akfireman

I have a 98 GMC 3/4 Ton 4X4 Ext cab short bed pickup, I have a question about the brakes, I got the truck about a year ago and when i pushed on the brake pedal the truck would stop fine, but i could keep pushing the pedal and it would go all the way to the floor. when i asked the used car dealer i got it from they said it was working fine, because it stopped with no problem.

Last week it developed a bad antifreeze leak from the intake manifold and since i have an extended warranty i took it to the dealer and had them fix the leak and the brakes. after replacing the front pads, the mechanic said it was fixed, i picked it up and took it back the next day because i could still push the brake pedal all the way to the floor. after going for a test ride with he mechanic he told me that from late 97 to early 98, GMC used a different 2 stage ABS system and that was normal for those models and that my brakes are working properly and that is normal so that no matter what the brakes cannot be locked up.

here is the question, has anyone had this problem or heard of this, i have never seen any vehicle that i could push the pedal all the way to the floor. i really think this guy is trying to pull one over on me, but you never can tell sometimes. thanks in advance

BOOZER

Excessive pedal travel is usually the result of the
rears being out of adjustment, but the pedal should never go to the floor. So there is a 2-stage abs system now, thats a new one, wonder when the 3-stage is coming out? If the pedal is spongy, bleed the brakes{do this every 4 years on mine} its good
maintance on a 4WAL system. If the pedal is firm &
slowly goes to the floor look at the master cylinder as the problem, assuming there is not an external fluid lead, there isnt one is there?
G.M. has a trend to build intakes dry, using little if any silicone, so if you get 5 years out of one your lucky. Stop in any dealer, ask any tech & if he isnt
doing one now he did one yesterday, I did mine last
month.

akfireman

Boozer, thanks for the reply, as far as the leak, there is none, the back brakes were adjusted and it was at the gmc dealer, I think its a master cylinder problem but the tech said no, thats why i am asking if anyone else has had this problem, how hard is a master cylinder r&r?

GMMerlin

Do you have a vacum booster or a hydraulic booster?
You should have no more than 3" to 4" of brake pedal travel depending on the booster system.
Take it back to the dealer and have them reference Doc ID #537536.
This document explains how to test brake pedal travel.
As far as a 2 stage ABS system....I'll have to call B*LLSH*T on that one..I have worked for GM dealers for 20 years and am still employed by a dealer...never heard of a 2 stage ABS system

__________________
Why do you never have enough time or money to do the job right the first time, but enough time and money to do it again?
Got all the certifications
25 years ASE Certified Master Technician
GM WCT
Yea..I work at a DEALERSHIP....got a problem with that?

Want to diagnose cars like the Pros? FOR SALE: OTC Perception 2 channel Labscope (badged as a Matco Reality)
This is my personal labscope that I use to diagnose waveforms, like new, comes with leads and training video
PM Me for details

akfireman

GMMerlin, thanks for the info, is there any place that i can get a copy of that in case they tell me that i am full of crap?

BOOZER

Brake Pedal Travel
Tools Required
J 28662 Brake Pedal Effort Gauge

Check the brake pedal travel at frequent intervals. Brake pedal travel is the distance the brake pedal moves toward the floor from a fully released position. Make this check with the brakes cold and with tool J 28662 .

With the engine off, press the brake pedal and release the brake pedal. Do this five times in order to release vacuum from the vacuum booster.



Measure the distance (1) from the bottom of the steering wheel to the brake pedal with the brake pedal released.
Apply about 455 N (100 lb) on the brake pedal. Use the J 28662 .
Measure the distance (2) from the bottom of the steering wheel to the brake pedal.
Subtract the first measurement (1) from the second measurement (2).
Compare the difference with the specifications below. If the brake pedal travel exceeds specifications, refer to Hydraulic Brake Diagnostic System Check .
System
Metric
English

Vacuum Booster
80 mm
3.1 in

Hydraulic Booster
110 mm
4.3 in

Four Wheel Disc Brakes
102 mm
4 in



--------------------------------------------------------------------------------
Document ID# 537536

akfireman

boozer, one question, i understand to push on the brake pedal 5 times while the engine is off to realease the vacuum. do i then start the engine to do the measuring or leave the engine off. here is why i ask, when the engine is off, the pedal stops normally, It is when the engine is running that i can push the pedal all the way to the floor, it wont do it when the engine is off.

ok, before i sent this i decided to go out with a tape measure and do some crude measuring, with the engine off, and having expelled all the vacuum, i had pedal travel of 3 3/4 inch. with the engine running i had pedal travel of 7 inches, which is if not too the floor its almost to the floor.

Again, I thank everyone for their help, I didnt believe the dual abs story that i got, after reading this board for a couple months i finally decided to register and post.

GMMerlin

The test is with the engine off.

__________________
Why do you never have enough time or money to do the job right the first time, but enough time and money to do it again?
Got all the certifications
25 years ASE Certified Master Technician
GM WCT
Yea..I work at a DEALERSHIP....got a problem with that?

Want to diagnose cars like the Pros? FOR SALE: OTC Perception 2 channel Labscope (badged as a Matco Reality)
This is my personal labscope that I use to diagnose waveforms, like new, comes with leads and training video
PM Me for details

akfireman

merlin,

Thanks, thats what i figured, so what about the pedal travel while the engine is running, thats when the problem shows up, maybe the vacuum booster?

GMMerlin

There is no official measurement for the pedal travel with booster assist.
If the pedal sinks with or without assist I would say its a leak in the brake system..most likely the master cylinder.
In your case, I would check the rear brake adjustment and the condition of the brakes in general.

__________________
Why do you never have enough time or money to do the job right the first time, but enough time and money to do it again?
Got all the certifications
25 years ASE Certified Master Technician
GM WCT
Yea..I work at a DEALERSHIP....got a problem with that?

Want to diagnose cars like the Pros? FOR SALE: OTC Perception 2 channel Labscope (badged as a Matco Reality)
This is my personal labscope that I use to diagnose waveforms, like new, comes with leads and training video
PM Me for details

akfireman

merlin,

Thanks, the tech put on new front pads and said the rear were ok then he adjusted the rear, it brakes alot better but still goes to the floor when running, i was thinking the master cylinder originally, I guess my problem now is how to get the dealership to look at the master cylinder and possibly change it. I would do it myself but i still have time on my extended warranty and i will only have to pay the deductable.

gmech

I had a similar problem on a different vehicle. After replacing the master cylinder, it corrected the issue.

__________________

gburian

i have the same year(98) gmc 3/4 ton extra cab truck you have and the same problem you have! mine has a vacum booster. does yours ?? (1) i replaced the master cylinder , but still have the same problem.(2) i have bleed the brakes again and again and again.(3)replaced and adjusted the back brakes.(4)replaced front pads(5)then more bleeding & bleeding and i am still having the exzact same problem you have described ??? anyone that has an idea or suggestions are more than welcome.thanks!

akfireman

Here is what i did to solve the problem, I traded in the truck for a 1999 1/2 ton, no problem with the brakes on it.