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

HotRod929 · 09-11-2002, 07:20 PM

So when I hit my brakes slowly sometimes my pedal will bottom out and the break warning light will go on. I read that this is because of a pressure loss somewhere. I have looked all over my truck for a leaky wheel cylinder or or anything. Checked the fluid level and that was also good. I have no idea what to do now. Do I need a new Proportioning Valve, master cylinder, or break booster? Please help me guys. THanks

HotRod

mikeoj · 09-11-2002, 08:31 PM

The light comes on because their is air in the system(being thru a break in the system or trapped).Bleed your brakes,starting from the farthest point away(Drivers rear,passengers rear,right front and then left front).
Before you do this,bleed the master cylinder.
Hope this helps.Mike

*Zkast* · 09-11-2002, 09:03 PM

When the pressure at the proportioning valve is not equal, it will activate the switch (obviously its more technical but ill leave that in laymans terms). If you have no history of problems you possibly have 2 things going on.

Master Cylinder: When the seals inside the master cylinder bleed off pressure(leaking internaly) you will notice a dropping pedal when the pedal is applied slowly. If you hit the pedal hard, the pedal stays high and tight. If the MC is bad enough it will still drop even when hit hard. When there is pressure bleed off, one of the ports is supplying full pressure while the other is not, this moves the Proportioning valve to the weaker side which causes the switch to turn the light on.

You may also have a external leak coming the rear seal of the mastercylinder which could cause a fading pedal.

SYstem Leak: If you have a leak in the system, your pedal will drop slowly anytime the brakes are applied. When this happens you usually wont be able to hit the pedal hard and obtain a good pedal. Same result, low pressure on one side of the proportioning valve, moving the valve to one side causing the light to come on.

Double check your fluid reservoir, make sure you have a good level. Then check all lines, connections, calipers, wheel cylinders and the rear of the mastercylinder for leaks. If you have no leaks, I recommend replacing the Master Cylinder as that is the most probable cause.

HotRod929 · 09-11-2002, 09:35 PM

From what you said Zkast about the master cylinder that is EXACTLY what is happening to me. I think that I am going to get a new master cylinder. What years are interchangeable? Can I use anyone? Thanks

HotRod

*Zkast* · 09-11-2002, 10:34 PM

Be sure to go over the entire brake system and check for leaks... never hurts to error on the side of caution. I'm not too sure about interchangability but I cant imagine a rebuild master cylinder costing more than 20.00 - 30.00.

palallin · 09-11-2002, 11:10 PM

Run, do not walk--and if you drive, drive something ELSE--to get that MC.

Why the urgency? See my "Requiem for a '67 SWB" thread. . . .

cableguy0 · 09-11-2002, 11:23 PM

theres one place it could be leaking that u cant see and that is into the booster if u have powerbrakes.and once brake fluid gets into a brake booster it will be trash.no question about it.unbold the mc and see if there is flud behind it. if there is i would recommend just getting a booster and mc combo and bolting it on.

Low68 · 09-12-2002, 12:12 AM

Warning lights? Who wants those. My front left hose split in half as I was coming up to a light. It went to the floor and I was like... What the HECK? I pumped them and stopped it and pulled into the parking lot. The light never once turned on. Stupid piece of......nevermind. Low68

barn9 · 09-12-2002, 12:37 AM

New master cylinder costs under $20.00, a small price to pay to save your truck and possibly your life.

HotRod929 · 09-12-2002, 12:43 AM

So you dont think that I should drive my tuck till I get it fixed? I dont have another vehicle. It still stops fine most of the time and all I have to do is pump it like once before I get to a light and it works fine.

HotRod

*Zkast* · 09-12-2002, 02:09 AM

Dont trust it.

JJR · 09-12-2002, 07:25 AM

Mine was doing the same as yours. It would stop fine but as I would be at a light the pedal would slowly go down until I pumped it a few times. Until the day I started to go to work and the pedal went to the floor and you could barely pump it up enough to stop. So like the others have said don't risk it and get a new master cylinder. Rebuilt go for under $15 and new for about $50. If the truck is a keeper, I would spend the $50 and get a new one.

HotRod929 · 09-12-2002, 09:06 AM

Thanks guys. I think I am going to get a new MC. Any info on how to put a new MC in? Thanks

HotRod

RodC · 09-12-2002, 09:38 AM

Not that hard, just bench bleed it. I bought one from Advance Auto $12.00 and it had all the plugs needed to bench bleed it. And some good instructions if you have not bench bled one before. Then I bled the whole system to change out the fluid.

Just have patients, if you have never bled one of these big MCs it will seem like you will never get all the air out of it. But a good bench bleed will keep the MC from indroducing more air into your system.

Keep everything clean! Dirt and water are brakes/fluids biggest enemy. Don't get fluid on your paint. I always cut out a section of cardboard that will fit around the lines going up to the MC and stuff plenty of rags around.

I like to keep my MC looking stock so I don't paint it. It will come with a coating of oil on it. I maintain that coating of oil because a little bit of brake fluid + water = lots of rust. If you want to paint it, now is the time. Just be sure you get all the oil off and use a good paint. (Im not sure if any can withsand brake fluid)

Brake fluid attracts moisture that is why you must keep a good seal on the top of your MC or you will get about an inch of rust in the bottom in no time.

09-11-2002, 07:20 PM	#1
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	Brake Warning Light So when I hit my brakes slowly sometimes my pedal will bottom out and the break warning light will go on. I read that this is because of a pressure loss somewhere. I have looked all over my truck for a leaky wheel cylinder or or anything. Checked the fluid level and that was also good. I have no idea what to do now. Do I need a new Proportioning Valve, master cylinder, or break booster? Please help me guys. THanks HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

09-11-2002, 08:31 PM	#2
mikeoj Senior Member Join Date: Nov 2000 Location: WI,USA Posts: 495	The light comes on because their is air in the system(being thru a break in the system or trapped).Bleed your brakes,starting from the farthest point away(Drivers rear,passengers rear,right front and then left front). Before you do this,bleed the master cylinder. Hope this helps.Mike __________________ 69 Lwb C-10 402/700R4 70 Lwb C-10 250/3pd Wisc.

09-11-2002, 09:03 PM	#3
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	When the pressure at the proportioning valve is not equal, it will activate the switch (obviously its more technical but ill leave that in laymans terms). If you have no history of problems you possibly have 2 things going on. Master Cylinder: When the seals inside the master cylinder bleed off pressure(leaking internaly) you will notice a dropping pedal when the pedal is applied slowly. If you hit the pedal hard, the pedal stays high and tight. If the MC is bad enough it will still drop even when hit hard. When there is pressure bleed off, one of the ports is supplying full pressure while the other is not, this moves the Proportioning valve to the weaker side which causes the switch to turn the light on. You may also have a external leak coming the rear seal of the mastercylinder which could cause a fading pedal. SYstem Leak: If you have a leak in the system, your pedal will drop slowly anytime the brakes are applied. When this happens you usually wont be able to hit the pedal hard and obtain a good pedal. Same result, low pressure on one side of the proportioning valve, moving the valve to one side causing the light to come on. Double check your fluid reservoir, make sure you have a good level. Then check all lines, connections, calipers, wheel cylinders and the rear of the mastercylinder for leaks. If you have no leaks, I recommend replacing the Master Cylinder as that is the most probable cause. Last edited by Zkast; 09-11-2002 at 09:06 PM.

09-11-2002, 09:35 PM	#4
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	From what you said Zkast about the master cylinder that is EXACTLY what is happening to me. I think that I am going to get a new master cylinder. What years are interchangeable? Can I use anyone? Thanks HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

09-11-2002, 11:10 PM	#6
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,239	Run, do not walk--and if you drive, drive something ELSE--to get that MC. Why the urgency? See my "Requiem for a '67 SWB" thread. . . . __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

09-11-2002, 10:34 PM	#5
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	Be sure to go over the entire brake system and check for leaks... never hurts to error on the side of caution. I'm not too sure about interchangability but I cant imagine a rebuild master cylinder costing more than 20.00 - 30.00.

09-11-2002, 11:23 PM	#7
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	theres one place it could be leaking that u cant see and that is into the booster if u have powerbrakes.and once brake fluid gets into a brake booster it will be trash.no question about it.unbold the mc and see if there is flud behind it. if there is i would recommend just getting a booster and mc combo and bolting it on. __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

09-12-2002, 12:12 AM	#8
Low68 Registered User Join Date: May 2001 Location: Arizona Posts: 758	Warning lights? Who wants those. My front left hose split in half as I was coming up to a light. It went to the floor and I was like... What the HECK? I pumped them and stopped it and pulled into the parking lot. The light never once turned on. Stupid piece of......nevermind. Low68 __________________ Build Thread

09-12-2002, 12:37 AM	#9
barn9 Half a bubble off! Join Date: Jun 2000 Location: Winfield, Ks, USA Posts: 5,588	New master cylinder costs under $20.00, a small price to pay to save your truck and possibly your life. __________________ Just call me LB. '71 Cheyenne, 402BB, hauls blondes, brunettes, or redheads.

09-12-2002, 12:43 AM	#10
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	So you dont think that I should drive my tuck till I get it fixed? I dont have another vehicle. It still stops fine most of the time and all I have to do is pump it like once before I get to a light and it works fine. HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

09-12-2002, 02:09 AM	#11
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	Dont trust it.

09-12-2002, 07:25 AM	#12
JJR Registered User Join Date: Apr 2000 Location: Leesburg, GA Posts: 746	Mine was doing the same as yours. It would stop fine but as I would be at a light the pedal would slowly go down until I pumped it a few times. Until the day I started to go to work and the pedal went to the floor and you could barely pump it up enough to stop. So like the others have said don't risk it and get a new master cylinder. Rebuilt go for under $15 and new for about $50. If the truck is a keeper, I would spend the $50 and get a new one. __________________ FOUNDING MEMBER OF THE DRUM BRAKE CLUB Second Owner 70 C-10 Second Owner 93 GMC C1500 Third Owner 99 C2500 Crew Cab Short Bed

09-12-2002, 09:06 AM	#13
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	Thanks guys. I think I am going to get a new MC. Any info on how to put a new MC in? Thanks HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

09-12-2002, 09:38 AM	#14
RodC Registered User Join Date: Nov 2001 Location: Fairmont WV Posts: 304	Not that hard, just bench bleed it. I bought one from Advance Auto $12.00 and it had all the plugs needed to bench bleed it. And some good instructions if you have not bench bled one before. Then I bled the whole system to change out the fluid. Just have patients, if you have never bled one of these big MCs it will seem like you will never get all the air out of it. But a good bench bleed will keep the MC from indroducing more air into your system. Keep everything clean! Dirt and water are brakes/fluids biggest enemy. Don't get fluid on your paint. I always cut out a section of cardboard that will fit around the lines going up to the MC and stuff plenty of rags around. I like to keep my MC looking stock so I don't paint it. It will come with a coating of oil on it. I maintain that coating of oil because a little bit of brake fluid + water = lots of rust. If you want to paint it, now is the time. Just be sure you get all the oil off and use a good paint. (Im not sure if any can withsand brake fluid) Brake fluid attracts moisture that is why you must keep a good seal on the top of your MC or you will get about an inch of rust in the bottom in no time. __________________ Rod Childers '71 C10 Stepside



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