vacuum leak? maybe break booster?

[kelz702]

Hello All!
First off I'd like to say this form has helped me out tremendously over the years. The knowledge and know how is next to none.
I have a 72 c20 rebuilt 350, old th300. I've been trying to track down what I believe to be a vacuum leak. After doing some searching, I found out about break boosters causing vacuum leaks due to a torn diaphragm.
The truck idles prefect accept for when I'm stopped at a light for a few minutes. It then start to surge. As soon as I accelerate, the surge goes away. Only when I stop at a light again, it starts back up. It only happens when I have the breaks on for over a few mins. On short stops, there is no problem.
Today I took the vacuum hose going to the booster off while it was surging. I had it on my street with the ebrake on, chucks under the tires, and in drive. Once I plugged the hose the idle went up and the surge went away.
Does this sound like I need to replace my break booster due to a torn diaphragm?
Thanks,
Kelz
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[Jurassic-1]

It sounds like it may be a possibility. There are usually some signs leading to the problem. Are the brakes getting harder to push? Do you hear any vacuum hissing noise. Is the one way valve at the booster attached to the hose good? It is a 1 way valve and should only work in 1 direction. Have you taken the master cylinder loose to see if it has leaked fluid into the booster? This will wreck the diaphragm. I would check for these things before changing it out.

[kelz702]

Originally Posted by Jurassic-1

It sounds like it may be a possibility. There are usually some signs leading to the problem. Are the brakes getting harder to push? Do you hear any vacuum hissing noise. Is the one way valve at the booster attached to the hose good? It is a 1 way valve and should only work in 1 direction. Have you taken the master cylinder loose to see if it has leaked fluid into the booster? This will wreck the diaphragm. I would check for these things before changing it out.

The breaks feel like they are getting hard to push. They seemed harder after I got the breaks done a couple months ago. I don't hear any vacuum hissing noise. Its a brand new hose connected from the break booster to the carb teed off to the pcv valve. I took a length of hose and attached it to the booster. I could suck in air but I couldn't blow threw. I won't be able to take the master cylinder loose to check for fluid until monday. Thanks for the reply!
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[motorcritter]

I vote for bad booster- the increased pedal pressure is a tip-off. Checking for a leak into the booster from the master cylinder will be easy, once the booster is removed. Be prepared to replace the master cylinder, too. PITA, but, it'll be cheaper than using some one else's car to stop with.
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[geezer#99]

Seperate the brake vacuum hose from the pcv. They aren't supposed to be teed togethor.

[GASoline71]

Yup... need a dedicated vacuum line to each one.

Gary

[kelz702]

I'm planing on buying a master cylinder with the break booster if it's bad. Sometimes working on these truck is a PITA but it's definitely worth it.
I only have one available manifold vacuum port on my carb. It's a quick fuel 650. Should I tie the PCV valve into the back of the carb and tied the break booster to the edelbrock intake? I see there's a place to screw in a brass fitting on the back of it.

[geezer#99]

Yes!
Plug pcv into carb.
Use a vacuum tree on the intake.
Like these.
https://www.paragoncorvette.com/c-22...aspx?pagenum=1

[kelz702]

I'll pick one up Sunday and fix thoes vacuum lines. Just to make sure. I have the automatic transmission vacuum hose going to the manifold vacuum on the front of the carb. I have the advanced timing going to the advanced port and the pcv will go to manifold vacuum on the back of the carb. The only hose I'm running to the intake is the break booster. Should I get a tree with the 3/8 port for the booster and cap off the smaller port incase I need an extra manifold vac in the future?
Also, could the pcv and the booster being teed cause the surge I'm having at stoplights?

[kelz702]

I know everyone likes to see pics. Here's a couple of the truck

[kelz702]

Couple of the motor

[geezer#99]

Originally Posted by kelz702

I'll pick one up Sunday and fix thoes vacuum lines. Just to make sure. I have the automatic transmission vacuum hose going to the manifold vacuum on the front of the carb. I have the advanced timing going to the advanced port and the pcv will go to manifold vacuum on the back of the carb. The only hose I'm running to the intake is the break booster. Should I get a tree with the 3/8 port for the booster and cap off the smaller port incase I need an extra manifold vac in the future?
Also, could the pcv and the booster being teed cause the surge I'm having at stoplights?

Yes, the two teed togethor can cause the surging!
Hook the pcv to your carb.
Hook the booster and your tranny to a vac tree on the manifold.
The tranny needs full time vac.
You might not have much room for a vac tree behind your carb.
Not a problem. The fix costs money but has two benefits. You need a carb spacer/heat insulator under your carb anyway so raising the carb an inch will give you room for a tee.
Your carb might be boiling a bit which also makes it surge.

Looks like you're gonna burn a couple plug wires on your header. You have enough length in the wires to run them underneath.
Also what fuel pressure are you running with your regulator?

[Mr Handy]

yep the factory hooked the pcv to the carb and the booster and trans to the manifold. I also second adding a 1" spacer under the carb. not 1/2" not 2" a 1" spacer. being a Holley you probably can pick one up at the parts store.
here are a couple of quick shots of how mine is hooked up, should be about the same for you.
BTW nice setup.

[kelz702]

Thanks for the info guys! I'm gonna go to the AP store tomorrow. Hopefully they carry the vac tree I need. If they dont, I'll have to wait until Monday and head over to the speed shop. I know it doesn't look like it in the first pics, but I have a 1" spacer already. It's a must out here in the desert. I learned a couple years back when I had the edelbrock on the motor and fuel was boiling in the bowl.

Geezer#99, I never thought about running the wires under the headers. If they're long enough, I'll do that tomorrow. I have the fuel pressure regulator set at 6.5psi. At least I hope that's where it's set. The gauge is all over the place. I think the heat messes with it because it's never consistent.

Mr Handy, your setup look really good too. I hope to have mine running like a champ soon.

I'll get back on tomorrow and let you guys know how everything goes.

[kelz702]

So after a week of rainy days off and work I had a chance to go grab a 3/8nmpt to 3/8 hose. Had to grab it from All Hose here in Vegas because none of the AP stores sold one that had a 90° angle. I tightened it to the intake and ran the break booster to it. Got rid of the "T" and ran the pcv to the carb. It seems to be running alot better now. I took the truck on a long test drive and it seems like the surging has stopped. I guess I'll see in the next couple of days if I still have a problem.

Thanks everyone for the help!!!
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[Custom/Ten]

Hope you got it worked out! You can put a vacuum pump on your booster, pump it up to 20 and wait a few minutes to see if it can hold the presure.

Also gotta ask, is that an orange truck with a black hood?
.....lol

[kelz702]

Originally Posted by Custom/Ten

Hope you got it worked out! You can put a vacuum pump on your booster, pump it up to 20 and wait a few minutes to see if it can hold the presure.

Also gotta ask, is that an orange truck with a black hood?Attachment 1424219
.....lol

Sweet, I'll see I can grab a pump and check that out.

Nope, close. It's an orangish, rustish, primerish truck with a gray hood. Close though haha!

Is that your truck? It looks sweet
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[Custom/Ten]

Haha, yea that's my beater. Camera makes it look 20 years younger though lol!

[Custom/Ten]

Hers a pic of mine. It was like 20 bucks, just in case you weren't familiar with them.

[kelz702]

Originally Posted by Custom/Ten

Hers a pic of mine. It was like 20 bucks, just in case you weren't familiar with them.

Perfect! I'll go pick one up after the weekend and check it out.