#1  
Old 11-06-2024, 06:45 PM
72lmsconv 72lmsconv is offline
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Default Bad master cylinder or brake booster or both

i have 72 lemans with drum brakes
the brake pedal slowing goes down to the floor but the brakes do stop the car
i bled the brakes and it still has the same brake pedal going to the floor
i start the engine with my foot on the brake and it goes to the floor.
what i have read its the master cylinder just wanted to know it that is the correct diagnosis and would you just go ahead and do both the master cylinder and brake booster
thanks Bill

  #2  
Old 11-06-2024, 07:16 PM
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67drake 67drake is offline
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Sounds like the master to me.
A little harder to diagnose a bad booster with a blown master, but in theory you should pump the pedal with the engine off multiple times to clear any vacuum out of the booster, then start the engine with your foot on the brake pedal. If it drops when the engine starts, the booster is working.

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  #3  
Old 11-06-2024, 08:54 PM
72lmsconv 72lmsconv is offline
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If your assumption is correct The brake pedal doesn't go down its the power booster then it should be the master cylinder

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Old 11-06-2024, 10:17 PM
72lmsconv 72lmsconv is offline
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thanks for the reply

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Old 11-06-2024, 11:22 PM
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ID67goat ID67goat is offline
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Have checked the brake adjustment?

I would start by pulling the drums and check the condition of the wheel cylinders, shoes, and hoses…then make sure that the shoes are adjusted properly and the system is properly bled before replacing any parts.

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Old 11-06-2024, 11:47 PM
72lmsconv 72lmsconv is offline
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The drums and shoes were all new a few years ago and i just bled the brakes
and i think its the master cylinder or booster at this point

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Old 11-07-2024, 02:17 AM
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Why did you bleed the brakes? If it ran out of fluid your gonna have a hard time getting the air out of it. If it did this before bleeding and fluid level did not go down
you have a bad master cylinder, bypassing inside the cylinder. If replacing master cylinder be sure to bench bleed it before install to booster.

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  #8  
Old 11-07-2024, 06:33 PM
72lmsconv 72lmsconv is offline
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the fluid was low and i thought the brakes needed to be bled and the brakes were spongy and went down quite far before the brakes stopped the car
Maybe i never didn't bleed them properly?

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Old 11-07-2024, 09:47 PM
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Could also need adjustment, being drums brakes if not adjusted, you will have low pedal. If master cylinder is bad it will usually sink with light pedal pressure
and will never build up pressure to stop car, sometimes if pumped quick it maybe build pressure.
Let me rephrase this, if you pump the pedal twice does is come up and stay firm or slowly go to floor?

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Last edited by Lemans64; 11-07-2024 at 10:06 PM.
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