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Hey guys I’ve got a problem with my 1987 300zx. When the car is idling and still, it runs fine, however whenever the brake is pressed, the car starts to sputter and eventually will die if you don’t let off the brake. I’ve heard this could be some issue with the brake booster, but I have no clue really what the issue could be. Whether it’s relevant or not, there is a leak in one of the brake lines and the master cylinder doesn’t hold brake fluid past the MIN line. Any help with this issue would be greatly appreciated! Thanks guys, -Retro
 

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You need to get that fixed before you run into something.

Sounds like the master cylinder booster isn't holding pressure - there is a check valve in the feed line which might be bad. It would be nice if you could fix the problem with a replacement check valve rather than a booster.

It's critical to fix the brake line as well. Your problems seem to be compounding the safety risk.
 

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Discussion Starter #3
You need to get that fixed before you run into something.

Sounds like the master cylinder booster isn't holding pressure - there is a check valve in the feed line which might be bad. It would be nice if you could fix the problem with a replacement check valve rather than a booster.

It's critical to fix the brake line as well. Your problems seem to be compounding the safety risk.
Thanks for the insight. I will say no worries about safety, I don’t drive the car at all really, I won’t drive it until it’s safe to drive. I appreciate your safety concerns though :). But I will ask where is that check valve located and how do I go about testing to see if it’s bad or testing for other possible issues? Thank you for the help though. -Retro
 

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The check valve should be in a vacuum line between engine and booster; it's a bit larger diameter than other lines and is usually easy to spot. If you can blow through it both ways, your check valve is shot. That would allow vacuum to leak out of the booster...and a vacuum leak can ruin the idle.

I'm not sure if you have replaced the vacuum lines, but this would be a good time to go over them in general.
 

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The check valve should be in a vacuum line between engine and booster; it's a bit larger diameter than other lines and is usually easy to spot. If you can blow through it both ways, your check valve is shot. That would allow vacuum to leak out of the booster...and a vacuum leak can ruin the idle.

I'm not sure if you have replaced the vacuum lines, but this would be a good time to go over them in general.
I will definitely investigate further, I appreciate the help. However, my idle is fine all the time except when the brakes are pressed when the car is still. I will definitely give that check valve a check for sure though! Thanks -Retro
 

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Discussion Starter #6
The check valve should be in a vacuum line between engine and booster; it's a bit larger diameter than other lines and is usually easy to spot. If you can blow through it both ways, your check valve is shot. That would allow vacuum to leak out of the booster...and a vacuum leak can ruin the idle.

I'm not sure if you have replaced the vacuum lines, but this would be a good time to go over them in general.
Okay I have checked the check valve and it is in perfect working order, I’m led to believe that my brake booster is broken. I’m not sure of this but after some research, I’ve found symptoms of a bad brake booster are: “BRAKE light illuminated on dash, brake fluid leakage, Insufficient braking pressure or hard brakes, and engine misfires or stalling when brakes are pressed” and I have all of those symptoms. However I’m not sure on that.
 

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Without a doubt you have a bad brake booster. Once the brake booster is being activated with a bad diaphragm in it, it provides a major vacuum leak to the engine. One of the biggest causes for a booster failure is a leaky master cylinder. Once you take the master off of the booster check the recess in the booster that gets exposed when the master is pulled. The recess should be dry, if it's wet, that's going to be brake fluid. That rots one of the rubber diaphragms in the booster. Most of the budget places that sell rebuilt boosters use A1 Cardone for the actual rebuild. Their internal work is usually ok, however they don't repaint the housing, they blast the housing, and contaminate it. To paint it you have to use a spray gun with anti-fish eye in the paint. Use a spray can and you'll see about 50 or so fish eyes in it. The company that I use to rebuild the boosters powder coats the housing before they assemble it. Their cost is significantly higher but the job is significantly better. My regular price is 280.00 and the zcar.com users price is 253.00. You'd need to send me your booster first, I don't have any cores unpacked for the Z31 cars. Those are still packed away somewhere in my abyss of parts. I have lots of core boosters for the 70-83 cars. Z man of Washington
 

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Without a doubt you have a bad brake booster. Once the brake booster is being activated with a bad diaphragm in it, it provides a major vacuum leak to the engine. One of the biggest causes for a booster failure is a leaky master cylinder. Once you take the master off of the booster check the recess in the booster that gets exposed when the master is pulled. The recess should be dry, if it's wet, that's going to be brake fluid. That rots one of the rubber diaphragms in the booster. Most of the budget places that sell rebuilt boosters use A1 Cardone for the actual rebuild. Their internal work is usually ok, however they don't repaint the housing, they blast the housing, and contaminate it. To paint it you have to use a spray gun with anti-fish eye in the paint. Use a spray can and you'll see about 50 or so fish eyes in it. The company that I use to rebuild the boosters powder coats the housing before they assemble it. Their cost is significantly higher but the job is significantly better. My regular price is 280.00 and the zcar.com users price is 253.00. You'd need to send me your booster first, I don't have any cores unpacked for the Z31 cars. Those are still packed away somewhere in my abyss of parts. I have lots of core boosters for the 70-83 cars. Z man of Washington
Dude thank you, that’s some very very useful information! The problem makes a lot of sense now. However, I was just going to buy a new brake booster (and of course a new master cylinder) and replace it, instead of rebuilding my old one. I’ve seen aftermarket ones selling for about $130 to $150 online, I don’t know if that’s a bad idea, but I see it’s definitely cheaper so I don’t know. I greatly appreciate the help. Thanks! and have a good one. -Retro
 
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