Brake issues!! At a lloss - Page 2 - Vintage Mustang Forums
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post #16 of 25 (permalink) Old 09-04-2019, 11:35 AM Thread Starter
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Originally Posted by Woodchuck View Post
With the engine off, does the pedal start firm then slowly sink under constant pressure from your foot? If so, you probably have a bum master cylinder.
Just replaced yesterday with new MC
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post #17 of 25 (permalink) Old 09-04-2019, 11:51 AM Thread Starter
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Originally Posted by Woodchuck View Post
With the engine off, does the pedal start firm then slowly sink under constant pressure from your foot? If so, you probably have a bum master cylinder.
Replaced MC yesterday and same results.
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post #18 of 25 (permalink) Old 09-04-2019, 11:57 AM
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Replaced MC yesterday and same results.
Just to eliminate:
Has the booster been tested? (No Vac leaks)
As Woodchuck eluded, the PR checked using a gauge?
All calipers oriented correctly?
Correct MC for 4 wheel discs?

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post #19 of 25 (permalink) Old 09-05-2019, 12:37 PM Thread Starter
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Booster is holding vacuum
Vacuum pressure is at 21 with gauge
Correct MC
I will check calipers to see if they are sticking today
Thanks!
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post #20 of 25 (permalink) Old 09-05-2019, 12:46 PM
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Have you tried bleeding with the engine running?

Cliff
1966 Blue Hardtop. 200 CID with 3 Speed Trans
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post #21 of 25 (permalink) Old 09-06-2019, 11:17 AM Thread Starter
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Have you tried bleeding with the engine running?
I have not. Running out of options tho. Will try
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post #22 of 25 (permalink) Old 09-06-2019, 12:15 PM
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I have not. Running out of options tho. Will try
Post a pic of your calipers

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post #23 of 25 (permalink) Old 09-11-2019, 09:54 PM
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We have the same problem, but......

Hi, new forum member here. We have a '66 disc in front drums in back. Orginially manual brakes, changed to power. Pedal has back pressure while engine off and sinks to the floor when on. My question is.....have you tryed them yet. I have and they actually work great although a little daunting at first. Our car stops with no problem but with hardly any pedal resistance. I even got the car up to 35 and locked up the brakes with no problem at all. I'm still trying to figure out why.....I'm thinking pedal shaft travel verses the boosted requirement. But since it works great I've moved on to other things like rear disc, efi, 4 link rear, etc.....
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post #24 of 25 (permalink) Old 09-11-2019, 10:08 PM
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Hi, new forum member here. We have a '66 disc in front drums in back. Orginially manual brakes, changed to power. Pedal has back pressure while engine off and sinks to the floor when on. My question is.....have you tryed them yet. I have and they actually work great although a little daunting at first. Our car stops with no problem but with hardly any pedal resistance. I even got the car up to 35 and locked up the brakes with no problem at all. I'm still trying to figure out why.....I'm thinking pedal shaft travel verses the boosted requirement. But since it works great I've moved on to other things like rear disc, efi, 4 link rear, etc.....
Pedal travel is in direct proportion to mechanical advantage and fluid volumes.

The mechanical advantage is determined by the ratio of pedal movement to master cylinder piston movement and by master cylinder bore size vs. wheel cylinder and brake caliper bore size. Fluid volumes are controlled by the amount of fluid displaced in the stroke of the piston and how much fluid must enter the wheel cylinders and brake calipers to move the brake linings into contact with the drum and rotor.

Mechanical advantage you control (primarily) by changing master cylinder bore size and/or pedal pivot point.

Fluid volume you control by adjustment of brake shoes to minimize wheel cylinder travel and residual pressure to prevent excessive return of wheel cylinder and caliper pistons.

Many times poor power brake travel goes along with very light application pressure as the master cylinder bore size is too small which provides a higher mechanical advantage to move the wheel cylinder and caliper pistons but requires more "stroke" to do so. Increasing master cylinder bore size increases the pedal pressure to apply the same braking pressure but shortens the master cylinder piston stroke to do so. Excess travel or, conversely, "touchy" brake application is frequently the result of incorrect adjustment of the booster to master cylinder pushrod, which must be to a specification matching the booster and master cylinder to function properly.

Brake pedal to booster pushrod adjustment/length can also result in excess travel or dragging brakes when the pushrod is too long.

Bart

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post #25 of 25 (permalink) Old 09-11-2019, 10:27 PM
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Thanks Woodchuck my feeling is the MC is a different bore size than the orginal. The travel of the pushrod would be a problem if the pedal were adjusted against a limiting stop so that it actuated the mc cylinder, the pedal. But my car doesn't have a limiting stop the pedal just comes further away from the firewall. Plus there is no drag on the brakes when tested with the tires off of the ground. Since I didn't change the pedal (as some kits have) I was thinking with the orginal configuration (hense pedal travel) along with a change in bore size in the boosted assembly is causing the problem. Mine is a show car, but not a trailer queen. So being streetable is important to me.
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