Human Vac Tester...

Long story short I think my Brake Vac Booster leaks like a boat. Car runs, but the carb spits when I initially stand on it (accelerator pump works), and my ignition timing settings for a "smooth ride" are a bit odd... (at 8deg BTDC she's jumpy at constant speeds on the highway)

Anyhoo!

As I understand Vac boosters; they work off of differential pressure (atmosphere versus MAP) and when you stand on the brakes magic happens and you have a smoother pedal.

That means when the brake pedal is not being touched there should be a constant vacuum on the manifold side of the booster right? So if I were to suck on the vacuum line connected to the booster I should not be able to pull air through it like a straw?

If I were able to pull air through the booster like a straw (using lung power) then would that mean I've been driving around with:

1) a big vacuum leak

2) a bad brake booster

?

(aside from lung power I have no vacuum tester, or access to one)

"Option Two" was to block off the line at the manifold and drive around and see if my right foot felt any difference. but I figured I'd ask ya'll first.

You're block off the line logic makes sense, although the simile about the leaking like a boat upsets me. LOL. Block off the vacuum line to the booster and see if it makes a difference. Just remember that you won't have the brakes you are used to so watch out where you do the test! Don't want to have crumpled sheetmetal!

I live on a pretty empty dirt road that dead ends into a field so I should be alright as long as I keep her pointed in the right direction.

Good times!

Yes, sucking on the booster without ever drawing a vacuum indicates a leak, which is a bad booster.

Understand that your lung/mouth capacity is small compared to that booster. You have to suck, exhale, suck again, repeatedly. If by 5-10 breaths you still haven't built up anything like a vacuum, it's leaking.

Don't get caught sucking on your vacuum booster. Fix a thousand transmission and you'll still be known as a vacuum booster sucker.

I'm with Pete, just disconnect the VB and plug the hose back to the engine. VB don't smooth out the brakes they help to apply more pressure. So when you take your test drive you will have to do much more work on the brake pedal. Be careful.

Engine issues aside, the piece of the puzzle that really got me looking at the vac booster came to me about a week ago when I was pressing on the brake pedal with the engine off. I started the engine and the pedal didn't feel any softer.

I think I've gone this long without thinking the booster might be bad because I figured the hard pedal was a side effect of having 4-wheel drums... In hind site I should have known better.

It's looking like I may not get a chance to look at the car tomorrow after all. I HAD some free time in the morning while the wife and kid were out, but now my boss is asking us to come in about three hours early.

figures

Sucking on the booster might get you a mouthful of brake fluid.

weeeeeeeeeeellllllllll, it looks like the vac booster is good. If I thought the pedal was hard to press before, then it was insane without the booster's assistance!

Is that what you locosters have to deal with when you stab the stop pedal?

So, it looks like I have other areas I need to be looking at as far as this hesitation goes (and the jumping at constant highway speeds).

Good times and thanks everyone! -Bill

RossD wrote: Don't get caught sucking on your vacuum booster. Fix a thousand transmission and you'll still be known as a vacuum booster sucker.

Reminds me of my idiot cousin who burned his lips trying to blow up a car

Hungary Bill wrote: weeeeeeeeeeellllllllll, it looks like the vac booster is good. If I thought the pedal was hard to press before, then it was insane without the booster's assistance! Is that what you locosters have to deal with when you stab the stop pedal?

Two other things.

1. Is there a check valve in the line to keep the vacuum there? You really need that. Not only does it keep the vacuum that gets built up in the booster there, but it also helps prevent the engine from seeing a huge and weird vacuum leak/surge.

2. Is the engine so modified that I cannot build up the degree of vacuum necessary? In which case a vacuum pump might be the best fix.

no, no check valve that I know of... The engine is completely stock and just a vacuum line that runs from the manifold to the booster and one to the vac advance.

I drove it a good distance yesterday and paid a bit more attention to the brake pedal than I usually do. I've got about an inch of soft brake pedal before it firms up and becomes rock solid. I'm thinking the booster is running out of poop? Maybe the diaphragm is leaking, but still holds a bit of vacuum (just not as much as it should)?

I've got a Megasquirt 3 that's about half built. I'm thinking I could use the MAP off of that as a vacuum tester. My plan is to measure vacuum at idle with the booster hooked up and then compare the results to the vacuum with the booster blocked off.

Good times!

Vacuum gauges are relatively cheap. Maybe splice the hose from the manifold to booster w/ a tee and adapt a vac hose to a gauge in cabin and monitor from yur seat while driving. my .02

I tried that and once I bought a combination Vac gauge fuel pressure gauge for 24 dollars at bumper to bumper I checked my lung capacity to realize I have a heart like a lion and lungs of a coalminer that smokes 3 packs a day. You will only pull about 10psi with your mouth. My GF got 15.... Hahaha.

I'd check for leaks with propane or you can use ether for a change in idle.

I have 2 check valves in my brake lines. I only pull 8psi vacuum at idle and have a massive canister I don't want emptying out.

When I pull my vacuum lines they whoosh. No leak there haha! I mean literally 2 weeks parked and I'm still holding Vacuum on the booster and lines.

Diagnosing an older motor with a vacuum gauge is like a modern day OBD2 reader. It can tell you a lot about a motor if you know how to read it. It is a must for working / diagnosing old cars. I still use mine.

I checked locally and it looks like the nearest vacuum gauge is $50 and 2-weeks away... I told the guy I'd pass and I went home and built a makeshift unit, but after reading Dean's post above I may go back and order it after all.

anyhoo, this is what I did:

With this rudimentary gauge in place I found that there was no vacuum difference between having the booster hooked up versus having it disconnected.

Bummer

I like your gauge.

thanks! Maybe one day I'll quit being such a cheapskate...

Hungary Bill wrote: thanks! Maybe one day I'll quit being such a cheapskate...

I'd be sorely disappointed if you did.