Author Topic: Car Stumbles when Applying Brakes - HELP!  (Read 3066 times)

Offline indetrucks

  • Posts: 41
  • Name: Christian
Car Stumbles when Applying Brakes - HELP!
« on: April 24, 2015, 04:43:27 PM »
I have a 59 Coupe Deville that stumbles when applying the brakes and wants to die.

The vacuum check-valve appears to be working fine. I can suck through the end that connects to the carburetor but cannot blow the other direction (towards Booster/Vacuum canister).  Also worth noting, when I remove the booster side vacuum hose from the check valve, I hear a release of air that was stored up in the vacuum canister.

One would think the booster has a leak (I did). So I just got done rebuilding the booster today. Tore it all down and reassembled with a new rebuild kit and used the supplied grease on the leather seal around the booster piston etc.

The master cylinder is also new, as are all four wheel cylinders and shoes (all done by myself last weekend).

So what gives?  The exact same amount of hesitation/stumble is happening when I apply the brakes as it did before the booster rebuild.
The only way to drive the car is to turn the vacuum check valve around so that you cannot suck air from the carburetor end.  Car still stops fine, but a little more effort is needed.

*worth noting*
New cap, rotor, wires, plugs and timing is all set.  The only thing I have not done as of yet is rebuild the carburetor (which is next week).

Any ideas?


My girl with the hiccups
« Last Edit: April 24, 2015, 04:56:57 PM by indetrucks »
C. Reedyhoff

Online James Landi

  • Posts: 2388
  • 2007 XLR
  • Name: James Landi
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #1 on: April 24, 2015, 05:24:19 PM »
You appear to be certain that the issue is with the brake booster.  Might try connecting a vacuum gauge into the car, so as you're driving and stopping you can assess the vacuum as you're applying the brakes and slowing down.  Vacuum is high when you're idling in gear and rolling to a stop.  A stumble that you describe might be something else, unrelated to your brake booster.   James

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #2 on: April 24, 2015, 05:30:35 PM »
Does pumping brake pedal in Park make the engine shake?
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #3 on: April 24, 2015, 05:47:38 PM »
Yes, when in park and applying the brake it stumbles.  I rebuilt the booster because it has never been done and "why not."  I'm thankful I did as there was about a cup (yes a cup) full of brake fluid in it from the previously leaking master cylinder/s. 

I checked vacuum at the carb where the line goes in from the booster and it's pretty good at idle (see pic)

« Last Edit: April 24, 2015, 05:50:45 PM by indetrucks »
C. Reedyhoff

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #4 on: April 24, 2015, 05:53:19 PM »
If you can get vacuum up higher by fine tuning the carburetor air mixture screws, that may help. Also what is the idle RPMs?
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline cadillactim

  • Cadillac Tim
  • Posts: 799
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #5 on: April 24, 2015, 06:25:53 PM »
Have you unplugged the check valve from brake booster after a few minutes of idling and listened for the "whoosh" sound? If you don't hear the air sound then your booster is leaking. Just for curiosity sake you might want to replace the check valve with a known good one. Sure sounds like a booster problem.

Tim
Tim Groves

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #6 on: April 24, 2015, 06:38:56 PM »
If you can get vacuum up higher by fine tuning the carburetor air mixture screws, that may help. Also what is the idle RPMs?

Current idle is at 520RPM (book calls for 450)
Timing is set at 5d ATDC (Book calls for the same)
I have tried advancing the timing to 10d ATDC which increases the idle and puts the vacuum gauge just in the green.  The idle still has a stumble when applying the brakes though.  This also causes the car to hard start.  So I put it back to 5d ATDC

I have also tuned the carbs air screws via the vacuum gauge as well (adjusted until highest vacuum is achieved.)

I will say this, the car runs rich no matter what I seem to do and the idle screw has minimal effect on the idle speed when I adjust it.  In fact, I have to turn it all the way in just to get the idle low enough to get near the books specs.  This is why a carb rebuild is up next.

Have you unplugged the check valve from brake booster after a few minutes of idling and listened for the "whoosh" sound? If you don't hear the air sound then your booster is leaking. Just for curiosity sake you might want to replace the check valve with a known good one. Sure sounds like a booster problem.

Tim

Yes and yes (see my original post)  The valve is working as it should that I can tell from the suck/blow diagnosis.
« Last Edit: April 24, 2015, 06:40:37 PM by indetrucks »
C. Reedyhoff

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #7 on: April 24, 2015, 06:46:57 PM »
One last question. After shutting engine off and sitting for 1/2 hour have you tried the brake to see if you get 2-3 additional vacuum assists before the pedal gets "hard?"
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline curly

  • Posts: 362
  • Name: T Lewis
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #8 on: April 24, 2015, 07:05:12 PM »
Sounds like the booster to me.  Unplug the vacuum port and cap it off. Now, apply the brakes and see if the engine stumbles.  If the problem is gone then the booster is most likely leaking around the brake pedal push rod, under the dash.

It is hard to tell from your picture, but the vacuum looks to be at 15". That is low, vacuum should be 18-20" at sea level.  Are you setting the idle speed in Drive? 450 in Dr will be about 550 or so in Park.

T Lewis

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #9 on: April 24, 2015, 07:17:40 PM »
One last question. After shutting engine off and sitting for 1/2 hour have you tried the brake to see if you get 2-3 additional vacuum assists before the pedal gets "hard?"

Hard to tell... I just tried it now (car was sitting for 30 min or so) and it didn't necessarily feel like it had a hole bunch of vacuum assist.

Sounds like the booster to me.  Unplug the vacuum port and cap it off. Now, apply the brakes and see if the engine stumbles.  If the problem is gone then the booster is most likely leaking around the brake pedal push rod, under the dash.

It is hard to tell from your picture, but the vacuum looks to be at 15". That is low, vacuum should be 18-20" at sea level.  Are you setting the idle speed in Drive? 450 in Dr will be about 550 or so in Park.

T Lewis

As mentioned in my original post, I just rebuilt the booster today.  It had no affect on the stumble (it's still present).  The vacuum at carb is about 16 at idle and yes, I am at sea level.
I did set the idle in drive (with a huge brick in front of the tire).  lol
What would cause low vacuum at the carb?  Maybe rebuilding it will help?
C. Reedyhoff

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #10 on: April 24, 2015, 07:25:48 PM »
I'm suspecting a vacuum leak - cracked hose, intake or carburator gasket, etc. vacuum at idle should ideally be around 20 lbs.
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #11 on: April 24, 2015, 07:27:58 PM »
I'm suspecting a vacuum leak - cracked hose, intake or carburator gasket, etc. vacuum at idle should ideally be around 20 lbs.

Great to know... I'll start again in the morning. Will let y'all know what I find.
THANK YOU EVERYONE for your help :)
C. Reedyhoff

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #12 on: April 24, 2015, 08:08:53 PM »
So I went back out to the garage.
I blew out the air mixture screw holes with compressed air (gas came out those tiny top holes). So I can assume those air mixture screw passages are cleared.

I threw her in drive and checked timing and idle etc.
Idle is at 450rpm, timing is at 5degree ABTDC and vacuum is now at a measly 12 (should be around 20 from what I understand).
I plugged all the vacuum lines except the one going to the vacuum advance and the same problem is still there (stumbles when brakes applied and low idle vacuum)

I pulled a can of  carb/choke cleaner and sprayed at the base of the carb.  The car stumbled and the idle dropped to about 400.  Brought the idle back and sprayed around the carb base again..same thing, idle dropped and car wanted to stall.  I think it's safe to say the carb should be rebuilt along with the base gasket replaced.

I'll tackle this tomorrow and report back. 
C. Reedyhoff

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #13 on: April 24, 2015, 08:20:35 PM »
Try tightening the 4 carburetor mounting bolts and see if vacuum improves.
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline bcroe

  • Posts: 3897
Car Stumbles when Applying Brakes - HELP!
« Reply #14 on: April 24, 2015, 09:40:29 PM »
I had a problem with stalling on one of those Hydro transmissions.  The problem was it didn't
downshift into low, because some of the plates in a clutch pack had worn grooves in the aluminum
drum and couldn't move freely.  Replacement plates had a wider tooth to reduce wear, but
the drum needed replacement.  Bruce Roe

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #15 on: April 24, 2015, 10:05:36 PM »
Try tightening the 4 carburetor mounting bolts and see if vacuum improves.

Pulled the carb and found a little bit of a problem.
The base plate/spacer has a crack in it and you can see the bulge in the gasket that was caused by it.  This was in the same area that I sprayed with the carb/choke cleaner that caused the car to stumble.

Looks like its time for a new spacer.
C. Reedyhoff

Offline Jon S

  • Posts: 2561
  • Name: J. Schapiro
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #16 on: April 25, 2015, 10:53:51 PM »
I had a feeling there was a problem with the carburetor gasket.  Make sure the hole on the driver' side front which feeds exhaust to pre-heat the carburetor is clear and the return hole on the passenger side.  Your picture #3 (at the curve of the "U" shaped channel).  Also, if you have a Carter AFB, there should be a thin stainless steel heat shield between the spacer and the carburetor.  If you have a Rochester, then there is no heat shield.
« Last Edit: April 26, 2015, 10:47:30 AM by Jon S »
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Offline indetrucks

  • Posts: 41
  • Name: Christian
Re: Car Stumbles when Applying Brakes - HELP!
« Reply #17 on: April 25, 2015, 11:51:02 PM »
Yep, that metallic shield was there. I ordered a new spacer.
Will report back with an update when it shows up.
C. Reedyhoff

 

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