engine and drivetrain Car visually farts when you step on the brakes

[jiggapoo]

'68 with '74 ti engine. Month ago replaced the intake-to-brake booster vacuum hose due to long brake pedal travel and much effort needed to stop the car. Immediately fixed that issue; car was very easy to stop. Two weeks ago brake pedal, over the course of one or two days, became very short in travel and, in fact, the brakes would remain slightly locked after stopping (typical of a booster saying bye-bye, I assume). About 10 days ago during a single stop it felt like I "lost" my brake booster--brake pedal travel increased a little but and I have to press a lot harder. In addition, the brakes don't feel 'on' after stopping.

Now for the fun part. After driving on the expressway for any distance when I put on my brakes (at just about any intensity) the car has a minor backfire-sounding but very visually dirty fart. I'm not sure what it smells like. It's entertaining when people tailgate me and I let them have it. Sometimes there are multiple little farts instead of one big one.

Engine doesn't use more than 1/2 quart between changes. Am I just blowing my exhaust to pieces with these farts or am I just seeing unburned / partially burned gas? I assume I'm somehow pushing air back into the intake when I press on the brake--is that assumption correct?

[jp02ti]

just a thought...is your brake fluid level going down? I've had this happen when the master leaks through into the booster and under hard braking the fluid going up the pipe, into the intake and through the motor. the smoke is usually white though.

[jiggapoo]

Not white smoke, brake fluid level rock solid. Forgot to mentioned I checked that.

[Guest Anonymous]

'68 with '74 ti engine. Month ago replaced the intake-to-brake booster vacuum hose due to long brake pedal travel and much effort needed to stop the car. Immediately fixed that issue; car was very easy to stop. Two weeks ago brake pedal, over the course of one or two days, became very short in travel and, in fact, the brakes would remain slightly locked after stopping (typical of a booster saying bye-bye, I assume). About 10 days ago during a single stop it felt like I "lost" my brake booster--brake pedal travel increased a little but and I have to press a lot harder. In addition, the brakes don't feel 'on' after stopping.

Now for the fun part. After driving on the expressway for any distance when I put on my brakes (at just about any intensity) the car has a minor backfire-sounding but very visually dirty fart. I'm not sure what it smells like. It's entertaining when people tailgate me and I let them have it. Sometimes there are multiple little farts instead of one big one.

Engine doesn't use more than 1/2 quart between changes. Am I just blowing my exhaust to pieces with these farts or am I just seeing unburned / partially burned gas? I assume I'm somehow pushing air back into the intake when I press on the brake--is that assumption correct?

Try adding this to your tank or brake fluid?

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[eurotrash]

They have '74 Ti's now?! I waaaant ooone...

[jiggapoo]

They have '74 Ti's now?! I waaaant ooone...

Well... I don't know. The VIN on the block fits in as being a '74... and it's a dual sidedraft Weber setup. I don't know crap about the difference between the years. The guy who sold me the car thought it was a '70, the owner before that claims it was a '69, and according to the VIN gurus on the board it's a '68.

[TJF]

Does this only happen when driving? If you are sitting still and apply the brakes can you make it happen?

[jiggapoo]

No, it's never randomly backfired after I've stopped... but at that point I'm not pressing very hard on the brake pedal. It happens most often when I'm slowing down on an exit ramp or slowing down for traffic or construction. In other words, coming down off of 4000+ rpm and slowing either quickly or slowly by applying a good amount of pressure on the brakes.

[TJF]

Ok. Another question, if you are going along and put the car in neutral, then put on the brakes does it make the noise?

What if you downshift to engine brake? Can you make the noise without touching the brake pedal?

Thinking that there may be a carb or timing issue at work and not a brake booster problem.

[jiggapoo]

I'll try that this morning and get back to you. I normally have the car in gear when I brake.

However, I recently did an entire tune-up (condenser, wires, plugs, cap, rotor, points) about 300 miles *before* I felt this 'emptiness' in the brakes and it had no such pooting. It seems that the pooting started almost immediately after the booster felt like it went bye-bye.

Compounding the contributing factors is that cylinder #4 has a leak at the exhaust manifold gasket though that has been around since the beginning, as well.

[jiggapoo]

Ok. Another question, if you are going along and put the car in neutral, then put on the brakes does it make the noise?

I couldn't make it fart when the car wasn't in gear--either moving or not. I tried a few high speed slow downs with the car in neutral and no farting. So you may be right--it could be a carb problem. There is some sluggishness in lower RPMs that feels like fuel starvation (minor) but that's about it.

[TJF]

Can the sound be reproduced without braking? When downshifting/engine braking or by quickly going from full throttle to off throttle at high RPM?

I think it may be related to your exhaust leak. Not enough back pressure in the exhaust system combined with a momentarilly rich condition when the throttle is rapidly closed, cutting down on the air leaving you rich, may be letting unburnt fuel into the exhaust during the overlap period when the intake and exhaust valves are both open.

It's a theory anyway.

[jiggapoo]

You hurt your what?

Just kidding, that could be it--a suck and boom on a small scale. I like it. I think the easiest thing to do would be to replace the exhaust gaskets, assuming the bolts aren't seized.

Still, it wasn't happening before the booster busted.