How do I troubleshoot my brake booster?

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Righty Tighty

Blame it on the dog
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Hey all, I apologize if this has been covered before, I tried searching a bit and couldn’t find really any specific info on this.

The other day I was moving cars around the driveway, and noticed my '74 Barracuda had little to no brakes. The first few presses of the pedal were okay, but I quickly lost all braking power. Luckily, I was only idling in and out of the driveway, and shifting into park was an easy solution.

I checked the fluid, level was good. This led me to the brake booster. The vacuum line is in good condition and connected firmly to the booster and carb. I'm wanting to know the proper procedure to troubleshoot the booster, as I suspect the diaphragm may be leaking. Any tips?
 
Easiest thing is start the car. Let it idle.

Shut it off and if there is no power assist for 1 or 2 brake applications, you have a leak somewhere.


  1. The one-way valve at the outside of the booster
  2. The valving inside the booster
  3. The diaphram

Define little to no brakes.

Even if there was no vacume in the booster you would have brakes it would just require more pedal pressure (like manual brakes)
 
Thanks, I'll try that when I'm home tomorrow. When I say little to no brakes, the pedal goes to the floor and the engine idling is enough to keep the car moving. Pumping the brakes does nothing, I have to shift into park to stop the car.
 
Thanks, I'll try that when I'm home tomorrow. When I say little to no brakes, the pedal goes to the floor and the engine idling is enough to keep the car moving. Pumping the brakes does nothing, I have to shift into park to stop the
Sounds like a brake fluid issue, not a booster issue.
 
Initially I thought the same thing, but the fluid level was good and I don't see any leaks anywhere.

I do have an FSM, forgot to consult that first...
 
If the master cyl was leaking internally, you would have no signs of a leak and no brakes.

Does your brake warning light come on?

Do you have an emergency brake?

What have you done to the car since you last drove it?
 
The brake warning light doesn’t come on, but then again nothing on my cluster works. No emergency brake, I still need to install the cable for that.

I haven’t been driving or working on the car much lately, but the brakes were fine last time I parked it last week.

What can I do to check the master cylinder?
 
You're describing a hydraulic issue.
 
Master cylinders can develop a problem where the rubber cups leak past internally. They won't leak, outwardly, and depending on how bad the problem is, the master may "stop" the car OK, or not, and may slowly, or quickly leak towards the floor. Sometimes "stomping" the pedal quickly will "seal" the cup temporarily, while often "easing on" the brake will cause it to leak right down.
 
Sounds more like a M/cyl issue. To test booster, see if it will hold a vacuum.
 
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Master cylinders can develop a problem where the rubber cups leak past internally. They won't leak, outwardly, and depending on how bad the problem is, the master may "stop" the car OK, or not, and may slowly, or quickly leak towards the floor. Sometimes "stomping" the pedal quickly will "seal" the cup temporarily, while often "easing on" the brake will cause it to leak right down.
That's kind of what happens. If I stomp the pedal fast enough, I get a little braking action. But depressing the pedal at a normal rate, it goes right to the floor and the engine pulls the car along with zero braking.

I've found a couple threads where they talk about what you describe; I'll keep digging. I'm also now wondering how difficult it would be to rebuild my master cylinder, or if I should just buy a new one (if that's what the problem is).
 
I would buy new/ rebuilt IF you can get a decent one and "look alike." The reason is warranty. Master kits have always been somewhat expensive, and if you buy a kit and find out it won't fix it, you have dumped a fair amount of money that a complete master costs

Be sure to follow destructions re: "pre" bleeding or "bench" bleeding the master. They have to be sort of "primed" to get air out of the piston area.
 
Usually, if a booster diaphram goes bad, you will have a vacuum leak when you step on the brake pedal. That will affect the engine idle causing it to idle higher, rougher, and sometime make the engine die. The valving in the booster can sometimes go bad too. The pedal going to the floor is a different problem.....probably internal master cylinder fluid bypass, air in the lines, or an external fluid leak somewhere.
 
I was just reading in MA how to test the booster.
Take a vacuum pump, and pull a vacuum. It takes a lot of pumping. See if it holds. No hold, no good. But, as stated before, when a booster is bad, and you step on the pedal, it will start idling rough.
 
Thanks everyone. With all of the input given, I’ll be looking at the master cylinder. Also good to know how to test the booster, should the need arise in the future.
 
Just wanted to update the thread. New master cylinder, car stops on a dime! Now if I can only fix that misfire…

Thanks everyone for your help.
 
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