Soft Brakes

Corey Owner/Technician Kenton, Ohio Posted   Latest  
Question
Braking
1998 GMC C1500 Sierra SL 5.7L (R L31) 4-spd (4L80-E)
Soft Brake

Hello Everyone,

I got this 1998 GMC C1500 that the truck feels like there is air in the brake system and the pedal goes to the floor. A little history. Customer bought the truck with a soft pedal and took it to another mechanic and they put new calipers and brakes on the front and also changed the brake lines. They still had a soft pedal afterwards and didn't know what it was so he brought it to me.

We have bleed all the brakes again and done the automatic procedure and replaced and tested the Master Brake cylinder. We got no better results so we pulled out ABS unit and bypassed it completely using a T and fittings to connect Master cylinder to brake lines going to wheels. After bleeding whole system again it was still a soft pedal. So we tried clamping the rubber lines and it still wasn't much better. So we went through and connected each line one by one to the master cylinder and bleed it and had a very hard pedal with every one individual. We also have replaced the Brake Booster and wheel cylinders in the rear. We have now just put the whole system all back together like normal and bleed the whole system with ABS auto bleed and still soft pedal going to the floor with it running and very spongy off but all air is out.

I'm at my ends on this truck everything has been replaced. Master cylinder, Booster, ABS bypassed, Wheel cylinders, Front calipers. Brake lines. Only thing we didn't replace was the rubber hoses but I've watched them and didn't see any expanding and we got a hard pedal with each wheel connected by itself.

ANY HELP would be appreciated I don't know where else to go.

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Tim Mechanic
Blytheville, Arkansas
Tim Default
 

If I remember correctly those trucks have what I remember to be called a "dump valve" in the rear brake circuit that can cause low pedal and spongy brake feel. Also, have you adjusted the rear brakes out?

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Agree
Rick Owner
Clinton Township, Michigan
Rick Default
 

Back to basics be sure calipers are installed correctly, bleeders on top

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Eric Technician
West Frankfort, Illinois
Eric Default
 

The older trucks with rear abs only had the dump valve. By 1998 it is full 4 wheel abs.

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Tim Mechanic
Blytheville, Arkansas
Tim Default
 

Thank you for clarifying. It's been awhile 🙂

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Eric Technician
West Frankfort, Illinois
Eric Default
 

No problem, I cringe every time an old comes in with wheel cylinders leaking and see a dump valve. Almost allows cause issues.

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Mike Owner/Technician
Concordville, Pennsylvania
Mike Default
 

Rear brake drums oversized and/or brake shoes faulty or out of adjustment

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Gary Owner/Technician
Peck, Michigan
Gary Default
 

I usually pinch (carefully) the rubber brake hoses off and try to determine where the air is. If rear drums, make sure the drums are the right size and adjusted correctly. Front, as Rick mentioned, bleeders up. On some extreme problems I've had to reverse bleed the system. Pumping the fluid from the bleeder up to the Master Cylinder.

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Garry Instructor
Pensacola, Florida
Garry Default
 

I've been in that same scenario. I't ended up being the ABS unit. Plug the lines coming from the unit and check the pedal. If hard, the problem is between the ABS unit and the brake units. If soft, plug the master cylinder and check pedal. If then hard, it's the ABS unit. If soft, seal leaking in master cylinder.

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Pat Technician
Carson, California
Pat Default
   

These trucks never had a good pedal but with the right pads and after learning how to drive it, the brakes can be adequate. This system uses what GM called a "low drag caliper", where the seal groove was cut with about an 8° dry side angle instead of the more typical 4ish°. This causes the seal to draw the piston back much farther upon release of hydraulic pressure than a more normal caliper…

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Helpful
Interesting
Jason Owner/Technician
San Luis Obispo, California
Jason Default
 

We have had luck with the old United brake master cylinders from NAPA. They have a different quick take up valve than the other manufactures. Its been a long time since we have had this problem in the shop, so I don't know if NAPA still offers the same master cylinder. Good luck.

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Pat Technical Support Specialist
Carson, California
Pat Default
 

At one point when I was at Centric I tore down about 30 of these cylinders from different manufactures and factories and the only one I found that had (in my eyes) a correctly designed QTU valve was from EIS, who has been gone now for 20 years at least. I had 2 of these trucks and played around with different sized vent ports...made some progress but in the end the solution was to buy a newer…

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

Thanks for the Imfo​.​Never knew why,only knew they were a peculiar brake system.

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Harm Technician
Fergus, Ontario
Harm Default
 

excellent write up and explanation. we have always had this issue on these trucks and have always sent them out the door. Its the 2 pump stop. This makes a lot of sense why. Its hard to believe why they never came up with an upgraded caliper and master cylinder

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Pat Technical Support Specialist
Carson, California
Pat Default
 

If you can find 'em, SSBC (went out of business, then I heard they're back up) made/makes a 2 piston aluminum caliper that is nice and stiff and performs very well on these...While they're designed for the 1/2 ton (origi…lly using a 75mm piston), I've succesfully installed them on 3/4 tons (origi…lly using an 80mm piston) as well. The 3/4 ton 8 lug stuff typically stops better, because while…

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Harm Technician
Fergus, Ontario
Harm Default
 

Makes sense. probably you can get something through willwood brakes but likely you have to replace the spindle and go to a big brake set up.

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Pat Technical Support Specialist
Carson, California
Pat Default
 

Yeah..the problem is the spindle. The slide pin holes aren't designed to carry any load...the load goes into the slide rails on the knuckle. That limits what you can do with the stock knuckle.

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

It's hard to believe that GM would design something like this and not have any recalls.

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Pat Technical Support Specialist
Carson, California
Pat Default
 

Not really. Lol

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Jeffrey Diagnostician
Lawrence Township, New Jersey
Jeffrey Default
 

That’s because this was back on the day before the snow flakes came into the world😂🤣

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Rudy Technician
Montebello, California
Rudy Default
 

Patrick- Fantastic explanation. Ive always wonder why they were so bad.

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Marlin Technician
Estacada, Oregon
Marlin Default
 

Excellent reply. I am very happy that these critters are rapidly becoming rare. As people generally become accustomed to drastically better brake systems, these stand out even more in their terribleness, which results in more complaints.

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Scott Owner/Technician
Troy, Missouri
Scott Default
   

Does this truck have 4 wheel abs or rear wheel abs? Your T comment makes me think RWAL. If you clamp all the rubber lines and it is still soft, then your problem is not at any of the wheels. Are you familiar with "quick take up" master cylinders and caliper systems? May or may not apply. I would start by plugging one side of the master cylinder at a time and narrow it down by using plugs in the…

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Corey Owner/Technician
Kenton, Ohio
Corey Update
 

To those who ask - yes we have adjusted rear brakes and yes we checked and were bleeding with bleeders up not down.

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Mark Mechanic
Georgetown Township, Michigan
Mark Default
 

This era of all GM Trucks were never known to have a good hard pedal before or after all brake work. But they would always stop fine although the pedal would seem very low. My only thoughts are what brand MC was put on and if it was rebuilt or a new one? Common Rebuilt MC's can often be too worn and pressure bypass between primary & secondary sections. With your line test, you must have put…

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

I hope I can join in and get an insight about this issue.I currently have a similar issue with the same vehicle,soft pedal​.​After replacing all the brakelines,that's when the customer brings up the issue of problems with the traction control also .The traction control would activate at the worst times like when trying to pass​.​Won't do it all the time though.I know the…

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Rudy Technician
Montebello, California
Rudy Default
 

Corey- Brake pedal was always doo-doo on these trucks. Before you go down the rabbit hole of replacing parts(BTDT early in my career) Try this: Pump the pedal a few times with the engine running, then shut it off. Now pump the pedal 2-3 times with the truck off. The pedal should get harder with each successive stroke and not sink to the floor.(might be spongy though) If it doesn't sink your…

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Nelson Diagnostician
Gastonia, North Carolina
Nelson Default
 

Agree with what others have posted. I have always found those brake pedal feeling/responsiveness a little soft, less aggressive compared to the later style. I never had an owner complain about it; maybe they just took it as being normal and never had any newer truck to compare it to. One kit i use and recommend for plugging off brake circuits when chasing brake problems is the Thexton 803P…

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Raymond Mechanic
Midland, Michigan
Raymond Default
 

As others have stated,those old trucks always had garbage brake pedals. If your customer is absolutely in love with the truck,one suggestion would be a HydroBoost conversion. It's a lot of work and not cheap but it does increase pedal height and provides a firmer feel. It has been a long time since I've done a conversion so I couldn't guess where you can get the kit anymore but like I said if…

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Troy Owner/Technician
Longbranch, Washington
Troy Default
 

This generation truck has a crappy pedal feel but if you can do the brake bleed procedure and the pedal will feel great during the bleed but when you stop it feels poopy, i spent hours trying to deal with one of those, you cannot go my it sitting you have to drive it

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Michael Mechanic
Montgomery, Texas
Michael Default
 

Read the whole thread... kept my attention riveted... mainly work on class 8 trucks and have never ran across this specific issue (thank you Jesus!). However, Corey... I suggest in intrest of customer relationship: sit him or her down for a few minutes in front of this thread and ask them to read this thread... it should do a lot of explaining for you. And naturally open the conversation as to…

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Rick Technical Support Specialist
Saint Charles, Missouri
Rick Default
 

I've been through the brake mill with these trucks numerous times. Replaced all the components stated and still had a lame pedal. They will drive you insane. You never pay attention to the pedal or stopping until after you did extensive brake work. By then its too late. It only takes (on average) 15-25 pounds of force foot pressure (measured many times with a pedal force gauge) to stop a…

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Agree
Scott Owner
Moreno Valley, California
Scott Default
 

Hi Corey, I'm a little late to the party, but I agree with the others that said the brake pedal was not great on these vehicles. This is one of the things that is best not to "attempt/start" to fix. There's a difference between the normal low brake pedal, or low/spongy/pulls when stopping... I don't see these vehicles all that much anymore, but when I do I have a decent feeling for this is as…

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Corey Owner/Technician
Kenton, Ohio
Corey Update
 

Thanks everyone for you input on this problem I have. It confirms what I told him earlier that everything on the system seem to be in working order and I think GM undersized the Master for the brakes but he wouldn't believe me. That is why so much got changed in a chase after a problem with a designer flaw. Customer stopped in today and I explained the situation to him and gave him a copy of…

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Corey Owner/Technician
Kenton, Ohio
Corey Resolution
 

Customer doesn't believe that it could be the Master Cylinder and is aggravated with me for charging him for the new parts and not having anything fixed. He said he is going to take it to the dealer and have them look at it and if they fix it he wants his money back. I told him if they fix it I want to know what I missed and what they did to fix it and if it was something I totally missed I…

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Pat Technical Support Specialist
Carson, California
Pat Default
 

all You had to do was put better pads on it.

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Corey Owner/Technician
Kenton, Ohio
Corey Default
 

He just had new pads and calipers and brake lines put on it before it came to me. There was no way I could have done that with him.

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Scott Owner/Technician
Troy, Missouri
Scott Default
 

$200 is all you charge for all of that? Or did I misunderstand something?

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Corey Owner/Technician
Kenton, Ohio
Corey Default
 

Yes I lost big time with him. I didn't get a cent of labor out of the whole deal. And just paid my cost on the parts so I was in the hole big time on the job. What can you do with such people? He says he shouldn't have to pay anything because truck is the same as it was when it came. These people just don't understand. I find it best just to loose on those jobs instead of having a bad name go…

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