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RPM drops when braking... Help?

jmbass98

jmbass98

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Not sure what is causing this issue... Replaced the hosing and intake manifold fitting from the brake booster. Next step I guess is the check valve or the entire brake booster?

What is weird is that I figured if it was vacuum, it would be consistent. The thing is. This issue doesn't occur at all on first start-up when cold. I can drive around for a while without the issue arising. Braking will return the RPM to normal idle of 900-1100 RPMs. Warmed up, though? Braking will dip down to 400~ RPM and almost stall the vehicle out. Super rough feel.

I disconnected the brake lights to see if it was somehow an electrical issue. Not.

Drums with factory brake booster. Anyone have any ideas?
 
Clarifying. Just pressing the brakes doesn't cause the issue (i;e just sitting still at idle). It's braking from speed.
 
Was this an issue before you started replacing things? Vacuum leaks are best detected by using a vacuum gauge. Are you sure the carb is not simply coming off high idle causing the rpm drop after the warm up.
 
Braking from speed there should be a high vacuum in the intake.
As above get a vacuum gauge.
 
This did not occur before I replaced intake manifold and carburetor. Not the fast idle, because the idle will somewhat quickly jump back up to 900-1100 normal RPM after the vehicle is completely stopped.

Idle feels okay otherwise... It's just that few seconds when braking.

I do have a vacuum gauge. What exactly should I do to test?
 
The float setting in the carburetor can cause an RPM drop. If it is set too low...
 
The intake shouldn't matter. I'm not familiar with the Edelbrock carburetor floats so I'll defer to those more qualified....
 
I was going to suggest electrical until I saw that you checked that, already. Kern raises an interesting idea.
Personally I’d temporarily disconnect the booster completely and drive around the block. Problem still there? Not booster.
 
My red car will feel like it is ready to stall when going downhill and slowing down. I had the carburetor rebuilt last year and didn't check his work. Maybe I should!
 
jmbass98 said:
This did not occur before I replaced intake manifold and carburetor. Not the fast idle, because the idle will somewhat quickly jump back up to 900-1100 normal RPM after the vehicle is completely stopped.

Idle feels okay otherwise... It's just that few seconds when braking.

I do have a vacuum gauge. What exactly should I do to test?
Click to expand...
Ok what does the vacuum gauge do when you let off the gas before applying the brakes? Increases, decrease or stay the same? You also did not mention but is this an auto or manual trans? Either way the vacuum should increase. You are taking the brake vacuum from the intake? Taking brake vacuum from the carb could cause a lean condition with the throttle plates closed. Someone else maybe better qualified to answer that one.
 
So it is not the brakes... I tested the brake booster, it's not the issue.

I tried messing around with idle mixture. Nothing that I do smooths out the idle. Always rough and drops when put into drive.

Brand new Edelbrock 1411.

I cannot for the life of me find a vacuum leak.
 
3978F325-8BE5-4F7B-899A-A72E2E89164D.jpeg
Some of your issues seem very close to what mine were last summer, and some not. Inconsistent idle/RPM’s and at times felt like vacuum leak (I had changed carb couple days prior and thought I had a leak) and at times braking did not feel quite right, check valve at booster was ok, then I pulled master and found this seal in the booster disintegrating
May not be your issue, just a thought
 
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