Jump to content

engine stumble when brakes applied


Recommended Posts

So I'm getting an engine stumble on my 720 when I apply the brakes at idle, pumping the brakes fully released or partially depressed makes it worse. The brake booster and check valve passed every test I could find. I blocked off the booster vac line at the check valve, stumble gone! So obviously I have a bad booster, right? Put in a re-manufactured unit and have the same issue. Side note: the MC is not leaking and I've triple checked all vac lines for leaks.  


Question is: is the new booster bad too? Or am I missing something?

  • Like 1
Link to comment
  • Replies 21
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Seems very unlikely two boosters in a row are at fault.



Manifold... hose... check valve... hose... booster


If you closed off before the check valve then the check valve and the hose between the check valve and the booster was not eliminated.


Close off just after the check valve and then just before the booster.

  • Like 2
Link to comment

I avoid rebuilt stuff any more, seems like it's all 3rd world crap these days.

My diaphragm developed a leak a few years ago, so I patched it.

That lasted a year (typical Band-Aid fix).

Now I have a Nissan rebuild kit that will go in once I have all pieces to put my Z32 brake on the front.

Have it blocked off for now. Makes for some exciting times when trying to slow down from 100 mph.


If you have a vacuum pump, you can test your rebuilt pod.

Doubt if it's the check valve, as they are supposed to free flow from the pod, to the intake manifold, so it would not effect idle.

Hoses are a strong possibility though.

Edited by G-Duax
  • Like 1
Link to comment

Thanks for the replies.


To clarify I blocked off the check valve port that the line from the booster connects to. So the only thing not physically tested in that case was the line from the check valve to the booster. I will recheck that line and all connections before moving forward. 


I've got a vacuum gauge on the way to test the booster. 

  • Like 1
Link to comment
  • 2 weeks later...

Update: still having the same issue. 


The new booster is tested and good. I replaced the lines and clamps from the manifold to the booster. The engine is pulling strong vacuum. I've checked for vac leaks everywhere on the engine I can think of. 


But the engine still stumbles at idle and at raised rpms when the brake pedal is pressed. It also stumbles when I release the brake pedal which really has me confused. But it's fine when the brakes are held. Vacuum keeps dropping significantly every time the brakes are pumped and doesn't return to normal until after the pedal is released.   



  • Like 1
Link to comment

look on fire wall for that vacuum line from motor that splits more than once and part goes to brake booster and part goes to electric device on fire wall above booster. 

mine had  hose break off at a T , .. but looked like line was not broke

cause T  broke off clean,

once i found vacuum line laying down behind other lines on fire wall and swung it up to see where it broke off.

i replace T  and idle improved .

  • Like 2
Link to comment

thanks for the tips.


I pulled the brake light switch wires, no change.


I can only find 2 vac lines, I believe everything else has been deleted.  One going from the carb to the distributor and another going from the intake manifold to the booster. no T's.


My next guess is something more severe: head gasket (which I have no evidence for) or intake gasket. I sprayed carb cleaner around the engine/carb/intake but didn't find any leaks.  

  • Like 1
Link to comment

Have you been able to vacuum test the booster alone both pressed and not pressed?

When my original went bad the idle was not controllable if I pushed the pedal,  you say yours stumbles, dies it raise and lower in RPMs, or does it just do one, raise or lower?

I bought a rebuilt brake booster that lasted a couple weeks, and another one that was bad out of the box, I have not got a lot of faith in rebuilt boosters anymore, but you have to keep buying them/returning them till you find a good one if you have no other choice.


  • Like 1
Link to comment

with the engine running and a gauge b/n the check valve and booster:

not pressed- gauge steadily reads ~-20in/hg. and will hold that with engine off.  

pressed- the gauge raises to like -15. If i pump the pedal without completely releasing, the gauge will raise about 5in/hg per pump and the idle gets progressively worse. 


rpms only go down.


if I apply the brakes lightly (like coming up to a stop sign) there is no stumble. If I slowly apply the brakes as far the pedal will go, there's no stumble or it's minimal. But when I release the brakes it will stumble. I'm guessing these are related to the amount of vac the booster is using. but the releasing part doesn't make sense to me. 


it's gotta be the booster right??

  • Like 1
Link to comment

I disagree. The idle mix (if properly set) is a close to perfect mix of gas and air. If you have a leak, letting more air in, the mixture goes leaner and the idle goes down. It simply has to.


The only way it could go up is if the idle mix was way too rich, then extra air could be tolerated and the wasted gas would be burnt increasing engine idle. A cold engine with the choke on would be another. I've pulled lots of vacuum lines off and had them fall off and it won't idle at all.


Pull the vacuum hose off the intake that goes to the booster and seal it. If this fixes the problem then the cause is obvious.

  • Like 1
Link to comment

Well as I recall, I had a PCV go bad and my idle rpms doubled,  to this day have no idea where the guts of that valve went, maybe they got stuck in that SU manifold balancing tube as I have never had any of it end up in any of the cylinders, I put another PCV valve in and all was good again.

So your saying if one pulls the power brake booster hose line off the manifold while it is running the rpms will not rise?

Edited by wayno
  • Like 1
Link to comment

A booster hose is huge. It will cause a massive vacuum leak and stall. I doubt you could even start it with it off. I've had the small vacuum line to the air filter housing fall off and it would stall at every stop but restart and keep running if you kept the revs up. A small leak of air wasn't enough to affect the air fuel ratio when driving but it sure does at idle.


The PCV valve is a controlled or metered 'leak' that is factored out by a proper adjustment of the idle mix screw.

  • Like 1
Link to comment

OK, here is a photo of my engine not running, you can see the fan blade.


Here are 2 photos of my engine running, you cannot see the fan blade in either as the engine is running, you can also see I disconnected the power brake booster hose in the lower right of the photos, and it is just hanging out in the open, it is making a massive amount of noise and the RPMs have gone up.



I said the guts of my PCV valve came out of it and it doubled my RPMs, and yes it even started that way.

Edited by wayno
  • Like 1
Link to comment

Then they must be running rich as extra air does not burn and produce more powder does it. The air leak is through that one way valve which is not as big a leak as the open hose would suggest... but a large leak just the same.


Maybe a property of an SU carburetor? SUs are CV or constant velocity carbs that sense intake vacuum and raise and lower the piston under the dome adding more fuel?  

Link to comment

That one way valve in the photos is on the hose going to the booster, that open hose you see in the photos goes directly to the intake manifold, it only has the 90 degree turns in the hose and fitting to enter the intake manifold.

The point of all this is I asked if the rpms rose or fell when the brakes were pressed, he says they fell, and you said it would not even run with a leak let alone raise the rpm.

I do not know if it is a SU thing, I have an E1 with a plug in the intake manifold that I could remove while it is running but I don't feel like doing that right now to see what happens, maybe tomorrow but as it is snowing outside right now so I doubt it.

I lost 2 power brake boosters, massive crack in one of them, power brakes didn't work anymore at all, the engine didn't die or even try to die when I stepped on the brakes, the engine rpm just raised slightly and when I let off it dropped to a normal idle rpm.

If an engine with a downdraft carb just dies with a small leak which I am skeptical about then it would mean he has another bad booster which I would not surprise me as rebuilt ones are a crap shoot at best, but the only way I know of to test a booster doesn't involve being connected to a running engine.

Link to comment

Right I see it now. SUs are weird animals.


I had the air filter off for something and the hose is so short it pulled off the intake. I started it up and drove away (choke on) so drove normally. When I slowed down in town ans stopped at a light she quit. Restarted ok but would not idle. When I got a chance I looked under the hood and noticed the hose off.


You would have to admit that introducing a vacuum leak is simply adding more air to the mix making it leaner. Same as turning the mixture screw in and reducing the fuel a idle.


Snow here today also. Not that much

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.