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Discussion Starter · #1 ·
I am re-doing 69 mustang with 302 crate engine. I have replaced all brake lines, disk brakes, proportioning valve, master cylinder and brake booster... When car is off I have a firm pedal but as soon as I start car pedal goes soft and to the floor. I have good vacuum and booster is holding pressure. I have bench bled MC and bled lines 4 times by vacuum an by gravity. At a loss!! Suggestions??
 

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Ok, It is normal, for the pedal to drop a little when the engine is fired and idling. Did you, at anytime, bleed them using a helper to depress the brake pedal method? If so, did you follow the prescribed method of starting at the RR, LR, RF, LF? During the process, did you received a steady stream without any bubbles?
Just seeking to rule out this step in the process.
Have you verified the brake MC pushrod is correct for your application?
 

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Make sure the bleeder on the caliper is pointing towards the rear of the car and not vertical. If they are vertical, you need to swap them side to side. Did this once, took a quart of brake fluid to figure out WTH I had done wrong....
 
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Discussion Starter · #5 ·
Ok, It is normal, for the pedal to drop a little when the engine is fired and idling. Did you, at anytime, bleed them using a helper to depress the brake pedal method? If so, did you follow the prescribed method of starting at the RR, LR, RF, LF? During the process, did you received a steady stream without any bubbles?
Just seeking to rule out this step in the process.
Have you verified the brake MC pushrod is correct for your application?
bled brakes 5 times now with helper and no bubbles. I even used the proportioning valve tool to prevent valve from moving when bleeding. Replaced MC today and no success. Adjusted push rod to .20
 

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What has been done on the rear brakes.
Is it plumbed up front bowl of master rear brakes
Rear bowl front brakes.
Proportioning valve in rear brake circuit.
Stock master with built in residual---Don't know?
 

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Disconnect the booster and see what happens when you start the car.
 

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So you are saying that if you push the pedal with the motor off it won't go to the floor? Since the booster is just that, I am bonking on this, you should be able to push it to the floor with the motor off, just harder. I like the rear brake out of adjustment theory or installed incorrectly, adjust till them till you just hear/ feel the shoes scrape. I would inspect the rear brakes carefully, make sure they are installed and adjusted correctly. Not much can go wrong with disks unless they are leaking or installed on the wrong side.



When bleeding, make sure you put a length of tube on the nipple, I actually put the other end in a tall salsa jar, tall but narrow so it is below the surface of brake fluid. Also, only back off the bleeders a 1/4 turn or less, if you back them too far they can pull air around the threads - at least that is a superstition I adhere to. You can bleed till your blue in the face but if the last motion you do lets a plug of air in each brake, you loose, the tube and limited wrench stroke stops that. You might not notice it as it would be the first to expel next time. Have your pedal person, lift the pedal slowly so the master doesn't pull air around the seals, unlikely your problem since you don't get bubbles in stream but...



Good luck...
 

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Rear brake adjustment, lack of residual valve in rear circuit, wrong bore size master cylinder or master cylinder pushrod adjustment. Here is tool dimensions.
 

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Bleeders are on top but pointing forward b/c calipers are on back of rotors.
If they are stock '69 discs, which you did not mention they were otherwise, then they are on the wrong side. The bleeder must be at the top of the caliper, horizontal and facing the rear of the car. If they are installed incorrectly, the bleeder passage is in the middle of the piston cavity, not at the top, so you'll never get all of the air out.
 

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Discussion Starter · #12 ·
When I disconnect the booster vacuum line I have a firm pedal. As soon as I plug it in it goes to the floor. The same as when I start the car from pedal but then goes to the floor after starting.
 

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Discussion Starter · #13 ·
What has been done on the rear brakes.
Is it plumbed up front bowl of master rear brakes
Rear bowl front brakes.
Proportioning valve in rear brake circuit.
Stock master with built in residual---Don't know?
Plumbed just like you said. Scott Drake master cylinder and tough stuff booster. New brake caliper on rear.
 

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Discussion Starter · #14 ·
So you are saying that if you push the pedal with the motor off it won't go to the floor? Since the booster is just that, I am bonking on this, you should be able to push it to the floor with the motor off, just harder. I like the rear brake out of adjustment theory or installed incorrectly, adjust till them till you just hear/ feel the shoes scrape. I would inspect the rear brakes carefully, make sure they are installed and adjusted correctly. Not much can go wrong with disks unless they are leaking or installed on the wrong side.



When bleeding, make sure you put a length of tube on the nipple, I actually put the other end in a tall salsa jar, tall but narrow so it is below the surface of brake fluid. Also, only back off the bleeders a 1/4 turn or less, if you back them too far they can pull air around the threads - at least that is a superstition I adhere to. You can bleed till your blue in the face but if the last motion you do lets a plug of air in each brake, you loose, the tube and limited wrench stroke stops that. You might not notice it as it would be the first to expel next time. Have your pedal person, lift the pedal slowly so the master doesn't pull air around the seals, unlikely your problem since you don't get bubbles in stream but...



Good luck...
I can push the pedal to the floor when the car is off. Disc brakes all the way around. I’ll take a look at the rear brakes and see if there’s an issue there. Bleeder valves are on the top so I can’t imagine they’re on incorrectly.
 

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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.
 

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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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Discussion Starter · #19 ·
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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