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Power Brake Booster Bad? |
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Rebel Machine 
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Joined: Jul/03/2007 Location: Western Il Status: Offline Points: 5562 |
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 Topic: Power Brake Booster Bad?Posted: Feb/07/2026 at 3:13pm |
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The brakes in my '69 AMX aren't working properly. The pedal travels a bit then it's really hard to push. I've suspected the power brake booster is shot but I can put a vacuum pump on it and it holds. If I depress the brake pedal when the pump is holding vacuum the vacuum goes away. The vacuum is still gone after releasing the pedal. Is this correct behavior for the brake booster? Is is supposed to bleed off vacuum after being depressed? Also the fluid in the rear reservoir of the master cylinder looks rusty. This is the second master cylinder I've had in this car, the previous one did the same thing. What's going on here? Thanks, -Steve-  |
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6PakBee
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Charter Member Joined: Jul/01/2007 Location: North Dakota Status: Offline Points: 5949 |
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Posted: Feb/07/2026 at 3:33pm |
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Steve, you should read the operational description in the TSM. Without the pedal depressed, vacuum is equalized on both sides of the diaphragm. When you depress the pedal the rear of the booster is vented to atmosphere which moves the diaphragm forward. When you release the pedal, vacuum is admitted to the rear from the front and the vacuum is equalized once more. As to pedal effort, two of my personal experiences seem similar to yours. One was with an internal failure of the vacuum valving in the booster and the second was a dragging master cylinder. I'd remove the master cylinder and see how the booster behaves. As to the fluid color, I'd take a syringe (turkey baster) and empty the reservoir. That way you should be able to tell if it is rust or some other contaminant. But bottom line is that it's either a reaction to something that was in the MC, or it came in through the discharge lines (unlikely), or it came in through a leaking cap.
Edited by 6PakBee - Feb/07/2026 at 3:37pm |
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Roger Gazur
1969 'B' Scheme SC/Rambler 1970 RWB 4-spd Machine 1970 Sonic Silver auto AMX All project cars. Forum Cockroach |
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Rebel Machine
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Joined: Jul/03/2007 Location: Western Il Status: Offline Points: 5562 |
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Posted: Feb/07/2026 at 3:54pm |
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I believe the vacuum booster is bad. This is a good explanation of how the booster works. When I press the pedal in after inducing vacuum the vacuum goes away so I think the valve in the diaphram that seals off the front chamber from the rear is bad. As for the fluid color I replaced everything in the car when I built it back in 2011. Brake lines are all stainless. Edited by Rebel Machine - Feb/07/2026 at 3:57pm |
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Rebel Machine
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Joined: Jul/03/2007 Location: Western Il Status: Offline Points: 5562 |
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Posted: Feb/08/2026 at 7:59am |
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Was wondering, this power brake booster is for a '68-'76 Corvette (which also has 4-piston calipers) but looks remarkably similar to the one for my AMX. The only difference I see is the connection rod to the pedal linkage. The AMC has an eye on the end. Could they possibly be the same other than that? -Steve-  |
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71SC360
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Charter Member Joined: Jun/29/2007 Location: Eastern TN Status: Offline Points: 5812 |
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Posted: Feb/08/2026 at 10:04am |
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The Corvette booster is basically the same booster but, in order to change that valve (the connector) you have to take the whole booster apart. AMC used to sell the rebuild kits for the valve. There are new rebuild kits available for your booster if you want to go that route. I've rebuilt a few of the 9 inch boosters for Gremlin/Hornets (they are not that hard to rebuild) but, I have not had any of the AMX Javelin boosters apart.
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Have2SC's
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Joined: Nov/30/2011 Location: Norwalk, OH Status: Offline Points: 1352 |
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Posted: Feb/08/2026 at 10:07am |
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Steve, When my booster went bad it did the same thing. It felt like you had to push the pedal very very firmly to get the car to stop. Rebuiled solved the problem. I might suggest you send your original booster and have it rebuilt. Lee
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1971 SC360 Hornet 4 speed 1967 Rambler 220 X code 343 4 speed 1969 AMX BSO 390 4 speed 1979 Spirit 401 4 speed
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1948kaiser
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Joined: Jan/25/2015 Location: south west flor Status: Offline Points: 1187 |
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Posted: Feb/08/2026 at 10:19am |
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when the booster went bad on the gremlin when the booster was only 4 years old. it came off and went into the nearest dump. drove over 200k without one
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71SC360
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Posted: Feb/08/2026 at 4:47pm |
Early gremlin/ Hornet boosters are getting tough to find. I am always interested in cores, especially disc brake boosters... |
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Rebel Machine
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Posted: Feb/08/2026 at 4:55pm |
I watched a video on rebuilding a booster, looks rather complicated. Granted it was for a Mopar it was a Bendix booster. AM I right in thinking the AMX booster is a dual diaphragm? |
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scott
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Joined: Jul/10/2007 Location: Wildwood Pa. Status: Offline Points: 3760 |
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Posted: Feb/08/2026 at 5:04pm |
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Since the engine in your AMX is supercharged, what do you do to prevent pressure from running into the power brake booster?
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1948kaiser wrote: