Intermittent Brake Issues - The 1947 - Present Chevrolet & GMC Truck Message Board Network

pilotXC · 03-01-2018, 04:58 PM

I looked all over the posts on brakes and didn't see anything relating to my issue so wanted to see if anyone has experienced this before...

The dummy light comes on some times and the pedal goes nearly to the floor, but the brakes work, just ok... Then they will be normal again. Yesterday this was basically every other time I stepped on the pedal, once would go to the floor, next would be normal, then back to the floor, then normal again. The light was on at first, then went off by the time I got home but the pedal kept going in and out.

I'm assuming I have a sticky wheel cylinder on one of my rear drums as the fluid is full in the master cylinder and the prop valve probably has zero influence on this particular issue if the light is turning off and on. BUT, I"m not sure and haven't dove into it, just curious if anyone has experienced this and/or has any ideas beyond this? Thanks!

**Steeveedee** · 03-01-2018, 05:11 PM

Go out and stab the brake pedal hard, then lighten up on the pressure, keeping some amount on it, and see if the pedal gradually sinks. If so (and I'd bet on it) your master cylinder is bypassing and will need rebuilding or replacement. You didn't report any pulling to one side or the other, so I'd say that the wheel cylinders (and calipers, if so equipped) are working properly.

B. W. · 03-01-2018, 05:48 PM

X2 - master cylinder is bypassing internally.

**HO455** · 03-01-2018, 06:43 PM

I had a similar issue that turned out to the anodizing on the master cylinder seal rod was flaking off and getting stuck under the lip seal one stroke and pop out the next. I wasn't having a problem with the pedal drifting down. It would just be nice one stroke and the next one it would go 3/4 of the way to the floor.
So master cylinder like all the folks above said.

demian5 · 03-01-2018, 07:14 PM

X3 MC. Prop valve switch detects uneven pressure front to rear only.

**Steeveedee** · 03-02-2018, 12:41 AM

Quote:

Originally Posted by HO455

I had a similar issue that turned out to the anodizing on the master cylinder seal rod was flaking off and getting stuck under the lip seal one stroke and pop out the next. I wasn't having a problem with the pedal drifting down. It would just be nice one stroke and the next one it would go 3/4 of the way to the floor.
So master cylinder like all the folks above said.

I've spent most of my life in the desert of California. You folks that live in rust country get to experience problems that I haven't seen. Though I did work as a mechanic in Idaho for a year, so I've seen some things there.

The real clue with the original post is that the master cylinder doesn't empty out. That's a classic clue for bypassing.

pilotXC · 03-02-2018, 01:40 PM

Thanks everyone, much appreciated! MC should show up at the parts store this morning, hopefully that's the fix! Haven't had a chance to run out and mash the pedal and see if it fades after that yet, but will try this weekend... Love this board for all the expert advice, have a great weekend guys!

pilotXC · 03-09-2018, 01:10 PM

So this is not going well... I drove it over to my buddies shop and had no problems at all with the brakes on the way. We put the new MC on, after bench bleeding it, bled the rears no problem, but could not get the air out in the front and the cap leaked. Went to a different parts store and got another MC. Bench bled the crap out of this one and was able to get solid fluid on all four corners, but had no brakes at all once it was started up and running. Jacked up the rear and had no brakes at all there. Jacked up the front and have brakes in the front. So, guessing it's the proportioning valve. Dummy light was not on at first, but was on after double checking the lines had no air. Ordered one up and will swap it and repeat the bleeding again.

Anyone else have any suggestions? I tend to get tunnel vision once I'm buried in a problem so outside perspective is helpful...

demian5 · 03-09-2018, 01:40 PM

Quote:

Originally Posted by pilotXC

Anyone else have any suggestions? I tend to get tunnel vision once I'm buried in a problem so outside perspective is helpful...

The valve inside of the prop valve can sometimes hag up. Pull the switch out of the prop valve and see what it looks like inside.

03-01-2018, 04:58 PM	#1
pilotXC Registered User Join Date: Feb 2016 Location: Portland OR Posts: 154	Intermittent Brake Issues I looked all over the posts on brakes and didn't see anything relating to my issue so wanted to see if anyone has experienced this before... The dummy light comes on some times and the pedal goes nearly to the floor, but the brakes work, just ok... Then they will be normal again. Yesterday this was basically every other time I stepped on the pedal, once would go to the floor, next would be normal, then back to the floor, then normal again. The light was on at first, then went off by the time I got home but the pedal kept going in and out. I'm assuming I have a sticky wheel cylinder on one of my rear drums as the fluid is full in the master cylinder and the prop valve probably has zero influence on this particular issue if the light is turning off and on. BUT, I"m not sure and haven't dove into it, just curious if anyone has experienced this and/or has any ideas beyond this? Thanks!

03-01-2018, 05:11 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,724	Re: Intermittent Brake Issues Go out and stab the brake pedal hard, then lighten up on the pressure, keeping some amount on it, and see if the pedal gradually sinks. If so (and I'd bet on it) your master cylinder is bypassing and will need rebuilding or replacement. You didn't report any pulling to one side or the other, so I'd say that the wheel cylinders (and calipers, if so equipped) are working properly. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

03-01-2018, 05:48 PM	#3
B. W. Registered User Join Date: Nov 2014 Location: Bigfork, Montana Posts: 1,137	Re: Intermittent Brake Issues X2 - master cylinder is bypassing internally.

03-01-2018, 06:43 PM	#4
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,393	Re: Intent Brake Issues I had a similar issue that turned out to the anodizing on the master cylinder seal rod was flaking off and getting stuck under the lip seal one stroke and pop out the next. I wasn't having a problem with the pedal drifting down. It would just be nice one stroke and the next one it would go 3/4 of the way to the floor. So master cylinder like all the folks above said. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

03-01-2018, 07:14 PM	#5
demian5 Registered User Join Date: Aug 2016 Location: Lake Forest, CA Posts: 2,177	Re: Intermittent Brake Issues X3 MC. Prop valve switch detects uneven pressure front to rear only. __________________ "Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC

03-02-2018, 01:40 PM	#7
pilotXC Registered User Join Date: Feb 2016 Location: Portland OR Posts: 154	Re: Intermittent Brake Issues Thanks everyone, much appreciated! MC should show up at the parts store this morning, hopefully that's the fix! Haven't had a chance to run out and mash the pedal and see if it fades after that yet, but will try this weekend... Love this board for all the expert advice, have a great weekend guys!

03-09-2018, 01:10 PM	#8
pilotXC Registered User Join Date: Feb 2016 Location: Portland OR Posts: 154	Re: Intermittent Brake Issues So this is not going well... I drove it over to my buddies shop and had no problems at all with the brakes on the way. We put the new MC on, after bench bleeding it, bled the rears no problem, but could not get the air out in the front and the cap leaked. Went to a different parts store and got another MC. Bench bled the crap out of this one and was able to get solid fluid on all four corners, but had no brakes at all once it was started up and running. Jacked up the rear and had no brakes at all there. Jacked up the front and have brakes in the front. So, guessing it's the proportioning valve. Dummy light was not on at first, but was on after double checking the lines had no air. Ordered one up and will swap it and repeat the bleeding again. Anyone else have any suggestions? I tend to get tunnel vision once I'm buried in a problem so outside perspective is helpful...

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