When engine is running I can pump up pedal after two to three pumps but then hear a loud hissing

Tiny
TMINER
  • MEMBER
  • 1998 CHEVROLET 1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 275,000 MILES
After replacing master cylinder twice (first new one shot fluid about three feet in the air when pushing down pedal) we flushed all lines and have all new fluid in the system. We then bled system using vacuum suction and we went from right rear and proceeded to left rear, right front and left front. When engine is running I can pump up pedal after two to three pumps but then hear a loud hissing (like air or vacuum) until I take my foot off pedal. Once I release pedal then press it, pedal goes back to floor. Checked for leaks and did not see any on any wheel. Blew out the combination valve and hear it clicking when doing so. When pressing brakes it will sometimes effect engine rpm's which to me is very odd. From what I am seeing online, I am guessing check valve and/or vacuum booster. Any better ideas before I spend $140.00 on a booster? Thanks in advance.
Sunday, December 10th, 2017 AT 9:39 PM

36 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Before getting a new booster (which is vacuum operated) check to see if it will hold vacuum. Follow this how to for checking for possible problems. I would suspect there is air in the lines. These guides can help you fix it.

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.
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Monday, December 11th, 2017 AT 6:38 PM
Tiny
TMINER
  • MEMBER
  • 49 POSTS
Trying all that, nothing was found. Running the engine and pressing the brake pedal down I get a hissing noise and pedal goes down a lot. After turning off the engine and pressing the pedal once it goes down due to stored vacuum pressure. After that initial press and I let back up then press down I get a good pedal and the rear brakes are heard working. It is hard to see the calipers so I do not know if fronts are working.
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Tuesday, December 12th, 2017 AT 9:09 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,512 POSTS
Hello,

The hissing is a bad booster you must fix that which will probably solve the low pedal problem.

Here is a guide that will help you bleed the system as well

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Here are diagrams to show you how to do the job.

Let us know what happens and please upload pictures or videos of the problem.

Cheers, Ken
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+1
Sunday, December 17th, 2017 AT 2:57 PM
Tiny
TMINER
  • MEMBER
  • 49 POSTS
Booster has been replaced and it fixed the issue. Now I have another issue I will post a question for. Thanks all!
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Sunday, March 18th, 2018 AT 12:58 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,512 POSTS
Nice work, we are here to help, please use 2CarPros anytime.

Cheers, Ken
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Monday, March 19th, 2018 AT 10:49 AM
Tiny
DYESCHICK
  • MEMBER
  • 1 POST
  • 1997 CHEVROLET 1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 185,000 MILES
Replaced master cylinder booster, rear wheel cylinders and calipers bled several times. Good pedal until engine is started. Problem started after hitting a pothole. Wheel bearings seem okay.
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Friday, April 9th, 2021 AT 2:20 PM (Merged)
Tiny
TOUGHDIVER
  • MECHANIC
  • 224 POSTS
Hi,

The brake pedal will drop when you start the vehicle this normal and the pedal will fade a little when sitting in the service bay it will feel different when driving. If the brake goes to the floor there is a leak or the brake system has air in it. So check for brake fluid leaks and repair if necessary.

Bleed the system starting at the master cylinder very important to bleed all air from the master cylinder. Start bleed manually at the master cylinder waiting 10 seconds between bleed cycles. Do the same for all components that is to be bleed. please do not judge the pedal until it's on the road.. I'm attaching repair guides for you to reference to. Let us know if you need any more information.

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor..

Thank you,
Joe T.
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Friday, April 9th, 2021 AT 2:20 PM (Merged)
Tiny
REDNECKDADDY0822
  • MEMBER
  • 1 POST
  • 1996 CHEVROLET 1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 250,000 MILES
Hello. I have read several threads on this issue, but I have not an answer for my issue. I have the truck listed above (C1500). This is my daily driver. The last mechanical work I had done was about six months ago when I had the transmission rebuilt. Out of the blue about two weeks ago, I get in my truck and the pedal goes to floor at starting. I had not had any brake issues to this point; no soft or weak braking. When the engine is off, pressure builds but when I start the truck, to the floor goes the pedal. I replaced the master cylinder and bled all lines at the wheel today. The end result is still the same. I checked the booster and it appears to be operating properly. I am stumped and looking for advice. I am interested in the hydro upgrade but I do not have the funds available at this time. TIA!
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Friday, April 9th, 2021 AT 2:20 PM (Merged)
Tiny
CARADIODOC
  • MECHANIC
  • 33,870 POSTS
Hydro-boost is definitely not an upgrade. It was the last-ditch effort to get decent power assist on diesel engines that don't have intake manifold vacuum. The early blocks were modified gas engine blocks with the hole for a distributor. They stuffed a camshaft-driven vacuum pump in there for the power brake booster.

In your case, the power booster is working just fine, as they usually do. They have nothing to do with how far the brake pedal goes, only how easily it goes there. GMs are well-known to need two men and a boy to push on their brake pedals when the engines are off. That's why it's so hard to notice the low pedal until the engine is started. To add to the misery, with a properly-working GM system, it is possible to push the brake pedal all the way to the floor when you're standing still. Had you been driving, that would equate to the wheels locked up and then some. This has the potential to lead to a master cylinder failure, especially given the age of the truck. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. Pushing the brake pedal to the floor, whether bleeding improperly, surprised by a sudden leak, or just when standing still with the engine running, runs the pistons over that crud and can rip the rubber lip seals. That leads to a slowly-sinking brake pedal or no pressure buildup at all, and that may not show up for two or three days. This applies to any master cylinder over about a year old, on any brand of vehicle.

The web sites are full of GM truck owners complaining of low brake pedal, and I have yet to read about a solution common to all of them, besides the things we normally look for. Most of the time people have already replaced the master cylinder, the logical suspect, but still have the same problem. One thing you might look for is a rear drum brake shoe lining that rusted off its frame and is loose inside the drum. That will make the shoes need to move way too much and will result in a low pedal. Look for signs of wetness on the back side of the rear drum backing plates. That would indicate a leaking wheel cylinder. The clue to a leaking wheel cylinder or rusted steel line is the fluid level will continue to drop in the reservoir even after it has been refilled. With internal leakage in the master cylinder, fluid level in the reservoir will not drop.

There's a simple trick that prevents the need to bleed at the wheels when replacing a master cylinder, but since you did bleed there already, did you see any air bubbles come out the instant you opened the bleeder screw? Normally you'll need to run through enough brake fluid to get the air bubbles all the way down to the calipers. That takes some time. If you see air bubbles right away coming from a caliper, suspect a torn dust boot on it. I did see one that was drawing air into the caliper through that torn boot. That can't happen according to all of my training, but a rebuild kit took care of the mushy brake pedal that kept returning about every week.

Did you bench-bleed the new master cylinder before it was installed? If the truck has four-wheel anti-lock brakes, you may need a scanner to command the computer to open two valves so those chambers can be bled, especially if there was air in the lines from replacing the master cylinder. This doesn't apply to the rear-wheel, (RWAL) system that was more common for the time. Air will bleed through the dump valve to the rear brakes like normal with no special procedures.
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Friday, April 9th, 2021 AT 2:20 PM (Merged)
Tiny
FELIXALEMAN0
  • MEMBER
  • 8 POSTS
  • 1995 CHEVROLET 1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 350,000 MILES
So I have change the brake shoes, the spring for the drum brakes, the brake calipers, brake pads, brake lines, master cylinder, the brake booster, also bleed all the lines. Still have a spongy brake pedal after all of that. I don’t know what to do.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

Did you do the automated bleed sequence for the ABS?

You need a scan tool to do this procedure or you will never get the air out of the ABS.

Roy

TWO PERSON PROCEDURE (PREFERRED)
Gravity and vacuum bleeding are not recommended for this ABS system.

PROCEDURE
1. Raise the vehicle to gain access to the system bleed screws. Install clear tubing on the bleed screws so that air bubbles in the fluid can be seen.

NOTE: Never pump the brake pedal as fluid cavitation may occur.

2. Begin by bleeding the system at the right rear wheel, then the left rear, right front and left front.
3. Open one bleed screw at a time 1/2 to 1 full turn.
4. Slowly depress the brake pedal until it reaches its full travel and hold until the bleed screw has been tightened. Release the brake pedal and wait 10-15 seconds for the master cylinder to return to the home position.

NOTE: Repeat steps 1 through 4 until approximately 1 pint of brake fluid has been bled from each wheel. Clean brake fluid should be present at each of the wheel bleed screws. Check the master cylinder fluid level every 4 to 6 strokes of the brake pedal to avoid running the system dry.

5. After bleeding all four wheels, use a Tech 1 Scan tool to run 4 functional tests while applying the brake pedal firmly.
6. Re-bleed all four wheels using steps 1 through 4 to remove the remaining air from the brake system.
7. Evaluate the brake pedal feel before attempting to drive the vehicle and re-bleed as many times as necessary to obtain appropriate pedal feel.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
FELIXALEMAN0
  • MEMBER
  • 8 POSTS
I'm trying to find a shop to do it but they don't want to do because they say that it is just going to be like that and it wont fix the issue. But i'm still trying to find other shops to do it.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Any shop with a scan tool can do this procedure.

Keep me updated.

Roy
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
FELIXALEMAN0
  • MEMBER
  • 8 POSTS
Not really because it need a special scanner tool that are for older trucks and they don't have it so that is probably why they tell me it wont work.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
A lot of the newer scan tools for the GM have the old system built into it.

Roy
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
FELIXALEMAN0
  • MEMBER
  • 8 POSTS
Found out that I need an ABS pump and I don't know where to get one from. Is there a place where I can get one new or a rebuilt one?
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
CANNON1349
  • MECHANIC
  • 670 POSTS
  • 1995 CHEVROLET 1500
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 349,000 MILES
Hi,

I did the front brakes on this truck, new rotors and wheel bearings, and the pedal is very spongy. With the engine off I can get a pretty stiff pedal, maybe with enough pressure it can be spongy but the real sponginess is with the engine on. The master cylinder/brake booster is very squeaky and also I can see the fluid inside the reservoir slush and bubble.

I've bled all four wheels in the proper sequence, and even bled the ABS unit looking thing. I'm almost convinced it's a weak master cylinder, it's original. Also the seals at the reservoir are seeping, just not pushing fluid out but they are looking a little wet.

And, when bleeding the brakes, as I crack the bleeder valve the brake fluid doesn't rush out, it just flows nice and smooth out of the tube.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Hi,

It could be a master cylinder. When you bled the brakes, did you have a helper and what procedure did you follow? Also, this truck has rear drum brakes. Are you sure the brakes are properly adjusted? If they are out of adjustment, it will cause a spongy pedal.

Now, if the master cylinder reservoir is leaking at the rubber grommets, they need replaced. Also, can you explain what you mean by squeaky? One last thought, there shouldn't be bubbles in the brake fluid. I have a feeling there is air in the system at some point.

Let me know exactly what was done and how it was done.

Take care,
Joe
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
I see nothing listed for new or rebuilt.

You can get a used one by going to this web site. WWW. Car-part. Com and enter your information.

I attached a partial page for you.

Roy
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Friday, April 9th, 2021 AT 2:21 PM (Merged)
Tiny
CANNON1349
  • MECHANIC
  • 670 POSTS
The shoes in the drum are like new, still have numbers on the shoes but I didn't do those, at any point in time but seem okay in regards to drag and braking. I did have a helper hold down the pedal, crack the bleeder, close the bleeder, then release the pedal and pump 3 times. I did it in this order: RR, LR, RF, LF.

When pressing the brake pedal there's a squeak like a cabinet hinge that needs some wd-40, might be the arm in the booster. Who knows. It's not leaking per se, but is deteriorating and has a shiny/wet appearance.
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Friday, April 9th, 2021 AT 2:21 PM (Merged)

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