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#1
11-06-2024, 06:45 PM
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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 |
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#2
11-06-2024, 07:16 PM
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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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 ‘63 LeMans- ‘69 400 w/ original transaxle. 2.69 gears. ‘55 Catalina ‘62 Mercury Meteor-all original, bought new by my grandfather 71' GTO -original 400/4-speed/3.23 posi 13.95 @ 102.1 on street tires @ 4055lbs.-now my sons ride |
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#3
11-06-2024, 08:54 PM
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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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#4
11-06-2024, 10:17 PM
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thanks for the reply
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#5
11-06-2024, 11:22 PM
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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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#6
11-06-2024, 11:47 PM
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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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#7
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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64 Lemans hardtop 4spd, buckets |
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#8
11-07-2024, 06:33 PM
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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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#9
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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64 Lemans hardtop 4spd, buckets Last edited by Lemans64; 11-07-2024 at 10:06 PM. |
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