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Discussion Starter #1
Hello everyone! I hope everyone is staying safe and healthy during this pandemic!

I have a 69 mustang and I noticed today that I had to press the brake all the way to the floor to really get the car to stop. When I listen closer I heard a hissing when pressing the brake. It sounded like it was coming from underneath the dash near the brake pedal.

I just started looking into it but I was curious if anyone has run into this before and have any ideas what it may be?
 

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Sounds like you might have a failing master cylinder and the hissing is the booster having to travel much more than normal. If the booster was failing you'd have a high, hard pedal instead. Low pedal almost always indicates a loss of hydraulic pressure in the system.
 

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Booster could be going out. Mine hissed when I stepped on the pedal initially, but the power function still worked. When I finally pulled the assembly to replace it the push rod and seal just fell out, all corroded....
 

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Booster have to exchange air internally. The hiss in the exchange. All operate this way. Extreme "hiss" is the pedal traveling further in the booster.
I think Mr Woodchuck might be correct????
 

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Discussion Starter #7
Sounds like you might have a failing master cylinder and the hissing is the booster having to travel much more than normal. If the booster was failing you'd have a high, hard pedal instead. Low pedal almost always indicates a loss of hydraulic pressure in the system.
I took the top off the master cylinder and it is completely full. I would think that if the master cylinder was going bad i would have lost some fluid. is that not the case?
 

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What if the internal seal that press again the cylinder walls of the bore fail. Will/Can fluid bypass the compression in the bore and still retain fluid in the upper chamber?
 

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Discussion Starter #9
Booster have to exchange air internally. The hiss in the exchange. All operate this way. Extreme "hiss" is the pedal traveling further in the booster.
I think Mr Woodchuck might be correct????
I know you will hear sound when pressing the pedal but the extreme "hiss" continues even when i press the pedal to the floor and keep it there. should i still continue to hear a hiss then?
 

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Discussion Starter #10
What if the internal seal that press again the cylinder walls of the bore fail. Will/Can fluid bypass the compression in the bore and still retain fluid in the upper chamber?
That i am not sure of. I do know that the car does stop eventually. right now i have only driven it up and down my driveway as i do not want to be on the main road and something happen.
 

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You have enough vacuum? You really should not here any noise inside the car. The vent is between the master cyd and the booster.. Pull the master out and see if you have any fluid on the booster edge. If so I would replace it or send it out. Also if it’s never been replaced or rebuilt it needs it as they usually don‘t last as long as they do now..If you here that noise inside I would have to say its leaking...
 

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Discussion Starter #12
You have enough vacuum? You really should not here any noise inside the car. The vent is between the master cyd and the booster.. Pull the master out and see if you have any fluid on the booster edge. If so I would replace it or send it out. Also if it’s never been replaced or rebuilt it needs it as they usually don‘t last as long as they do now..If you here that noise inside I would have to say its leaking...
Thank you! Ill check that out.
Here is what i have gathered so far
1.) with the car off, if i keep pressing the brake, it never gets firm, i am always pressing the same amount
2.) the fluid is completely full in the master cylinder
3.) I just found out that i can hear some hissing on the outside of the master cylinder. is it possible that the check valve is going bad?
 

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I know you will hear sound when pressing the pedal but the extreme "hiss" continues even when i press the pedal to the floor and keep it there. should i still continue to hear a hiss then?
If the air noise continues that is not right. Holding the pedal at a stationary position and hearing air is not good.
 

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Discussion Starter #15
I wanted to update everyone, I replaced the booster today and that 100% took care of this hiss and the pedal did feel more firm, but the pedal is still going pretty far to the floor before braking begins.

I have a new master cylinder as well and I plan on replacing it in the next couple days (whenever my buddy is free), but I wanted to share some new information with everyone to get your opinion.

I recently purchased a Brake Fluid tester

When I tested my brake fluid it registered as red >4%

Clearly I need to replace my brake fluid, but could the brake fluid be causing the issue with the pedal going so far to the floor before braking or is that still pointing to the master cylinder?
 

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No, the water in the brake fluid wont cause that problem. Air in the lines will, or an internally leaking master, or rear brakes that need to be adjusted or replaced.
 

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Where did you get the replacement booster?
 

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Discussion Starter #19
Just an update to everyone, the master cylinder is replaced as well. I noticed on the test drive that when i press the brake that that the front right and the back left catches more but the front left and the back right do not, or atleast they do not catch as much as they should.

@Hemikiller I think you are right, my drums need to be adjusted. I am going to do that today hopefully and ill update everyone.

Thank you everyone for all your input and help!
 

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Discussion Starter #20
All, just wanted to update you. I have the brakes adjusted and everything is working MUCH better. I think i have a few adjustments to do on the drums but all in all everything is working great!

Thank you everyone for all your help and direction!

I do have one other question. While I was getting ready to adjust the drums, I found a drum brake caliper tool or a resetting tool. I called up to my local auto parts store to see if they had it and they did not, but the guy working there told me i could measure the distance of the brake pads with a tape measure and that would do the trick for me. He tried to explain it to me but i didnt quite follow so i just went with the old school adjust and test method.

Has anyone ever heard of using a tape measure for calibrating the drum brakes? If so, how do you go about doing that?
 
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