68 GT350 Power Brake Issue

[Corey Bowcutt]

I am getting bready to start the winter projects on my 68 GT350.  On the list is doing some brake work.  I have an issue that I would like to fix but need help determining where to start.  The issue is when I press my brakepedal lightly for a standard stop they feel great.  But if I push harder for a more extreme stop the pedal goes hard and the braking does not get any better.  I could not lock up the brakes if I wanted to.

I was told by a friend that if the pedal is soft it is a master cylinder issue but if the pedal is hard it is a power booster issue.  Does this sound correct?  Is the booster the first place to look?  Is there trouble shooting that can be done to determine if in fact the booster is bad?

I feel like this problem started after I did some brake work and blead the brakes a few years ago but not 100% sure.  I did do a very thorough rebleeding of the brakes and it did not make a difference.

Any suggestions or recommendations would be greatly appreciated.

Corey

[Coralsnake]

The booster is probably a good place to start

Do you have your calipers on the correct sides?

[Bigblock]

I'll be waiting to hear how this is solved!

Had a 67 Cougar that did the same thing. Great power brakes if you pushed slowly. Had Dewey rebuild the booster and got a another rebuild booster/MC from NAPA and still the same problem. Tried vacuum pump and tank (20in.), power bleeding. Never fixed, sold the car.

[Steve McDonald Formally known as Mcdonas]

First check everything mechanical, calipers on the correct side, everything is free and moving smoothly, also check the rear wheel cylinders to make sure they aren't stuck or sticking. Next due an "Air entrapment test". Have someone to pump up the pedal about 10 times and hold, then take the cap off the master cylinder, hold the cap slightly over the master cylinder to prevent "splash" Release the pedal quickly and look for a geyser of fluid from either chamber, if its in the back chamber is air in the from system, if its in the front chamber its in the back brakes. You should see a slight "wave" as the brakes release. I'll look through my old stuff for the Ford Brake Certified Training Program book, it has lots of tests and tips for brakes

[Corey Bowcutt]

I am pretty sure I have been in this discussion before and my calipers are on the correct side.  But I have attached a photo so if it looks incorrect please let me know.

[Steve McDonald Formally known as Mcdonas]

Looks right

[shelbydoug]

I do know that one of the recommendations of brake modifications when you go racing is to increase the size of the bore of the master cylinder.

It changes the proportion of the fluid pressure to the brakes and gives the pedal a harder feel.

At one time I had gone to manual brakes and eliminated the power booster. With that set up I was using a 1-1/4" bore master. It was a Ford part and originally had a Ford Truck application.



When I went back to the power brakes, I also went to the "Trans-Am" "Big Ford" front calipers, so I was in no-mans land, all by myself determining how to set this mess up, with NO ONE knowing  WTF I was talking about.

What I discovered was that the 1-1/4" bore master could no longer be used with the power brakes. I also had discovered an inexpensive source of master cylinders. I started at 1-1/4" bore and went down in size in 1/8" increments.

I went all the way down to 7/8", which I believe was the original bore size new in the car. Through trial and error I arrived at 1" as the best balance between stopping, pedal feel and lock up resistance.

The car now stops HARD and has a firm feel BUT you can't lock up the brakes.



The point of this entire post by me is that reading the symptoms that you list, it strikes me that it sounds like someone messed with the master cylinder in your car and the cylinder bore of the master is too big for the feel that you want.

If you WANT to be able to lock up the brakes (and I don't know why you would want to) you need the original 7/8" bore master. Locking them up isn't going to make it stop any faster and in fact may make you spin the car since the rear will come around.



One of the characteristics of the 65 GT350's Shelby changed was that no matter how hard you step on the brakes, you can't lock them up.

[Corey Bowcutt]

It is not that I want to lock up my brakes at all.  But the feel I have with my brakes does not feel correct to me.  The pedal feels like it hits a brick wall but the brakes do not feel great.  I just do not believe it is correct.  The last time I had experience driving these cars (Mustangs) was in the late 70's so I do not have a rich history to draw from.  But in my opinion things are not correct. 

Nothing is impossible but I would be shocked if the bore of my cylinder was changed by the PO.  I do know the master cylinder in my car is correct from the outside but what is going on on the inside I do not know yet.

But I do appreciate all replies because they all give me something to think about.

Corey

[shelbydoug]

Ford power brakes of the era came from the factory as soft powder puffs.

They were deemed adequate by the servicing  mechanic if when he stepped on the pedal he could put his left foot under it, the pedal didn't squash his foot and the car stopped.

They were set up so that a 85 pound 85 year old lady could operate them without needing a trip to the Emergency room. There were no exceptions. So 85/85 brakes are stock.

If you wanted them to compliment the rest of the performance of the car, then they needed "just a little work".


I just hit a deer with my Audi a month ago. I go so far back with these cars, the last time I hit something was a dinosaur.

[corbins]

Corey... what did you ever figure out Anything ?

[Corey Bowcutt]

I removed both the master cylinder and booster and sent them to Jim Cowles to rebuild. I will let you know when I get them back if that does the trick.

[Royce Peterson]

From the symptoms described it would lead me to check the pushrod length to make sure it is not too short. Next would be a thorough bleeding of the system. Finally make sure the rear shoes have working adjusters, and that the adjustment is proper. Final adjustment of the rear brakes is done by reversing, slamming the brakes, reversing, slam the brakes, repeat, repeat, repeat.

The brakes on your Shelby should be plenty adequate if they are in good shape and adjusted properly.

[Corey Bowcutt]

Can anyone tell me what the correct length of the pushrod is?

[KR500]

Corey
Check page # 2-4 fig.6 and page # 2-34 of the 68 service manual. I made My own gauge.

Rodney

[1967 eight barrel]

You should have sent the booster to Power Boster Exchange in Portland, OR. They had all the phenolics tooled and have every internal item new.
They're about 155.00 for the booster rebuild providing all the phenolics are good to go. Most here will tell you they're the best. Jim would be better for the finish on the master, providing it's an original Bendix unit.
                                                             -Keith