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Brake HELP .... soft pedal to floor intermittent????

D

dwdooley

Full Member
Joined
Jun 13, 2005
Messages
59
Location
Houston, Texas
Having issues and I can't figure this out on my 05 Ava Z71?  I can have a good firm pedal and then all of a sudden the pedal will softly go to the floor but will hold the truck to a stop. I can pump the brakes and it will slowly ... with each pump get a higher and higher pedal .... but still a little spongy.  After quite a while of driving it will slowly get back to a firm pedal again and when it is solid I can slam on the brakes and it will be solid and firm.    Then later down the road it will all of a sudden start all over again. After it starts acting up it seems like if as I push on fist push and if I feel it is spongy and I don't go all the way down but as soon as I feel it spongy I can let off and pump it 3 or 4 times I will have a High pedal stop but it will be a softer hold and not rock firm.
I have no leaks around hydro boost or inside cab, fluid is full on both brake booster and power steering pump. I am getting no codes or brake light warnings.  I have tried to determine if it is power steering pump or hydro boost.  At one point when it was acting up I did hear a slight whine from power steering pump. When this happens I also don't have any issues with my steering feeling different.
Any help would be appreciated!!!!!
 
could be master cylinder leaking internaly.
 
Will check. Rumor is If leaking by you will see bubbles in tank through fill cap off and observe turbulence in tank????
But normally you will always see a little turbulence.
Is this true?
 
Is the brake fluid clear ?
 
dwdooley said:
Will check. Rumor is If leaking by you will see bubbles in tank through fill cap off and observe turbulence in tank????
But normally you will always see a little turbulence.
Is this true?
Click to expand...
DEPENDS on how it failed but do not count on it as fluid is bypassing the spool seals from one side to the other and tank does not matter so much...


 
Well .... if I stick my finger in and the run off of my finger is clear, but looking in tank it would be a darker clear.
This is some old fluid though.
 
If it were internal of the master cylinder, when the brake pedal goes down .... wouldn't I get a brake light check come on?
Since I have not seen brake light ..... That"s why I was going after power steering pump or hydro-boost.  On one incident when it went to floor I did hear a slight whine off the power steering pump that I've never heard before, but then it went away and the pedal slowly built back up.
I'm at 150k miles now and around 80K miles I had a master cylinder failure ..... BUT when that time occurred the cups blew(found bore all pitted) ...The pedal slammed to the floor, the brake light came on, and I never got ANY brakes back.  It stayed gone and any pedal press would slam to the floor with NO resistance.
I slowly get my pedal back and then sometimes it's solid as a rock again.
I am real good a trouble shooting and since my pedal slowly comes back and I can always feel a little resistance, that's why I felt it wasn't the master cylinder.
 
dwdooley said:
If it were internal of the master cylinder, when the brake pedal goes down .... wouldn't I get a brake light check come on?....
Click to expand...

I am Not sure what makes the BRAKE light comes on.

How does it feel with engine off?
Maybe bleed brakes?

When a master cylinder fails it does not have to fail like you describe where it goes to floor quickly... and no pressure.
It can fail where the bore gets slight score and pressure is bled from one side to other very slowly...
(the old issue: "I sit at red light and pedal ends up on floor...... and ruck creeps forward...")



If mine...I would:
Check all lines
Bleed all 4 corners
MAybe check calipers for smooth moving piston cups.
check booster plumbing (could it have been reversed?)
Check Master Cylinder and booster


 
  Have you checked the rubber brake lines?
 
MS03 2500 said:
Let me ask another question does the brake fluid look like this, it's still clear but greenish.

normal_0624181345.jpg
Click to expand...
Yes ... It looks clear .... but greenish.
And No broken lines ..... NO FLUID LEAKS ANYWHERE.
I wish it would stay down, but then after a while it gets solid and firm again and can sometimes go a few days with NO issues. WTH  ???
 
JVZL1 said:
  Have you checked the rubber brake lines?
Click to expand...

What I'm suggestion rubber line will swell with age. Just something to look at. 
 
JVZL1 said:
  Have you checked the rubber brake lines?
Click to expand...

Yes ..... I have checked all rubber hoses.
Once they would soften and swell .... then that .... would let me find it and not go back to normal and firm pedal,
 
If it's greenish then the seals are bad in the master cylinder. When mine went bad like that I bleed the brake front and back 2 weeks later it turned greenish again.

Replaced the master cylinder and rebleed then brake went back to firm better than it ever was.
 
I'm pretty sure that's what your problem is, With mine is would stop fine I just couldn't tell how far the pedal would go down. I have no clue what the green stuff is I tried siphoning it out of the Reservoir a couple of times but it still turned green, flushed the whole system it still turned green. Other than the fact it was 15 years old, something dissolved inside of the master cylinder.

Now press the pedal about a inch and a half its happening, half way down is about as far as it goes without using some serious leg muscle. it's going to be interesting driving in snow LOL something tells me the ABS will be kicking in a lot.
 
Years ago I had a power brake booster fail and that is how it acted. Normal brakes for most stops but once in a while the brakes would go to the floor!
 
How can you tell if the brake booster is bad?
Start the engine and let it idle. 7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine. The pedal should hold its position, if not, there's a leak in the brake booster, valve, vacuum hose or intake manifold.
 
Z66 BUTCH said:
How can you tell if the brake booster is bad?
Start the engine and let it idle. 7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine. The pedal should hold its position, if not, there's a leak in the brake booster, valve, vacuum hose or intake manifold.
Click to expand...
He has POWER steering powered booster not Vacuum.
 
I would do the master cylinder and all 5 of the flexible brake lines. Then bleed bleed bleed.

This would be the easiest fix to do first and likely to get the problem.

Make sure you don't cheap out on the master cylinder as it can save your life.

The flexible brake lines when plugged act like a bad caliper but it certainly won't hurt to put on good quality brake lines and replace the factory ones that are pretty old by now and insure they are not an issue.

Rodney
 
Ordered one click on Amazon Prime ... Be here Saturday.

New ACDelco #18M2404 Professional Brake Master Cylinder Assembly
by ACDelco

Price: $102.28  ... Prime w 2 day shipping.
 
I see that one comes with the bleeder kit, don't skip that step manually bleed the master cylinder or you'll never get the air out of it.



Oh yeah spray some penetrating oil on the bleeder valve now so it has time to work. I like PB Blaster
 
MS03 2500 said:
I see that one comes with the bleeder kit, don't skip that step manually bleed the master cylinder or you'll never get the air out of it.



Oh yeah spray some penetrating oil on the bleeder valve now so it has time to work. I like PB Blaster
Click to expand...

STOP ,,,, I got wrong part!!!  Double checked but getting ready to take old unit off and noticed my reservoir has a hose off front of tank going down to ABS module.  I don't see any pics w part numbers that have that feature.
I will keep looking for new AC Delco part # today.
 
Does it look something like this
 

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MS03 2500 said:
Does it look something like this
Click to expand...

Yes .... ACDelco part #18M2395 ;  I just got in a hurry and first ordered 18M2404 which is identical except is without outlet port for traction control module. I will receive replacement on Tuesday.  I am trying to stick with OEM since a friend of mine that has a shop told me he was working on a 2500 hydro-boost system that after using aftermarket parts and changing hydro-boost also; didn't fix it until he finally got a OEM master cylinder.
 
dwdooley said:
Yes .... ACDelco part #18M2395 ;  I just got in a hurry and first ordered 18M2404 which is identical except is without outlet port for traction control module. I will receive replacement on Tuesday.  I am trying to stick with OEM since a friend of mine that has a shop told me he was working on a 2500 hydro-boost system that after using aftermarket parts and changing hydro-boost also; didn't fix it until he finally got a OEM master cylinder.
Click to expand...

  Thanks good to know.  (y)
 
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