Fox Brake booster keeps failing.

93CalypsoConvert · Jul 18, 2023

So I've had brake issues the entire time I've owned this car. Now, I can't seem to find a brake booster that will actually work.

Here is what is happening. Its weird, bear with me:

- The VERY FIRST time I press the brake pedal after the car is completely cooled off, the brakes work great. After that, that the performance is crappy.
- After the engine (and engine bay) is hot from driving, the brake pedal sticks. I must use my foot to draw the pedal back to the top. The hotter it is under the hood, the worse this gets.
This happens every time I drive and is not intermittent.

NOW, the worst part is, this has happened to the original and TWO NEW BOOSTERS. They work great for about a week, then they begin to stick. Like I said this has happened with two brand new brake boosters. I have verified multiple different times that there is NO fluid leaking from the back of the master cylinder and the pedal pivot is free.

I know its the booster because I can unbolt the master cylinder and the brake pedal will still be stuck.

I don't think its a vacuum issue either. I installed an auxiliary vacuum pump for brakes with no difference.

I am at a total loss here.

Noobz347 · Jul 18, 2023

93CalypsoConvert said:
I know its the booster because I can unbolt the master cylinder and the brake pedal will still be stuck.

I'm not sure what this means. Does this mean that when you separate the MC from the booster that the pedal is [still] stuck?

If so, wouldn't that indicate a problem with the MC?

For s and grins, my first guess is that the stroke of the MC is too long and it's jamming it into the MC.

93CalypsoConvert · Jul 18, 2023

Yes. The pedal remains stuck with no master cylinder attached.

When it is stuck, the brakes release when the MC is unbolted, just leaving the pedal stuck at that point.

Noobz347 · Jul 18, 2023

Have you kept the same MC through each of these boosters?

Praetorious · Jul 18, 2023

How long have you owned the car? Since you've had the issue since you've owned it sounds like the P.O monkeyed around with the master cylinder etc and it hasn't been the same since, correct me if I'm wrong, aren't these Master cylinders pushrods adjustable? Could it be too long or short or what not? It's definitely a strange problem.

Just wanted to add that I reread your post and you say it gets stuck even with the master cylinder off,I believe the brake linkage has some plastic spacer to keep everything smooth, maybe it's gone?

93CalypsoConvert · Jul 18, 2023

Different MC. Owned two years. Pushrod is adjustable, and is adjusted. Maybe there is a linkage bind, but how would that effect it when the engine is hot?

General karthief · Jul 18, 2023

Next time it gets stuck loosen the nuts for the booster and see if anything happens. I suspect the linkage between the peddle and the booster or the peddle swing getting in a bind or catching on something.
Don't recall, stick or auto?

AeroCoupe · Jul 18, 2023

Can you provide some details on the car (year etc.) and the brake system (stock MC, stock booster, etc.). Was the car converted from an AOD to a T5 or AOD to T5? Have there been any modifications to the brake system like a 3 to 2 conversion, removing the stock combination (proportioning) valve, etc.?

The above information helps with narrow the focus down. For instance if you told me the car has always had the same type of transmission (manual or auto) I would ask if you have you inspected the brake pedal where it pivots in the assembly that hold the pedal(s). This is due there are bushings in there and if they are no longer there and the rod is metal to metal on the housing then the holes could be oblonged and causing bind.

That was just one example. We need more info to narrow this down.

Praetorious · Jul 18, 2023

The hot and cold issue could be a expanding and contracting situation, I had a vacuum leak once that was awful when the car was cold and nearly gone when hot for obvious metallurgical reasons.

Your gonna have to play with it till you figure it out, maybe get under the dash while a Friend moves the pedal and all you attention is on the linkage and see if you can see anything strange happening.

I have a extra auto pedal set at home,I know it's not a standard but its still the same brake setup for the most part ,when I get home from work I'll look at it and see if I can see anything that might aid your problem.

93CalypsoConvert · Jul 18, 2023

Good idea. I'll loosen them up and see what happens.

1993, always been a manual. Stock brakes. Drums have been rebuilt, rubber lines replaced, original proportioning valve, replaced master and booster, and resurfaced rotors.

I think the pedal pivot is metal on metal. I'll have to check. I don't recall a bushing

Car has 42k actual miles.

AeroCoupe · Jul 18, 2023

That's not a lot of miles for the bushings to be gone so that may not factor in but its a good to check. This guy has a pretty good video on how to go about replacing the bushings on the clutch pedal and the brake pedal is pretty much the same:

93CalypsoConvert · Jul 18, 2023

Just checked. No bushings.

93CalypsoConvert · Jul 18, 2023

I loosened the bolts. No effect. Under the dash, you can completely disconnect the brake lever from the booster and the booster still sticks (MC off). This concludes that it must be the booster... Again.

The MC shouldn't be responsible for returning the booster, correct? Every booster I've seen goes back by itself.

Is it possible I'm just that unlucky with boosters??

General karthief · Jul 18, 2023

It's possible to get consecutive bad boosters, unlikely but possible.
Is the vacuum line still hooked to the stock location? (Those few miles it likely is)

93CalypsoConvert · Jul 18, 2023

From the tree labeled BB

General karthief · Jul 18, 2023

Will it stick with the engine off? (Still hot)
Will it stick with the master cylinder off?

93CalypsoConvert · Jul 18, 2023

Will not stick without vacuum.
Will stick without MC

Noobz347 · Jul 18, 2023

93CalypsoConvert said:
Will not stick without vacuum.
Will stick without MC

Do you mean that the booster is sticking with or without the MC?

Noobz347 · Jul 18, 2023

This is really starting to feel like, "Green Eggs and Ham"

93CalypsoConvert · Jul 18, 2023

Both. MC does not affect if it sticks. If it's hot and has vacuum, it sticks.

Fox Brake booster keeps failing.

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wonder how much it would cost to ship you a pair

lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

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Active Member

wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

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Stangnet Facilities Maint Tech... Er... Janitor

Stangnet Facilities Maint Tech... Er... Janitor

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