Author Topic: 68 GT500KR Brake Woes  (Read 1219 times)

EmeraldMC

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68 GT500KR Brake Woes
« on: October 05, 2019, 03:02:27 PM »
So here's what I've got: The car has been in the family since new, from my uncle to my dad to me.  It's got under 46K on it.

I went thru the brakes last year, changing the fluid, replacing MC, calipers, and rotors, and adjusting the shoes, and bleeding it.  Stopped pretty decent.  Then a few weeks later the booster went out, so I had it rebuilt and the proportioning valve and distribution block rebuilt as well. After reassembling everything and bleeding it there's a good pedal feel when the motor is off. 

As soon as I start it, the pedal goes waaaay down, and then sinks a little further.  It stops, but pulls to the right.  When I start checking things, there's no flow to the LF caliper, explaining the pull tot he right.  It has good flow on all other three wheels, the rear brakes are properly adjusted, the bleed screws on the calipers are in the right position, I bench bled the MC.

I've bled a couple of gallons thru it, and I'm stumped - its as if the piston in the distribution block is going all the way to the front, blocking the LF port.  I even got a repro distribution block and proportioning valve assembly from Tony Branda to see if the rebuild of my originals was not up to snuff.  Exact same behavior.

Any ideas on what to check, as I'm out of ideas, and frustrated over not being able to drive the car.

Thanks for any help,

Eric Mullins

Coralsnake

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Re: 68 GT500KR Brake Woes
« Reply #1 on: October 05, 2019, 03:07:38 PM »
Did you put the calipers on correctly?

Can you share a picture of them installed?
« Last Edit: October 24, 2019, 07:46:22 AM by Coralsnake »
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acman63

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Re: 68 GT500KR Brake Woes
« Reply #2 on: October 05, 2019, 04:18:23 PM »
did you have the special tool in the junction block when you bled the brakes. Theres a shaft in there and you take the plastic switch out  and if the hole is empty,  insert the tool,  bleed the brakes then put the switch back in

NPD  has em  $10
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EmeraldMC

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Re: 68 GT500KR Brake Woes
« Reply #3 on: October 05, 2019, 04:26:12 PM »
yep, I've got the tool. 

It'll be Monday before I can get a picture of the calipers.  I'll do it then, anything else that would be helpful w a pic?

Coralsnake

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Re: 68 GT500KR Brake Woes
« Reply #4 on: October 05, 2019, 05:44:36 PM »
Lets start ther, as installed from the inboard side so we can look at the bleeders
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Corey Bowcutt

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Re: 68 GT500KR Brake Woes
« Reply #5 on: October 05, 2019, 07:10:08 PM »
acman63,

What happens if you bleed your brakes and did not use this tool?  I have never heard of this tool before and I have bled my breaks twice.  I do have an issue with my brakes but sounds significantly different from what the OP has.  My breaks feel nice when you press lightly on the pedal.  But if you push harder the brakes to not get any better and is seems the peddle bottoms out.  I could never lock up my brakes if I wanted to.  Would this be the result of not using this tool when bleeding the brakes?  If not any idea what the issue is? Master cylinder? Power booster?...

Corey

TLea

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Re: 68 GT500KR Brake Woes
« Reply #6 on: October 07, 2019, 06:39:22 AM »
Calipers installed backwards nor not using the lock tool will isolate left side from right. The tool keeps plunger from shifting front to back if brake failure but not side to side.
You say no fluid to LF caliper but yes to right. Start there. Loosen hose from LF caliper slightly and step on pedal. If flow caliper issue, if no move back to cracking brake line from LF at distribution block. If flow line problem. If no problem with distribution block.
As a sidebar I have seen many times calipers installed backwards (left and right) In correct position bleeder points towards rear of car not up

69 GT350 Vert

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Re: 68 GT500KR Brake Woes
« Reply #7 on: October 07, 2019, 09:26:05 AM »
Calipers installed backwards nor not using the lock tool will isolate left side from right. The tool keeps plunger from shifting front to back if brake failure but not side to side.
You say no fluid to LF caliper but yes to right. Start there. Loosen hose from LF caliper slightly and step on pedal. If flow caliper issue, if no move back to cracking brake line from LF at distribution block. If flow line problem. If no problem with distribution block.
As a sidebar I have seen many times calipers installed backwards (left and right) In correct position bleeder points towards rear of car not up

Great information in this thread.  I didn't know about this tool to replace the shaft.  What if the hole is not empty and the mechanism is not centered in the hole?  How can I re-center it?  After I replaced all my brake lines and rebuilt the entire system, I left my brake light harness connector disconnected because it always turns on.   

shelbydoug

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Re: 68 GT500KR Brake Woes
« Reply #8 on: October 07, 2019, 09:33:58 AM »
I didn't read the entire thread but if it wasn't already suggested, you probably just need to change out the bleeder valves in the calipers. Those things freeze closed in time.

Donmo

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Re: 68 GT500KR Brake Woes
« Reply #9 on: October 10, 2019, 09:15:27 PM »
What's the chance of a collapsed brake hose to the LF caliper? 

1967 eight barrel

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Re: 68 GT500KR Brake Woes
« Reply #10 on: October 24, 2019, 01:13:20 AM »
Everyone missed one of my recent issues on my 67 GT 500. The centering tool is ABSOLUTELY necessary.  You'll have to recenter the valve if the light is on that says "Brake" on the cluster.
I can provide directions if you need. in a PM.
I had a pair of Chinesium hoses from a well known parts vendor When the hose was crimped to the fitting they pinched off the rubber line most of the way closed. Even with the tool the car had poor breaking. When I found I could hardly blow through the brake hoses I bought a pair from Napa.  It resolved the issue.

GT350AUS

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Re: 68 GT500KR Brake Woes
« Reply #11 on: October 24, 2019, 03:03:05 AM »
Is this only a 67 issue or KR also?

Coralsnake

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Re: 68 GT500KR Brake Woes
« Reply #12 on: October 24, 2019, 07:20:24 AM »
If the dash light is in on, the valve should be centered. The procedure is in the shop manual. It was mentioned multiple times in this thread (including second reply)

Yes, it applies to all 1968s.

« Last Edit: October 24, 2019, 07:47:10 AM by Coralsnake »
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Steve McDonald Formally known as Mcdonas

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Re: 68 GT500KR Brake Woes
« Reply #13 on: October 24, 2019, 11:29:07 AM »
An old test for “air entrapment” is to have someone pump up the brakes and hold the pedal down, then carefully remove the master cylinder cap and quickly have them release the pedal, you should see a slight rise as the fluid returns to the master cylinder. If you have a tall geyser of fluid then you have “air entrapment” in what ever side of the brake system has the air. As someone mentioned if the calipers are mounted on the wrong side, the bleeder location will not allow all of the air to be removed. If I remember correctly the bleeder should be horizontal to the ground if is correct, if it points up then it’s on the wrong side. The centering tool was never used at the dealer level. If the brake light is on , the valve is not centered, if it’s off it’s centered. We used to have someone sit in the car and open a line on one side of the master cylinder and slowly push it down until the light went out, if one side did not do it, you tried the other side line. The idea was to create a low pressure and the valve would move to shut off the pressure on that side
Hope this helps
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1967 eight barrel

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Re: 68 GT500KR Brake Woes
« Reply #14 on: October 24, 2019, 02:24:05 PM »
The dash brake light is OUT if the bias valve is centered.  It comes on when the key is turned to the START position and extinguishes when the key is released to run. Unless of course the valve isn't centered and it stays lit in that case.

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