not suffice. Hope the system will not have to be reprogramed.

good luck

You almost have it. The pedal barely moves when starting the car after it sits for awhile. Once it's started, the pedal and brakes behave normally. It feels like someone has pumped the brakes when the car is off and expended the vacuum boost (even though I know without a doubt nobody has done so).

 

[sk47]

Hello; Back a couple of years ago, maybe three by now, I replaced both rear calipers on my pickup truck. Did all the work including the bleeding at all four corners with the engine off. Got a good firm pedal after doing such. Soon as i started the engine the pedal got softer and I had an ABS light on the dash. Turned out I had allowed too much brake fluid to drain out due to a problem with one rear caliper ( the bras washer on the banjo fitting was wedged tightly on the fitting.) Any way seems i had allowed too much fluid out and the anti-lock system wound up with air pockets.
Truck drove OK and I had brakes, but they were soft + the dash warning light. I made a long thread about the incident on here. Almost took it in to a shop to get it bleed but found a shade tree workaround.

Back to the topic. Two things. One is to pull codes to see if any show up about brakes.

Next, take the car for a drive and use the brakes some but not real hard. After a bit pull into some place where a light application of the brakes will stop the car. Get out and gingerly touch each wheel. Checking to see if one or more are extra hot. A sticky caliper would get everything hot and maybe build up extra pressure in the hydraulics?????

 

[tomservo92]

No codes.

No excessive brake heat.

Again, brakes are functioning properly when the car is running. It just won't hold booster vacuum once the engine is off.

 

[Mike Pfeifer]

Yeah, most likely the brake booster has a slight vacuum leak and the vacuum that is held there while the car is off bleeds off over time. It will probably get worse eventually. I have not replaced one on a mustang, but have done countless BMWs. The gist is disconnect brake pedal, remove brake lines from master cylinder, remove master cylinder from booster, unless they are able to be removed as a unit, and then remove booster from firewall. There may be things in the way that will also need to be removed, like wiper linkage, but not sure on the S550.

 

[Keeffa]

When I first start my car in the morning, the brake pedal is very stiff. Once started, the pedal acts normally and there is no degradation in brake performance. When I turn it off, the pedal acts normally for awhile but will stiffen up again over time (I've seen it do it in less than 2 hours). I replaced the check valve since it was easy and inexpensive but no luck. SInce the check valve is new and functioning properly, I don't think the hose is the issue. IMO that leaves the brake booster itself. Am I missing something else?

What your describing sound's normal to me. The pedal will be hard until you start it and vacuum builds up then when you turn it off there's no vacuum it'll go hard again.

 

[tomservo92]

What your describing sound's normal to me. The pedal will be hard until you start it and vacuum builds up then when you turn it off there's no vacuum it'll go hard again.

It wasn't always like this. In the past, it would hold vacuum overnight but now it doesn't. In fact, it won't hold it more than a couple of hours.

 

[Keeffa]

It wasn't always like this. In the past, it would hold vacuum overnight but now it doesn't. In fact, it won't hold it more than a couple of hours.

I've never noticed how long it takes for the pedal to get hard again, so long as you feel it change as soon as you start your car and the brakes are working correctly when driving it should be ok.

 

[sk47]

It wasn't always like this. In the past, it would hold vacuum overnight but now it doesn't.

the brakes are working correctly when driving it should be ok.

Hello; I now understand. Yes to the vacuum ought to hold for a while, even days. I do not know how long is normal but it is much more than a few hours.
That said you will still have functioning brakes even if the brake booster fails has been my take. They will be essentially manual brakes but ought to work.
If my thinking is correct there are at least two areas for such a leak. One being the canister/diaphragm. The other some sort of check valve. Both ought to be part of the booster but newer cars have tricked me.

 

[tomservo92]

Hello; I now understand. Yes to the vacuum ought to hold for a while, even days. I do not know how long is normal but it is much more than a few hours.
That said you will still have functioning brakes even if the brake booster fails has been my take. They will be essentially manual brakes but ought to work.
If my thinking is correct there are at least two areas for such a leak. One being the canister/diaphragm. The other some sort of check valve. Both ought to be part of the booster but newer cars have tricked me.

I replaced the check valve so it must somewhere in the booster itself (like the canister or diaphragm as you stated).

 

[S550HPP]

Silly suggestion but maybe the booster cap is not sealing they can cross thread easily, and even strip, or gasket may have fallen out...

 

[tomservo92]

Silly suggestion but maybe the booster cap is not sealing they can cross thread easily, and even strip, or gasket may have fallen out...

Are you referring to the master cylinder cap (where you put in brake fluid)?

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