GT500 Brake issues

[kbraitbe]

I have had my GT500 for nearly 6 years and have had a number of mechanics attempt to get my brakes in working order.  After spending $1000s the brakes continue to be a mess.  Zero feel and with one touch they lock up.  What is different about GT500 brakes Vs a normal 390 mustang? 

[Coralsnake]

There is no difference. Its not a complicated system.

Do you have any "upgraded or after market" parts in the system?

Are your calipers correctly installed?

[TLea]

Slight difference. 2.5 vs 1.75 rear brakes but that won't make it do what you are describing. Check prop valve function or possible blockage in lines/hoses

[67350#1242]

Malfunctioning power brake booster can cause super sensitive braking with little modulation.   Also make sure you have correct disc/drum master cylinder.

[kkupec02]

Ever put a vacuum gauge on the booster? Low vacuum may be the problem

[S7MS427]

Ah, this brings back memories from when I bought my car in 1977. After driving a 1965 Mustang for some years, I found that the GT500's brakes felt like they wanted to put me through the windshield when I touched the brake pedal. The difference between the two cars was truly startling. I soon learned that this was normal for the car and, once I got used to the brakes, they began to feel more normal and I didn't have a problem. So see if using a light touch doesn't fix the problem. If that doesn't do it, then it is time to start looking deeper. Best of luck solving your problem.

[shelbydoug]

At this point you pronably need to actually measure the amount of pressure the calipers are.recieving from the master.

With no boost, they should be around 500psi. With vacuum boost, around 700psi.


Two things.can efect the pressure. Number one is going.to be the bore of the master cylinder.

Stock it should be 15/16". Smaller bore will give more pressure. Larger bore, less pressure.

66 Mustang is 3/4" and wrong for a 67.
I ll bet you the aftermarket lists them with the same part number.

The variation is going to be 3/4", 15/16" and.1".

It is too commom for the 3/4" bore to be substitied and marked as a stock replacement, which it is not.


The sensativity you describe is related to the bore size of the master, not the booster which is obviously working correctly.

You can reduce boost.by lowering the vacuum to it.

Originally, they are intended to work with 20.to 22 inches of vacuum.

Under about 14, they stop working.


A stock.67.GT 500.uses a.very mild cam profile and really is mismatched to the booster as originally supplied.

You will find that if you change to virtually any aftermarket cam, the idle vacuum will drop to about 15, wbich will reduce the boost the system provides and help the brake sensativity.


Sorry for the long post but you.have run into an engineering issue and explanation is necessary.



Additionally, pedal travel changes with master bore. Originally that travel was "correct" if at full travel, you could put one foot under the pedal at.full lock.

The only way you can change that is with bore change. The.larger the bore, the less pedal travel.


Race cars typocally go up to 1-3/16" which makes the pedal travel very short, in fact just touching it, but they are manual with no boost.
That os.more that a race engine typically will idle with about 8 imches so there isnt enough vacuum supplied.

This is all.about the master bore. The rears do not care. They are just along for the ride.


There is a huge differece just between the 15/16" and 1" alone and the only way YOU can decide is to try the combinations.
There really is no simple answer to this.

Comander Scott once commented "you remind me of the man who demanded to be taught the wisdom of the world while standing on one foot".
Don't be that guy.

[KR500]

Make sure the brake booster push rod is adjusted to the correct length. If it is to long the brakes can be very sensitive.

[68blk500c]

Is there a specification for the length, or trial and error?

[69 GT350 Vert]

Is there a specification for the length, or trial and error?

Yes, it's in the shop manual.

[TA Coupe]

Also, if your breakhoses are original or have not been changed for quite a number of years, I would change them as collapsing Breakhoses can cause issues, and you cannot see that they are bad inside. Not saying that that is your issue, but definitely something to think about While working on your brake system. Better safe than sorry.

      Roy

[JD]

Shop Manual... also do you have the proportioning valve

In the shop manual there is instruction for adjusting the brake booster push rod

[roddster]

And: has the proportioning valve been rebuilt?  The spring inside actually rusts apart causing issues.
  One final comment: On my tribute car which sat for 30 years, there was a ridge inside the prop valve bore that was impossible to see.  That too caused problems for a while.

[Road Reptile]

Would be nice to see a list of parts replaced, and that may help determine what is going on.
You mention lock up, yet do not say front or rear. Going back to base or verifying part numbers should point you in the proper direction. Roadster has mentioned a very commonly overlooked part.
R.R.

[Coralsnake]

The op left the chat and hasnt returned