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

lukecp · 03-16-2003, 12:33 PM

I have been having problems with the brakes on my truck. Mostly, they suck. The pedal has to go down about an inch before i get any real brakeing feel, and then it gets very hard to push, and the truck barely slows down. It is a '72 C-10 with power disc brakes. I think that on a disc brake truck, the proportioning valve gives pressure to the rear brakes first, and then the front ones, so woldn't this mean the problem is in my rear brakes? The truck has a rebuilt master cylinder, new wheel cylinders on the rear, and the brake shoes on the rear are about 8 months old. I though they were out of adjustment, so i pulled off the wheels, and there is a slight drag when turning the drum on each wheel. I poped off the master cylinder top, and when the brakes are depressed and then released, fluid jumps back out of the front resivour, but fluid barely jumps at all out of the rear resivor. The brakes have also been bled and shouldn't have any air in them. No bubbles come out of the master cylinder either. I am thinking that either the rebuilt master cylinder is junk, or possibly the proportioning valve, sine it is the only thing i haven't replaced.

Gordo · 03-16-2003, 12:49 PM

Could be the power booster is bad and or the vaccum supply is restricted.

lukecp · 03-16-2003, 12:57 PM

I think the booster is OK. If i turn off the motor, pump the brakes 5-6 times until they are hard, then start it up the pedal sinks about an inch......

Andy4639 · 03-16-2003, 01:28 PM

I would check all your lines for damage. Check to make sure you haven't ran over something that may have flatened out one of your lines. I had this happen on a old truck years ago. Also check your front flex hoses to see if they are in good shape.

Xzavior240 · 03-16-2003, 06:10 PM

check your lines for pinholes as well I had a truck one time where you could stand on the brake pedal and dance and it would take it a block or so to stop if you were doing over 20. I found I had a really really small hole in my rear brake line where the line runs along the frame. since most people who use trucks for work dont spray out under it the mud and stuff from it being used just piled up on the line. and over time the moisture the mud held there rusted a hole in the line.

cdowns · 03-16-2003, 06:18 PM

if your having to pump the brakes5or6 times till they're hard i suggest bleeding the brakes again

lukecp · 03-16-2003, 07:31 PM

Well, i looked for obstructions in the lines, and found none. I doubt i have any pinhole leaks, because the master cylinder has been continusiouly full of fluid for several months, and i haven't had to add any. Would a bad proportioning valve cause this?

I can still drive the truck, but i hauled a large load of firewood in it yesterday, and the brakes got kinda scary at times.

LudwigTheMad · 03-16-2003, 07:43 PM

Tell us, how did you bleed your brakes....had similar probs when I fist started doing my own brakes, till I found out that although I wasn't doing it wrong, there was a better (more efficient) way.

Pumping 5 or 6 times till it gets hard is too much, a good set of brakes should be getting difficult during the second pump and near imposible by 3...

ZOOMY · 03-18-2003, 02:49 AM

What's the more efficient way ?????.............Zoomy

7T1BURB · 03-18-2003, 09:26 AM

Just finished a complete brake system rebuild (booster, MC, prop valve, ss lines and ss flex hoses, cylinders and calipers) on our 71 BURB and initially had difficulty getting all the fluid to the rear wheel cylinders and front calipers. Discovered that the proportioning valve pin (located in the front of the new prop. valve) must be held "IN" when bleeding (from the 1971 Service Manuall, " NOTE: When bleeding the brakes; the pin in the end of the metering portion of the combination valve must be held in the open position (not allowed to close). This can be accomplished by installing tool J-23709 under the mounting bolt and depressing the pin a slight amount. Be sure to retorque the mounting bolt after removing tool J-23709). I fabbed one (shaped like a "L") and then had success in bleeding the system.....if you are using rubber flex lines you might also consider checking the flex lines as they sometimes collapse inside with no apparent outside visual clues.....Also, 5-6 times brake pumping appears too much---one or two is max to gain pedal pressure when bleeding. Hope this helps!

03-16-2003, 12:33 PM	#1
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Brake problems I have been having problems with the brakes on my truck. Mostly, they suck. The pedal has to go down about an inch before i get any real brakeing feel, and then it gets very hard to push, and the truck barely slows down. It is a '72 C-10 with power disc brakes. I think that on a disc brake truck, the proportioning valve gives pressure to the rear brakes first, and then the front ones, so woldn't this mean the problem is in my rear brakes? The truck has a rebuilt master cylinder, new wheel cylinders on the rear, and the brake shoes on the rear are about 8 months old. I though they were out of adjustment, so i pulled off the wheels, and there is a slight drag when turning the drum on each wheel. I poped off the master cylinder top, and when the brakes are depressed and then released, fluid jumps back out of the front resivour, but fluid barely jumps at all out of the rear resivor. The brakes have also been bled and shouldn't have any air in them. No bubbles come out of the master cylinder either. I am thinking that either the rebuilt master cylinder is junk, or possibly the proportioning valve, sine it is the only thing i haven't replaced. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-16-2003, 12:49 PM	#2
Gordo Registered User Join Date: Jan 2001 Location: Killingworth, CT. USA Posts: 3,378	Could be the power booster is bad and or the vaccum supply is restricted. __________________ 1971 C10 swb stepside 350/700R4/3.73posi (retired as of 4/22/03) 1998 S10 short bed 2002 S10 Blazer 1942 Oldsmobile 1958 Massey Harris Pony 1951 Wife Killingworth, Connecticut May those who love us, love us, any of those who do not love us, may God turn their hearts. And if God is unable to turn their hearts, may he turn their ankles so we may know them by their limping. A man who works with his hands is a laborer; a man who works with his hands and his brain is a craftsman; but a man who works with his hands and his brain and his heart is an artist.

03-16-2003, 12:57 PM	#3
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	I think the booster is OK. If i turn off the motor, pump the brakes 5-6 times until they are hard, then start it up the pedal sinks about an inch...... __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-16-2003, 01:28 PM	#4
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Brakes! I would check all your lines for damage. Check to make sure you haven't ran over something that may have flatened out one of your lines. I had this happen on a old truck years ago. Also check your front flex hoses to see if they are in good shape. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

03-16-2003, 06:10 PM	#5
Xzavior240 Registered User Join Date: Oct 2002 Location: enterprise kansas Posts: 230	check your lines for pinholes as well I had a truck one time where you could stand on the brake pedal and dance and it would take it a block or so to stop if you were doing over 20. I found I had a really really small hole in my rear brake line where the line runs along the frame. since most people who use trucks for work dont spray out under it the mud and stuff from it being used just piled up on the line. and over time the moisture the mud held there rusted a hole in the line. __________________ 71 chevy C10 LWB 2WD runs and everything 79 chevy 1/2 ton Big Ten silverado gonna part it out since it left me walking. 69 f100 real good shape with lots of go go juice 89 plymouth reliant (daily driver) 86 thunderbird sitting in the weeds almost covered (anyone need parts?) WWW.xschunkofthenet.freeservers.com nothing there I promise " tell me I'm a sinner I got news for you. I spoke to god this morning and he dont like you" OZZY

03-16-2003, 06:18 PM	#6
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	if your having to pump the brakes5or6 times till they're hard i suggest bleeding the brakes again __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

03-16-2003, 07:31 PM	#7
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Well, i looked for obstructions in the lines, and found none. I doubt i have any pinhole leaks, because the master cylinder has been continusiouly full of fluid for several months, and i haven't had to add any. Would a bad proportioning valve cause this? I can still drive the truck, but i hauled a large load of firewood in it yesterday, and the brakes got kinda scary at times. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-16-2003, 07:43 PM	#8
LudwigTheMad Registered User Join Date: May 2000 Location: Ramona, CA Posts: 2,033	Tell us, how did you bleed your brakes....had similar probs when I fist started doing my own brakes, till I found out that although I wasn't doing it wrong, there was a better (more efficient) way. Pumping 5 or 6 times till it gets hard is too much, a good set of brakes should be getting difficult during the second pump and near imposible by 3... __________________ That's right, I'm back in the game again. No it's not pink..... 1972 Blue and White Suburban.

03-18-2003, 02:49 AM	#9
ZOOMY Senior Member Join Date: Feb 2003 Location: WIS Posts: 1,752	efficiency What's the more efficient way ?????.............Zoomy

03-18-2003, 09:26 AM	#10
7T1BURB Registered User Join Date: Aug 2002 Location: Newport News, VA Posts: 190	Just finished a complete brake system rebuild (booster, MC, prop valve, ss lines and ss flex hoses, cylinders and calipers) on our 71 BURB and initially had difficulty getting all the fluid to the rear wheel cylinders and front calipers. Discovered that the proportioning valve pin (located in the front of the new prop. valve) must be held "IN" when bleeding (from the 1971 Service Manuall, " NOTE: When bleeding the brakes; the pin in the end of the metering portion of the combination valve must be held in the open position (not allowed to close). This can be accomplished by installing tool J-23709 under the mounting bolt and depressing the pin a slight amount. Be sure to retorque the mounting bolt after removing tool J-23709). I fabbed one (shaped like a "L") and then had success in bleeding the system.....if you are using rubber flex lines you might also consider checking the flex lines as they sometimes collapse inside with no apparent outside visual clues.....Also, 5-6 times brake pumping appears too much---one or two is max to gain pedal pressure when bleeding. Hope this helps!



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