Engine miss when using power brake

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JimMac

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I have had my 65 valiant 225 engine off the road for a few years and I did add power brakes. I now have the engine running but when I hit the brake pedal the engine misses. I assume it has to do with the vacuum portion of the brake system. any Ideas?
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Looks like you have it hooked to one of the intake runners. That will lean out one cylinder. The power brakes need to be hooked up at the base of the carb where it will pull equally from all the cylinders as said above.
 
I would just take off the power brakes don't really need it on a slant six. But that's just me.
 
I’m on the other (old & lazy) side. Love my power steering & power brakes. Oh, & don’t forget the a/c either.....
 
Mike69cuda said:
Looks like you have it hooked to one of the intake runners. That will lean out one cylinder. The power brakes need to be hooked up at the base of the carb where it will pull equally from all the cylinders as said above.
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I do have a place on the carb that is blocked off. I will try that. thanks.
 
Every morning at start up, the engine evacuates the entire booster, both sides of the diaphragm. As long as the control valve is closed, and the check valve is working,the engine cannot suck air thru the unit.
When you step on the pedal, immediately the control valve separates the forward chamber from the rearward chamber. Then it lets atmospheric pressure into the rear chamber in proportion to how far you are depressing the pedal. The vacuum side of the chamber is supposed to remain sealed. So stepping on the brakes and getting a rough-idling engine points to a failure of the vacuum chamber, in that somehow air is getting from the power-side of the booster to the vacuum side.
It may be a perforated diaphragm, or it may be a leaky control valve. In either case a replacement booster should fix it.
Now; as the power piston moves, a small and brief hick-up in engine rpm or quality can be expected as the vacuum side adjusts; but as soon as your foot becomes stationary, you can expect the engine to recover.
But yes re-plumbing the charge hose to the plenum will at least put #6 back on line; but now all cylinders will share the leak. At least maybe you can compensate by mixture screws..... until the leak in the booster gets really bad,lol.

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Y'all are leading him astray. That tap on the № 6 runner is exactly where the power brake vacuum hose is supposed to go. That's where it went on every single vehicle ever made with a Slant-6 engine and power brakes. There is no port on the carburetor that can supply adequate vacuum to evacuate the booster's front compartment quickly enough—this is a safety issue, so @JimMac, do not move the brake booster hose to the carburetor or anywhere else.

You have a vacuum leak when you apply the brakes. That's because you have a faulty brake booster—its diaphragm has ruptured and/or one or more of its internal seals has failed. The type of booster you're using, which appears to be a '73-'75 A-body item, is somewhat notorious for not liking to be stored for long periods of time, whether under the hood of the car or on a shelf somewhere.

Skip the poor-quality "remanufactured" garbage you might still be able to find at parts stores or on RockAuto. Get in touch with Booster Dewey for a properly-rebuilt booster.
 
Dan,
Thanks for the correction. On my 360, the brake hose was hooked to a port on one of the intake runners and I was advised to connect it to the base of the carburetor. Was that incorrect?
 
Mike69cuda said:
On my 360, the brake hose was hooked to a port on one of the intake runners and I was advised to connect it to the base of the carburetor. Was that incorrect?
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Yes, that was incorrect. The power brake booster vacuum hose goes to the intake manifold vacuum tap on V8 motors, too, not just Slant-6s. Never to the carburetor.
 
Dan, I went out to the garage and looked at mine and it is hooked up to the manifold tap as you describe. Must have gotten confused with the PCV hose. Sorry, senior moment, galloping senility, brain damage, etc......
 
JimMac said:
I have had my 65 valiant 225 engine off the road for a few years and I did add power brakes. I now have the engine running but when I hit the brake pedal the engine misses. I assume it has to do with the vacuum portion of the brake system. any Ideas?View attachment 1715410714
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I have to disagree with some of the posts on this.
Most every vacuum operated power brake system introduces a bit of extra air into the intake system intermittently when the pedal is pressed or released.
Here's how to tell normal from not normal.
Any of us with power brakes can press and release the brake pedal and get a change in idle speed or a light miss for a second.
If you press the brakes and have a continuous miss when holding the pedal down, chances are the diaphragm has some sort of leak.
If you press the pedal and release it and have a miss for a second and then it recovers, this is normal.

Anyone one of us with vacuum power brakes can prove this by idling your engine and pump the brakes a couple of times and notice the idle either increases a little momentarily, or there is a miss for a second but it recovers right away.

Just because there is a miss when doing some operation of the brakes does not prove the booster is bad.
This has to be diagnosed by when and for how long the idle changes, the miss occurs or a vacuum leak down test is done on the booster.
 
Mike69cuda said:
I’m on the other (old & lazy) side. Love my power steering & power brakes. Oh, & don’t forget the a/c either.....
Click to expand...
Yes, I added power steering and power brakes.It is a convertable so did not add air. I am an old timer whose first car was a 1934 Chev Master sedan. Did not even have hydraulic brakes and the steering was really bad. when I finally got a car with an automatic trans. (a 52 ford) it was great and then in 1964 I bought a new Acadian (chevy ll Canadian model) it had power brakes and power steering. At my age you do not want me on the road without those things.
 
TrailBeast said:
I have to disagree with some of the posts on this.
Most every vacuum operated power brake system introduces a bit of extra air into the intake system intermittently when the pedal is pressed or released.
Here's how to tell normal from not normal.
Any of us with power brakes can press and release the brake pedal and get a change in idle speed or a light miss for a second.
If you press the brakes and have a continuous miss when holding the pedal down, chances are the diaphragm has some sort of leak.
If you press the pedal and release it and have a miss for a second and then it recovers, this is normal.

Anyone one of us with vacuum power brakes can prove this by idling your engine and pump the brakes a couple of times and notice the idle either increases a little momentarily, or there is a miss for a second but it recovers right away.

Just because there is a miss when doing some operation of the brakes does not prove the booster is bad.
This has to be diagnosed by when and for how long the idle changes, the miss occurs or a vacuum leak down test is done on the booster.
Click to expand...
I think you are right. I did pick up a used unit and it has been not used for some time. When I hit the brake on the idle it did miss and continued until I removed my foot. Yesterday I did move the connection the carb and it worked O.K. but I think I need to get it checked.
 
JimMac said:
I think you are right. I did pick up a used unit and it has been not used for some time. When I hit the brake on the idle it did miss and continued until I removed my foot. Yesterday I did move the connection the carb and it worked O.K. but I think I need to get it checked.
Click to expand...

This is where I would agree on it being a possible booster problem.
You hold the pedal down and have a continuous miss until it is released.

Moving the connection to the carb spread the leak over the rest of the cylinders instead of just that one where it was connected, so it makes sense that it would seem a little better.
 
slantsixdan said:
Y'all are leading him astray. That tap on the № 6 runner is exactly where the power brake vacuum hose is supposed to go. That's where it went on every single vehicle ever made with a Slant-6 engine and power brakes. There is no port on the carburetor that can supply adequate vacuum to evacuate the booster's front compartment quickly enough—this is a safety issue, so @JimMac, do not move the brake booster hose to the carburetor or anywhere else.

You have a vacuum leak when you apply the brakes. That's because you have a faulty brake booster—its diaphragm has ruptured and/or one or more of its internal seals has failed. The type of booster you're using, which appears to be a '73-'75 A-body item, is somewhat notorious for not liking to be stored for long periods of time, whether under the hood of the car or on a shelf somewhere.

Skip the poor-quality "remanufactured" garbage you might still be able to find at parts stores or on RockAuto. Get in touch with Booster Dewey for a properly-rebuilt booster.
Click to expand...
 
Thanks Dan. I did move it to the carb because the carb on mine did have an extra port that was blocked off. It did run better when I hit the brakes but it did not feel the same and it is in the shop so I don't know if the booster actually worked but I am sure the problem is the booster as you said. It did seem logical to me that the plug on the intake manifold was where it should be connected. So are you sure it is from a 73-75. I will have to check it and replace it.
 
While fooling with it, you can buy an inline carbon filter for that vacuum hose which is supposed to keep gas fumes from getting to the booster, in HELP shelves at auto parts. But, I put one on my 65 Chrysler and still got a hole in the rubber diaphragm, right where the vacuum port is. The rubber was hard and cracked there, but soft everywhere else. Worst thing is I couldn't find anywhere to buy a replacement diaphragm or just its molded rubber. Booster Dewey must have a source since he rebuilds them. The Midland-Ross type (post 1) are easy to take apart with the V-ring. The Bendix type (bent tabs) are harder (I haven't). There is a check valve where the hose connects, which is supposed to stop gas fumes. Next pass, I'll add a 2nd in-line check valve.
 
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