Let’s diagnose a hard brake pedal

[oldbmwfan]

I'll whip the horse one more time.

Shut your engine off. Push the brake pedal a few times. Does it get hard? Now start your car, Does the pedal go back to normal?

Long straight at WOT and OP is using brakes a few times, at WOT there is virtually 0 engine vacuum, so it is like the engine is off. When at the end of the straight there is not enough vacuum to operate the brake booster, = hard pedal. When he lets off the throttle for 1 or 2 seconds before the end of the straight the engine vacuum replenishes the booster and the brakes work as normal.

Walk around the pits and I guarantee every large cam V8, non VVT with power brakes has a vacuum can, or a vacuum pump.

In the 70's and early 80's lots of factory cars came with vacuum cans due to all the vacuum operated accessories.

Yep, especially turbo cars which had positive manifold pressure a lot of the time. My '86 Saab 900 turbo had a vacuum canister behind one fender.

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[NightmareMoon]

I've had the same issue on my GT for a while. A friend's GT has never encountered it. He drove my car at an autox and plowed through a corner entry due to the surprise hard pedal.

It only happens after good full throttle run (usually up to high RPM) followed by an immediate stab on the brake pedal. Giving the car a few moments to respond and brake pedal is always fine.

It can be really pucker inducing when it happens on track in a hot braking zone. The pedal is rock hard when it happens. Usually a quick pump gets some brake force back, I guess because vacuum is building again.

I blamed rev hang engine tuning, but if there is a part that we can replace to fix this I'd love to hear it. I have a little trouble believing that its not a bad part if some cars do this and others don't? Ford was useless when I brought it up with them

(No, its not a brake pad, fluid bleeding, or bedding issue)

 

[honeybadger]

I've had the same issue on my GT for a while. A friend's GT has never encountered it. He drove my car at an autox and plowed through a corner entry due to the surprise hard pedal.

It only happens after good full throttle run (usually up to high RPM) followed by an immediate stab on the brake pedal. Giving the car a few moments to respond and brake pedal is always fine.

It can be really pucker inducing when it happens on track in a hot braking zone. The pedal is rock hard when it happens. Usually a quick pump gets some brake force back, I guess because vacuum is building again.

I blamed rev hang engine tuning, but if there is a part that we can replace to fix this I'd love to hear it. I have a little trouble believing that its not a bad part if some cars do this and others don't? Ford was useless when I brought it up with them

(No, its not a brake pad, fluid bleeding, or bedding issue)

Is anything done to your engine besides the tuning?

 

[K4fxd]

Here is a link to a different one than my first link

https://www.amazon.com/JEGS-63011-V...uum+canister+automotive&qid=1617213811&sr=8-3

 

[Caballus]

I had a catastrophic failure that was very similar.

How did you ultimately gain control?

 

[NightmareMoon]

Is anything done to your engine besides the tuning?

Not even tuning. Stock drivetrain.

 

[K4fxd]

Back in the day we would put cans on large cam street cars. Without one braks would be iffy.

If you have a bone stock S550 GT you shouldn't need one on an auto X course.

If you let your car sit for a few days is your pedal hard before you start your car?

 

[Rapid Red]

I'll whip the horse one more time.

Shut your engine off. Push the brake pedal a few times. Does it get hard? Now start your car, Does the pedal go back to normal?

Long straight at WOT and OP is using brakes a few times, at WOT there is virtually 0 engine vacuum, so it is like the engine is off. When at the end of the straight there is not enough vacuum to operate the brake booster, = hard pedal. When he lets off the throttle for 1 or 2 seconds before the end of the straight the engine vacuum replenishes the booster and the brakes work as normal.

Walk around the pits and I guarantee every large cam V8, non VVT with power brakes has a vacuum can, or a vacuum pump.

In the 70's and early 80's lots of factory cars came with vacuum cans due to all the vacuum operated accessories.

Same test different method, key off, press brake and hold. Start engine peddle will drop, booster good.

 

[NightmareMoon]

Same test different method, key off, press brake and hold. Start engine peddle will drop, booster good.

Key off, press brake and hold (goes to floor), start engine, peddle already on the floor.

Key off, pump brake thrice for hard pedal, start engine, peddle drops.

 

[Rapid Red]

Key off, press brake and hold (goes to floor), start engine, peddle already on the floor.

Key off, pump brake thrice for hard pedal, start engine, peddle drops.

Soup sandwich


Nope, brake peddle should not go to the floor, ever.

 

[Alain]

Subscribed! Good luck dude !!
sounds to me like some sort of vacuum issue.

 

[svttim]

How did you ultimately gain control?

I didnt. Gravel trap, tire wall and a body shop. Fortunately I was in a full competition car with legal cage. Cosmetic damage. Not bad for an off at 143 mph.. Walked away

 

[oldbmwfan]

I didnt. Gravel trap, tire wall and a body shop. Fortunately I was in a full competition car with legal cage. Cosmetic damage. Not bad for an off at 143 mph.. Walked away

Sounds like a true "code brown" - ouch! Glad it was only cosmetic.

 

[honeybadger]

Well, I think this tells the story.

On hot idle

At 1000 RPM or so

At one point when it was running particularly rough

 

[Alain]

Well, I think this tells the story.

On hot idle

At 1000 RPM or so

At one point when it was running particularly rough

Elaborate a little for those of us that are not as mechanically inclined.
mim assuming there’s not enough vacuum.
what would be normal?

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