brakes completely failed .help!!! - Vintage Mustang Forums
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post #1 of 30 (permalink) Old 06-05-2019, 10:42 PM Thread Starter
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brakes completely failed .help!!!

the brakes on my 1970 mustang completely failed while driving... power brakes, disk in front and drums rear... it was working fine but while driving last night slowly faded and just completely stopped working ... no visible leaks, brake fluid level not low ...but there is a hissing sound now every time you press/pump on the brake pedal

where do i start?
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post #2 of 30 (permalink) Old 06-05-2019, 10:44 PM
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My guess is that the brake booster has failed. has the engine been 'stumbly' too when you step on the brakes? If you push super hard, does the car slow/stop?


Being that the '70 has a dual master cylinder, it's unlikely that both sides of the master cylinder have failed at once, so my bet is that you either have a leaky hose, or that the booster itself has a leak. They have a big rubber diaphragm inside, and those will get brittle and fail over time.

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post #3 of 30 (permalink) Old 06-05-2019, 10:55 PM Thread Starter
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My guess is that the brake booster has failed. has the engine been 'stumbly' too when you step on the brakes? If you push super hard, does the car slow/stop?


Being that the '70 has a dual master cylinder, it's unlikely that both sides of the master cylinder have failed at once, so my bet is that you either have a leaky hose, or that the booster itself has a leak. They have a big rubber diaphragm inside, and those will get brittle and fail over time.
the engine does not stumble if pushed or aye and if i stand on the brakes i get very minimal braking .....barely
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post #4 of 30 (permalink) Old 06-05-2019, 11:04 PM
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If the booster failed, you’d have a firm pedal and hard braking effort, but you’d still have brakes.
Sounds to me like you master cylinder has failed internally and you need a new one.
Did the brake warning light come on?
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post #5 of 30 (permalink) Old 06-05-2019, 11:06 PM
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Seals on the piston in the master cylinder gone, I reckon. Time to buy, bench bleed, and install a new one, I reckon.
Then ya gotta bleed all them brakes far to near, I reckon.
Be careful.

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post #6 of 30 (permalink) Old 06-05-2019, 11:19 PM Thread Starter
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Seals on the piston in the master cylinder gone, I reckon. Time to buy, bench bleed, and install a new one, I reckon.
Then ya gotta bleed all them brakes far to near, I reckon.
Be careful.
yea might as well get a new booster too , is there a way i can make sure its the master cylinder or Booster?
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post #7 of 30 (permalink) Old 06-05-2019, 11:26 PM
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You can test the booster with a vacuum pump, pump it up and see if it holds. You can pull the master cylinder, empty it and see if it will bench bleed at all.

My 1st car...
'66 Tahoe Turquoise/ Aqua coupe
•289 / 4100 •C4 Auto •Disc Brakes
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•Dealer A/C • Console
•Dual Exhaust • Power Steering

Younger son's 1st car...
'66 Nightmist / Blue & white deluxe coupe
•289 / 2100 •C4 Auto
•Factory A/C •Console
WAITING FOR RESTORATION

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post #8 of 30 (permalink) Old 06-05-2019, 11:43 PM
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It's actually rather common for the master cylinder to start failing, dribble brake fluid onto the diaphragm inside the booster, and then both of them fail. Regardless of which has the problem, inspect them both, and you'll get it sorted.

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post #9 of 30 (permalink) Old 06-06-2019, 12:37 AM Thread Starter
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It's actually rather common for the master cylinder to start failing, dribble brake fluid onto the diaphragm inside the booster, and then both of them fail. Regardless of which has the problem, inspect them both, and you'll get it sorted.
sounds like ill be looking into a new booster & master cylinder ....
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post #10 of 30 (permalink) Old 06-06-2019, 01:37 AM
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sounds like ill be looking into a new booster & master cylinder ....

Unless you're lucky and only one has failed! =)

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post #11 of 30 (permalink) Old 06-06-2019, 02:44 AM Thread Starter
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sounds like ill be looking into a new booster & master cylinder ....
Unless you're lucky and only one has failed! =)
Mate either ways which ever failed shouldn't there be loss of brake fluid ? The guild was full ?
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post #12 of 30 (permalink) Old 06-06-2019, 03:09 AM
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No- the internal seals failed
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post #13 of 30 (permalink) Old 06-06-2019, 09:39 AM
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No- the internal seals failed
You can test for a bad master by rapidly pumping the brakes. You'll usually feel the pedal come up to a firm feel, then if you hold it, it'll gradually lose pressure and go to the floor. AS long as there is no fluid loss, that's a 99% certainty you have a bad master. If the master is leaking out the rear, it'll usually show below it on the booster. FWIW, in 30 years of Mustangs, I've never run across a failed booster or had one fail on me. Lost plenty of master cylinders though.

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post #14 of 30 (permalink) Old 06-06-2019, 09:53 AM
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Sounds like he needs a new Booster and Master Cylinder.... been there ...done that....

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post #15 of 30 (permalink) Old 06-06-2019, 12:54 PM Thread Starter
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Originally Posted by Hemikiller View Post
You can test for a bad master by rapidly pumping the brakes. You'll usually feel the pedal come up to a firm feel, then if you hold it, it'll gradually lose pressure and go to the floor. AS long as there is no fluid loss, that's a 99% certainty you have a bad master. If the master is leaking out the rear, it'll usually show below it on the booster. FWIW, in 30 years of Mustangs, I've never run across a failed booster or had one fail on me. Lost plenty of master cylinders though.
thanks mate, will check them out this Saturday and aye get new ones if needed , just going to miss driving her this weekend
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