When braking fast, let's say stopping last second for a red light, my car's vacuum drops from 18" in drive to around 5-10" and car wants to really stall. Slow braking, a gradual stop..the car is fine. It's got a HT383 and the brake booster is hooked up to outlet in the back of the intake. At idle the car's vacuum is 20" but what would make the vacuum drop that much and make the car wanna stall? Too much pedal travel? The brakes have been completely redone with all new lines, calipers and since I have drums on the back, they also been totally rebuilt. Any ideas? ??

 

How's the pedal feel? Is stiff/harder than normal? If you have a leak in the diaphram, when you crack the brakes, it would equalize more or less on both sides, causing a vacuum leak, but in turn, it would stiffen the pedal feel.

 

The pedal feels normal, not a two feet on the brake pedal, just normal...a nice firm pedal.

Any clue what could cause this?

 

Vacuum leak
Check the vacuum check valve on the booster. Blowing through it's hose end, you shouldn't pass any air. Blow through the booster side and you should have no restrictions. Is it an automatic?

 

I replaced the booster valve. What about that canister on the brake hose? It's for vapors?

Yes it's an auto, I have a T350. The vacuum line from the modulator goes to the holley carb. Checked that for leaks, all seems solid.

 

It is!

Are you running off a ported vacuum source?

 

Thanks Murco, I read that and put it back on but doesn't make a difference.

The tranny is hooked up to a full time vacuum source.

 

Sounds to me like you have the order of events confused.

You stomp on the brakes; the gasoline in the carb all sloshes forward, uncovering the jets; it leans out; the engine hates that; the idle speed goes way down; the vacuum drops accordingly. But, since the booster had plenty of vacuum when you first applied the brakes, and that vacuum is trapped, the brakes continue to work fine so you don't really notice any change in them.

Then, you take your foot off the brakes; the deceleration goes away; the gasoline sloshes back rearwards to where it belongs; the engine starts running right again; idle speed goes up; vacuum rises accordingly.

It has nothing to do with vacuum advance, vacuum leaks, modulator hookup, phases of the moon, or any of that.

Everybody's carbed car does that to some extent.

 

Pretty close and thinking in a different direction, but I disagree.
The vacuum isn't trapped when applying the brakes, it's continually sourced from the engine. When stopping you do lose RPM but gain vacuum, especially in an automatic car. That initial spike of vacuum may make the brakes feel stronger and then disappates as the vacuum stabilizes. If the car has a stock torque converter, especially if it's a lock-up converter, it may drag the engine down below 500 rpm before downshifting on a hard stop and kill the vacuum assist. Another theory?

 

Really.....

My car has a check valve in the vacuum line going to the brake booster, for this exact purpose. The system is designed such that it contains a sufficient reservoir of vacuum (yes, I know that sounds stupid ... a container of nothing) to provide for a couple of applications of the brakes, even after engine vacuum goes away.

Maybe your car is different?

I stand by my diagnosis.

 

Thank you both for the both response.

The TC is a Saturday Night Special from TCI. Approx 2k stall.

 

It's there to allow you application of power assist in case of a no vacuum situation. If a naturally-aspirated engine is running there will be vacuum and the check valve doesn't come into play. It is there as a safety measure in case of engine shut-down while driving, allowing typically 2-3 applications with power assist.

How low is the RPM getting during these hard stops?

 

Right, I believe that's what I said. In any case, the check valve WILL come into play any time the vacuum drops. It's a CHECK VALVE, which means that any time the vacuum in the intake tract is lower than the vacuum in the booster, vacuum will flow (!) from the intake to the booster; where it will be stored until such time as it is released during brake usage, or until it leaks down, which may take days.

The check valve does not look out at the engine and see whether it's running or not, and decide whether to do its thing accordingly. Rather, ANY time engine vacuum is greater than that stored in the booster, regardless of the reason for the inequality, more vacuum will be supplied to the booster and captured by the check valve. Therefore, whether the engine is "running" or not, under the circumstances at hand, the booster will contain a plentiful supply of vacuum collected during the period of engine operation immediately prior to brake use; thus, when the driver stomps hard on the brakes, he will have full assist regardless of whether the engine subsequently starts running poorly or not. The low-vacuum condition will not cause brake operation to change at that time.

So, as stated before, the "low vacuum during heavy braking" complaint is not caused by the brakes, as such; but rather by engine operation (fuel delivery) being disturbed by the forces applied to the car.

It has nothing to do with the torque converter, unless the transmission's control hydraulics are severely impaired; since the VERY FIRST THING the transmission is supposed to do when the driver touches the brakes, is to release the converter clutch. Under no circumstances should it be possible to slow the car at such a rate with the brakes that the clutch can't release fast enough to avoid affecting engine speed.

I still stand by my original diagnosis; that being a very commonly observed behavior.