Vacuum brake booster testing - Chevelle Tech
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post #1 of 16 (permalink) Old Oct 13th, 19, 10:37 PM Thread Starter
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Tom
 
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Vacuum brake booster testing

So I have installed a new Wildwood master cylinder, all new stainless hard lines and stainless flex lines at calipers and all new rear disc conversion (The Right Stuff). I have bled the brakes and all seems good. But when the car is running, my pedal goes to the floor. At that point, then the brakes will engage and work, but I'm guessing that's beyond the power assist portion of the pedal travel.

The only thing that is old in this system is the Delco 11" vacuum brake booster. When I purchased the '66 in 1990, it was there.

When I use a MityVac vacuum pump and hook it up to the vacuum hose going to the brake booster, I can't draw any vacuum, gauge stays at zero. Should I be able to?! I found one post in the internet that indicates I should be able to, but want to confirm. If so, then that would confirm my booster is bad.

Otherwise, maybe my master cylinder is bad (brand new!). Or I have issues with the pushrod...though when I installed this thing many months ago, I was sure I got that all right.

Thanks,

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post #2 of 16 (permalink) Old Oct 13th, 19, 11:07 PM
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Re: Vacuum brake booster testing

Bad boosters usually result in a very hard pedal with the engine running. Your problem may be the mismatch between the booster push pin and the master cylinder actuating plunger. Remove the master cylinder from the power booster and see if there is a short push pin going into a deep hole in the master cylinder. You don't need to remove the brake lines just move the master cylinder forward. The problem should be pretty evident.

My solution was to cut an aluminum dowel to a length that would fit in the deep hole and allow for correct pedal free play and plunger actuation. It was trial and error to get it right so I don't remember the exact length of the dowel.
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post #3 of 16 (permalink) Old Oct 14th, 19, 12:48 AM
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Re: Vacuum brake booster testing

What you're describing doesn't sound like a bad brake booster.FWIW it's extremely hard to draw a vacuum with a hand held pump.You have a 3/4 inch pump drawing vacuum on an 11 inch booster so no matter how fast you pump it doesn't work.
You say the brake eventually work after the pedal sinks to a certain point ? If you recently replaced the M/C the internal pin could be the wrong size or if you have a disc/drum brake system the rear drum brakes could be way out of adjustment either one could cause a really low brake pedal.
If this car had 4 wheel drums when you first got it ? there's a good chance the M/C isn't matching up to the brake booster or the internal pin is the wrong size.

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post #4 of 16 (permalink) Old Oct 14th, 19, 9:52 AM
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Re: Vacuum brake booster testing

things to consider,is the combination valve the proper one,brakes bleed properly,haul the hose of the vaccuum booster ,not running,if it hisses its generally ok.Make sure its adjusted properly,pin length and free play ,right pedal hole position,some have 2 options.good luck

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post #5 of 16 (permalink) Old Oct 14th, 19, 1:00 PM Thread Starter
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Re: Vacuum brake booster testing

Thanks for the replies. It does seem like my booster is OK according to what you guys say and my tests.

So to attempt to describe my symptom more accurately.. With the car running, when I begin to press on the pedal I can hear a whoosh of air as I press it. The pedal will travel a couple of inches with little effort and no effect on the brakes. Then the pedal stops. If I then continue to press ever harder, the car will stop. But at that point, I'm really pushing hard on the pedal and I'm very near the floor.

I had front disc, rear drum. I now have 4 wheel disc. I also changed the M/C to the Wilwood version (1-1/8" bore) with their proportioning valve. Used the Motive pressure bleeder to bleed the brakes, and then afterward even did the 'ole fashioned method with another person pressing on the pedal...

When I installed the M/C I was very deliberate to measure the length of the pushrod on the booster, and stuck a dowel into the M/C and marked it. I measured both of these and it seemed perfect. Though I can't recall details on that this time...(this was several months ago). I can do what was suggested, disconnect it and measure again.

Thanks,

Tom

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post #6 of 16 (permalink) Old Oct 14th, 19, 5:11 PM Thread Starter
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Re: Vacuum brake booster testing

Ok, so I removed the Wilwood master cylinder from the booster and took some measurements. I used some dial calipers for all measurements, hence the decimal readings...

My pushrod on the OEM 11" Delco booster measure 1.80" in total length from back of booster to tip. The Wilwood instructions show measurements that show that the total bore depth is 1.82". So that seems pretty good when the brake is at rest. There is only a 0.02" gap between end of pushrod and M/C.

Without the M/C connected, my total booster rod travel is 1.44" (1.80" with brake pedal free, 3.24" with brake fully pressed).

With the M/C connected, my total pedal travel at the brake pedal connecting rod under the dash is 7/8" (0.875"). So the M/C is limiting my travel, which the instructions say is good as you don't want to bottom out the pedal -before- the M/C is fully compressed.

OK...so now where to go from here?!! Can it still just be air in the system? I can't believe that, but I'm stumped and willing to try that some more.

Thanks,

Tom

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post #7 of 16 (permalink) Old Oct 14th, 19, 5:42 PM
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Re: Vacuum brake booster testing

What size bore in the master cylinder? Should be 1.125 for power brakes.
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post #8 of 16 (permalink) Old Oct 14th, 19, 5:59 PM
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Re: Vacuum brake booster testing

Ok, if your 2 mating points are correct and there is no air in the system, the only other thing is on some disc brake setups the parking brake adjustment plays a major part in how the brakes operate. Does your parking brake interact with the main system?

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post #9 of 16 (permalink) Old Oct 14th, 19, 8:16 PM
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Re: Vacuum brake booster testing

Is your rear disc system from Right Stuff Detailing? If so, you will need to adjust the rear parking brake.There are online videos to help you with this.
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post #10 of 16 (permalink) Old Oct 14th, 19, 8:40 PM
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Re: Vacuum brake booster testing

Quote:
Originally Posted by Gunn317 View Post
So to attempt to describe my symptom more accurately.. With the car running, when I begin to press on the pedal I can hear a whoosh of air as I press it. The pedal will travel a couple of inches with little effort and no effect on the brakes. Then the pedal stops. If I then continue to press ever harder, the car will stop. But at that point, I'm really pushing hard on the pedal and I'm very near the floor.
I had this exact problem. I installed 6 piston front/4 piston rears and Wilwood recommended their 1" master.

Then they suggested it was bad and I sent it in for pressure testing. No problem.

Then they said go to their 1-1/8" master. This is where I started to suspect they were just guessing. It was ever so slightly better-but still unacceptable and un-driveable.

Along the way, they kept bringing up vacuum, the pushrod, the brake arm hole, the booster and how I bled everything-none was the problem.

I then determined Wilwood makes great calipers but they know nothing about master cylinders.

I installed a 1-5/16" master from a truck and my brakes are just awesome. Wilwood can't explain why. They need to make at least a 1-1/4" master cylinder.
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post #11 of 16 (permalink) Old Oct 14th, 19, 8:52 PM Thread Starter
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Re: Vacuum brake booster testing

Quote:
Originally Posted by oldcutlass View Post
Ok, if your 2 mating points are correct and there is no air in the system, the only other thing is on some disc brake setups the parking brake adjustment plays a major part in how the brakes operate. Does your parking brake interact with the main system?
Quote:
Originally Posted by modern malibu View Post
Is your rear disc system from Right Stuff Detailing? If so, you will need to adjust the rear parking brake.There are online videos to help you with this.
I have a 1-1/8" bore size Wilwood master cylinder (Part #260-8556-BK). And YES I do have the Right Stuff Detailing rear disc system! I also installed a brand-new stainless emergency brake system but have not adjusted it at all, really. When I tested it, it barely worked to slow the car from a slow roll. I just added that to my list of TODO's, but never thought it would have an affect on the overall brakes...but now I can totally see why it would. Thanks! I will check out the adjustments and report back!

Tom

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post #12 of 16 (permalink) Old Oct 14th, 19, 8:55 PM Thread Starter
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Re: Vacuum brake booster testing

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Originally Posted by paul bell View Post
I had this exact problem....

I installed a 1-5/16" master from a truck and my brakes are just awesome. Wilwood can't explain why. They need to make at least a 1-1/4" master cylinder.
Paul, I love this setup! If this all goes south with me too and I can't figure it out, would you mind giving me more info on this master cylinder? What year and what truck? I will definitely go that route next if I need to. And is that a 9" standard vacuum booster that it bolted right up to? I assume it would bolt right up to my 11" one too then.

That's awesome. I love that you can see the fluid level, and that it has a simple twist lock cap (which might just hook right up to my Motive pressure bleeder with an adapter I have for my '04 Silverado 2500HD).

Tom

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post #13 of 16 (permalink) Old Oct 14th, 19, 9:19 PM
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Re: Vacuum brake booster testing

Sure Tom, I'd have to look back in some threads.

It's a 9" dual diaphragm booster.

The truck master's piston sits out (rearwards) more than the original, I shimmed it away from the booster with a few washers. There's a metric line nut that fits the master outlets that's made for our 1/8" lines.

With any power brake system, you need at least 16 inches of vacuum. My engine only made like 11 so I get my vacuum from a high capacity Hela vacuum pump, same as used on trucks. Getting my brakes right took months-no thanks to Wilwood.

Honestly, I would have preferred the look of the Wilwood master but good functioning brakes are way more important than looks. And it's a conversation starter.

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post #14 of 16 (permalink) Old Oct 14th, 19, 9:22 PM
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Re: Vacuum brake booster testing

It's a 2009~2010 1500 Express/Savana master cylinder without the active brake control option.
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post #15 of 16 (permalink) Old Oct 15th, 19, 5:55 PM Thread Starter
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Re: Vacuum brake booster testing

Quote:
Originally Posted by paul bell View Post
It's a 2009~2010 1500 Express/Savana master cylinder without the active brake control option.
Awesome, thanks Paul! I will keep this in my back pocket in case I continue to have troubles.

Also love your fuse and wiring layout, very neat.

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