# Engine Vacuum



## TinIndian68 (Mar 22, 2015)

Last year, I installed front disc brakes on my '68 convertible. I have bled them several times, and there is never any fluid missing, but sometimes I need to just drive the car around the block before my brakes really work well. If it sits for like a week, it feels like I have no brakes when I start it up. The booster holds vacuum, so I am assuming my engine just isn't creating enough vacuum for this? Here are my two questions:

1. Other than making sure my gaskets are good, and all the vacuum ports are capped, is there a trick to making your engine create more vacuum? Other than my brakes, I also have everything setup perfectly on my hide-away headlights, and I just don't think the vacuum my car is pulling can hold them up either. I have tried the manifold vacuum port, as well as a couple carb ports. Instead of sucking the exhaust from my carb to the valve cover, would it help to cap that port off and just use a breather?

2. Has anyone tried one of those electric vacuum motors? What about adding a vacuum canister to assist in the psi? Anything that could be the culprit?

This is a YS coded 400 engine, with everything pretty much stock in terms of cam, compression, etc...but it does have an aluminum edelbrock manifold with edelbrock 1406 carb.

Any help is greatly appreciated!


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## chuckha62 (Apr 5, 2010)

Doesn't sound like a vacuum issue to me. Sounds more like a master cylinder/air issue. Also, assuming you have front discs, make sure your rear brakes are adjusted properly. They can have a huge effect on brake feel.

Edit: Sorry. Apparently, reading comprehension isn't my forte'. I totally missed the first part where you said you upgraded to disk brakes. You did change your master cylinder to a disk brake cylinder, right? ...and the prop valve? ...and for '67 and '68, the "hold off" valve? There are a lot of variables to verify before going after vacuum, in my opinion.


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## 67Twistytee (Feb 24, 2014)

Do you have a vacuum gauge? If not get one. Hook it up to a manifold source at idle and give us your readings. If it's less than 14" at idle, you probably do have a leak and it will affect your brakes. You should adjust the idle mixture screws and shoot for a reading in the mid to high teens. I would then troubleshoot with one device connected at a time (cap the open ports) and search for a drop in vacuum. This can help you pinpoint leaks by device. 

I agree with chuckha. If the MC is ok (did you upgrade for the disc brakes?) it could be a bad check valve or booster. At start-up, you should feel a slight drop of peddle and then immediate firming. 

Also check for leaks at the calipers and cylinders. You should have a tight seal at hoses and bleed nipples. It's possible to strip threads here and let air into the system. 

Electric vacuum motors or extra canisters should only be necessary with modified motors/longer duration cams.


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## 05GTO (Oct 6, 2004)

When you changed to front disk brakes did you also change the proportioning valve?


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## TinIndian68 (Mar 22, 2015)

Lol Chuck. Yea I bought the full front disc brake conversion kit from inline tube, so the master cylinder, booster, proportioning valve, etc...is all new.

05GTO - yes sir, the prop valve is the correct one.

Twisty - I just purchased a vacuum gauge today, thanks for the advice. I will go through everything one-by-one to see where the leak is coming from, if there is one. I will post my results of the vacuum test as soon as I get a chance to do it. I did notice a small leak in one of the wheel cylinders on my drum brakes. You think that could be letting air in and causing this whole issue? I was planning on replacing that one anyway, since the bleeder valve is stripped which makes it a pain to bleed the rear.

In terms of vacuum, my carb should be sufficient for everything I am running, right? I ma thinking of all of the things on my car that run off of vacuum, and I have brakes, hide-away headlights, wipers, and I guess something with the AC? My car is an original AC car, but I don't have anything hooked-up on it right now. I will just trace everything with it to make sure I am sealed-off.

Thanks for all of the help as always.


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## PontiacJim (Dec 29, 2012)

Carb should be fine and you should have good vacuum with your set-up as its nothing too radical in my opinion. Did you hookup your vacuum line for your booster at the rear of the carb? This is the one to use. I read that there may be one at the front on an AFB as well and this is for the PCV valve. If reversed, it won't work. 

1. It is best to bench bleed the master cylinder prior to installing it and then bleeding the system. You can have air trapped inside the master cyl that you may not get out by simply bleeding the brakes. Remember, when you bleed the brakes, you are doing one system (front or back) and you may not be getting full travel of the piston within the master -and I believe this is why you bleed it before installation. I always bleed my master cylinder prior to installation.

2. There are different length brake rods that go from your pedal to the power booster. Did you change out your factory booster? Sometimes the aftermarket boosters require a different length (longer) pushrod.

Not saying either of these are your problem, just throwing them out there.


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## 67Twistytee (Feb 24, 2014)

TinIndian68 said:


> In terms of vacuum, my carb should be sufficient for everything I am running, right? I ma thinking of all of the things on my car that run off of vacuum, and I have brakes, hide-away headlights, wipers, and I guess something with the AC? My car is an original AC car, but I don't have anything hooked-up on it right now. I will just trace everything with it to make sure I am sealed-off.
> 
> Thanks for all of the help as always.


Your heater control assembly is vacuum controlled, so even if the A/C vacuum canister is not connected, you need the vacuum hose attachment to control your dash vent settings. You should have the rear manifold port or intake stud running to the brake booster, another line running your distributor advance can, one running to the transmission modulator (if auto), one to the choke pull off, and one to your headlight control canister. The wiper motor is electric. 

And yes, if you have an air leak at one of your brake cylinders, it can cause a spongy peddle. You want the system as air tight as possible and free of bubbles which can compress and act like a spring rather than transmitting fluid pressure to the calipers or cylinders. It takes a lot of fluid bleeding to rid even a small amount of air introduced to the lines, so bleed it extensively after replacing the bad cylinder.


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## Lemans guy (Oct 14, 2014)

Sometimes air gets trapped in the very top of those disc calipers. I have had to remove the caliper and hold the whole thing so that the bleeder nozzle is at the absolute peak, even angle it a little when you crack it open and bleed. Sometimes a small a out of trapped air is there.

But if your Vacumn is good at the manifold, brake check valve Ok?

Vacumn problems at booster have different feel than air trapped in brake lines..

Good tips, start with vac reading and work slowly....

Air does not compress in hydraulic lines....so it makes it drop, or feel soft.

Different style bleeders can help as well....hang in there you will get it...


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## BearGFR (Aug 25, 2008)

It does sound like a "bleeding" issue, but it's a little weird that the situation improves with time/driving. If the issue was air in the system I don't think that would happen. What pads are you using? I'm sort of wondering if the issue is that the disc pads haven't been "bedded" and have to get some heat in them before they start working well.

Try this:
https://www.tirerack.com/brakes/tech/techpage.jsp?techid=85

Bear


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## Goat Roper (Oct 28, 2014)

BearGFR said:


> It does sound like a "bleeding" issue, but it's a little weird that the situation improves with time/driving. If the issue was air in the system I don't think that would happen. What pads are you using? I'm sort of wondering if the issue is that the disc pads haven't been "bedded" and have to get some heat in them before they start working well.
> 
> Try this:
> https://www.tirerack.com/brakes/tech/techpage.jsp?techid=85
> ...


This is what I was thinking, when I installed my discs the brakes hardly held until the pads were seated.
After they were I adjusted the proportioning valve to where the rears locked up under hard braking then backed it off two clicks.
Now the car squats under hard braking like it should instead of diving in the front.


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## geeteeohguy (Feb 2, 2008)

I'll chime in: it's a bleeding issue, not a vacuum issue. The reason the pedal gets better when driving is because it pumps up. Same reason the pedal is lousy when it's been sitting. I would bleed the master first, then gravity bleed all 4 corners. A bad power booster or weak vacuum will give you a hard pedal, not a soft one.


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## Lemans guy (Oct 14, 2014)

I use a machine to bleed it. But I have done it everyway like we all have. But one thing you can do is take it to a shop and have them bleed it with the power bleeder. Should not be much charge and may save you a lot of time.


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## Goat Roper (Oct 28, 2014)

I think he has two problems going on since he states that the vacuum operates the headlights slowly.
I would start by clamping the hose that goes from the manifold to the heater control and see if the headlights move faster.
When I did this on my car it just crumbled, I went to the parts store and got a bunch in all 3 sizes and just replaced it all.
It was a combo of bad 47 year old hose and improper connections at the control that was causing my leaks.


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## TinIndian68 (Mar 22, 2015)

Wow, very informative - thank you all for the information and advice. I will replace the rear wheel cylinder that is leaking, troubleshoot the vacuum stuff and let you know how it turns out/what the culprit was. 

I bleed the brakes myself with my hand-pump, but you are probably right that it would be worth maybe $20 - $25 to have a shop power bleed them. I never really heard of the "bedding" before, but it makes sense.

Thanks all.


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## jmt455 (Mar 26, 2012)

One additional comment; vacuum doesn't hold the headlight doors in position. 

The vacuum actuators move the headlight doors to the open or closed position, but the doors should be held in place by the over-center spring in the door mechanism. If the doors sag or droop, you probably need to replace the door pivot bushings and/or adjust the door mechanisms.

If the doors move very slowly, you probably need to rebuild or replace the actuators. If the actuator seals and dust boots are leaking badly, they might contribute to your brake issue. Same goes for the vacuum reservoir which is mounted beneath and outboard of the master cylinder, on top of the inner wheelhouse under the left fender. If that is leaking, the headlight doors will not work properly.

And finally, make sure that your vacuum check valves are properly installed and working correctly. I don't think this is your problem, but you might as well make certain that all vacuum system parts present and are working properly.


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## Lemans guy (Oct 14, 2014)

Some of those hand pump bleeders are pretty good. Some use Vacumn others reverse pressure. I used a reverse pressure one for a long time. But it still would not take the air trapped in the very top of a caliper, sometimes.

The Vacumn chamber bleeders I used a lot, bolt to master cylinder and pressure bleed. The best have an electric motor pump, no rubber Vacumn chamber. Constant pressure you set at the master and get under and crack the bleeders.

That is what a shop will have....if all else fails. It doesn't take much air to make it act up.


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