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Old 10-13-2008, 05:06 PM   #1 (permalink)
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68 Original Front Disks

First time out for 'longer drive' today. Entire brake system has been redone, as car had 'sat' for almost 30 years. I'll skip details and go to point:

Front brakes are evidently not totally releasing, which is keeping tension/friction on the front pads > heat > almost locking.

When they cool, the wheels will turn by hand, but barely.

I loosened bleeders, tightened, and they were fine.

Took it out, and they got tight again.

Pedal seems fine, and brakes operate fine, but it is not driveable as is.

Rear turned fine, so it is all front oriented.

Rebuilt calipers (Advance or wherever).

Service manual indicates more of an issue of the pistons not releasing in caliper sleeve?

If I'm not mistaken, I'm using original master cylinder with a rebuild kit in it, that had also been sleeved by Apple.

Is there anything in master cylinder operation that could lead to holding pressure on the fronts?

If so, the easiest thing to first replace is to put a different MC on it, as I have both a fully rebuilt orginal, as well as a parts store rebuild.

Thanks for thoughts....

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Old 10-13-2008, 05:49 PM   #2 (permalink)
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Manual or power brakes ?

Sounds like the master cylinder piston may not be completely returning to the back of the bore.

If you have power brakes, make sure that the rod between the master cylinder and the booster has the correct amount of clearance (it's a really small amount...but there has to be a few thou clearance, at least).

You may have an adjustable push-rod in the brake pedal linkage. See if you have a turnbuckle-like adjustable pushrod...stick your head under the dash and look at the thing that is connected to the pedal itself, right by the brake light switch. If it's adjustable, make sure that there is a little bit of "free play" in the linkage.
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Old 10-13-2008, 06:41 PM   #3 (permalink)
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Front Power Disk

Thanks
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Old 10-13-2008, 06:55 PM   #4 (permalink)
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I'm installing '68 disc brakes in my '67. I' ve been working to learn as much as possible about the swap. One thing I learned is about residual pressure valves included in master cylinders used on drum/drum vehicles.

The residual pressure valve keeps brake fluid pressure at the drum for quicker response. If you always had disc brakes and everything is original, this is not likely. However, you asked if there is anything with the m/c which would do this and a residual pressure valve would.
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Old 10-13-2008, 06:55 PM   #5 (permalink)
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When you opened the bleeders did fluid spurt out?
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Old 10-13-2008, 07:04 PM   #6 (permalink)
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'slight' pressure/squirt, i.e. not a 6" spurt, but yes, got a slight release, and then nothing but a 'slight dribble'.
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Old 10-13-2008, 07:10 PM   #7 (permalink)
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Quote:
Originally Posted by JohnTheTruckGuy
Manual or power brakes ?

Sounds like the master cylinder piston may not be completely returning to the back of the bore.

If you have power brakes, make sure that the rod between the master cylinder and the booster has the correct amount of clearance (it's a really small amount...but there has to be a few thou clearance, at least).

You may have an adjustable push-rod in the brake pedal linkage. See if you have a turnbuckle-like adjustable pushrod...stick your head under the dash and look at the thing that is connected to the pedal itself, right by the brake light switch. If it's adjustable, make sure that there is a little bit of "free play" in the linkage.

+1

When I finally got mine road worthy, I spent an afternoon fine tuning the adj. rod till it was just right. If there is the slightest pressure on the plunger, you will have your symptoms.
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Old 10-13-2008, 07:31 PM   #8 (permalink)
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im leaning towards the possibility of a faulty proportioning valve....


....

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Old 10-13-2008, 09:30 PM   #9 (permalink)
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Quote:
Originally Posted by Flye
68 Original Front Disks

First time out for 'longer drive' today. Entire brake system has been redone, as car had 'sat' for almost 30 years. I'll skip details and go to point:

Front brakes are evidently not totally releasing, which is keeping tension/friction on the front pads > heat > almost locking.

When they cool, the wheels will turn by hand, but barely.

I loosened bleeders, tightened, and they were fine.

Took it out, and they got tight again.

Pedal seems fine, and brakes operate fine, but it is not driveable as is.

Rear turned fine, so it is all front oriented.

Rebuilt calipers (Advance or wherever).

Service manual indicates more of an issue of the pistons not releasing in caliper sleeve?

If I'm not mistaken, I'm using original master cylinder with a rebuild kit in it, that had also been sleeved by Apple.

Is there anything in master cylinder operation that could lead to holding pressure on the fronts?

If so, the easiest thing to first replace is to put a different MC on it, as I have both a fully rebuilt orginal, as well as a parts store rebuild.

Thanks for thoughts....
All the ideas presented are good areas to check
out carefully.
As you mention yourself.....rebuilt calipers.
Don't discount that they may be playing into your
situation.
The square cut seal and the overall condition of
the seal's land in the caliper bore are the main
determining factors
as far as how much the piston
returns back into the bore after you release the
brake pedal..... at least when you're purely
talking about the caliper, apart from the system
as a whole.

Akebono Corporation
Brake Systems Engineering Center
Farmington Hills, MI
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Old 10-14-2008, 09:19 AM   #10 (permalink)
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If the hoses are old they could have collapsed internally and are holding pressure.
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Old 10-14-2008, 09:40 AM   #11 (permalink)
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Many years ago my 68 coupe with PDB would build up more and more pressure as I drove and wouldn't release the pressure after stopping, it ended up being a bad brake booster.
Just another possibility to add to your list of possible things to look at.
Good luck with the fix.
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Old 10-14-2008, 11:57 AM   #12 (permalink)
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Bad brake booster? Would you explain this? Could have been the rod that goes with
the booster was improperly adjusted.
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Old 10-15-2008, 02:12 AM   #13 (permalink)
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If your brake pedal is a little soft, you might still have air in the system. The air will expand with a little heat and cause pressure to extend the caliper pistons. You might consider more brake bleeding as the first step.

Doug
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Old 10-15-2008, 10:30 AM   #14 (permalink)
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Quote:
Originally Posted by slim
Bad brake booster? Would you explain this? Could have been the rod that goes with
the booster was improperly adjusted.
The rod was never adjusted and it was the original booster and MC in a low mileage car.
We changed the booster and problem was fixed.
It built up so much pressure the car wouldn't move.
I was literally stuck on the Ambassador bridge between Windsor and Detroit. Talk about a hefty towing charge.
That's what happened to me, but his problem could be completely different.
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Old 10-15-2008, 10:44 AM   #15 (permalink)
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Next time they stick loosen the nuts holding the m/c to the booster ans see if the calipers release. If they do then the pushrod needs adjusted. Also check pivot points on any linkage for the booster for rust.

Steve
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