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the wonderful brake issues.....

Dirtautoguy

By Dirtautoguy
in Ask A GM Technician

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Dirtautoguy Enthusiast

Dirtautoguy

Posted
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I have a 1996 chevy k1500 with the 350 vortec. My problem is the brakes. it has a new master cylinder, drums, hardware, shoes, wheel cylinders, rotors, pads, calipers, and some new lines. no matter how much I bleed it I cannot get a hard pedal. I understand these trucks dont have very good brakes to start with but I have driven several other GMT 400 trucks and the brakes were tons better than mine. I have checked that the drums were adjusted out as well.

 

I read this forum and tried it

 

http://advrider.com/index.php?threads/97-gmc-1500-with-spongy-as-hell-brakes-fixed.919964/

 

and I did get some pretty nasty fluid out as well as some air but it didnt seem to help the pedal much.

 

I took it to a shop with a tech 2 scanner to see if maybe I had air in the ABS and they said that their scanner would start the test however something wouldnt alow it to do the whole test and they didnt know why. they said they tried it 3 times and it would not do it, however the pedal does feel better than it did.

 

any ideas?? I have been reading alot but not finding many answers. I did find two more things with rubber caps on them under the proportioning valve anyone know what they are?? could it be my proportioning valve?? they fronts seem to grab pretty good and the drum brakes dont seem to be grabbing like they should. with the truck off the brakes are rock solid and the pedal wont bleed down. but soon as I start the truck the pedal goes way down. I have about 3-4 inches before the brakes start to grab and about 5-6 inches untill they are grabbing better. If I slam on the brakes it stops fairly well but still not like they should. I feel like I am about at worlds end with this does anyone have an idea??

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

To rule out any mechanical issues with the brakes, block off the flex lines with vice grips. Do one flex hose at a time. Block the hose off, start the engine, see if the brake pedal is any better. If you find no difference, move the vice grips to another flex hose and try again. This test will rule out any issues from that brake system beyond the flex line.

 

The link you posted contains a fair bit of what I call "guess logic". Not one of the posters in that thread bothered to mention that brake fluid is hygroscopic. That means it will absorb water or moisture. Once the fluid becomes contaminated with water, the boiling point will drop dramatically. Get the brakes hot, and the water will boil out of the fluid, leaving air in the system. Brake fluid with no water in it will be virtually clear, with a slight bluish tinge to it. As it starts to become contaminated, it will darken up in the brown range of colour. Once it changes to the black colour, all bets are off in predicting brake action. If the shop you took the truck to is unable to figure out why the Tech 2 is not helping, it is time to change shops. Diagnostic tools are only as good as the technician using them. They are not a problem solving tool, they are simply a diagnostic tool. It is the technician that is the problem solver, not the scan tool.

 

You may want to try testing the master cylinder as well. To do this, start engine, and put a light pressure on the pedal. Do not apply hard pressure. Under light pressure if the pedal slowly begins to drop away, and continues to drop until it reaches the floor, the master is likely starting to fail. Applying high pressure to brake pedal will likely stop the pedal from dropping to floor. This is due to the design of the seals (cups) on the master cylinder pistons. Higher pressure will force the seals to press tighter to the bore, light pressure will allow the fluid to leak past the seals. The fluid will not likely leak out of the master cylinder unless the last (or first depending on your logic) seal is worn. If you loosen the master cylinder you may see fluid between master and brake booster. If there is fluid in there, it is time to replace master.

sdeeter19555 Senior Enthusiast

sdeeter19555

Posted
Posted

If there is nothing obvious, replace all the rubber lines...the two for the front calipers, the drop from the frame to the rear axle, and any other rubber lines in the brake system. ..they swell and swell shut with no visible indications. This limits the fluid that can pass through and will cause poor braking.

 

I've had the rear line swell to the point that the rear drum brakes did nothing.

 

Sent from my SAMSUNG-SM-G900A using Tapatalk

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

You can test the flex hoses by applying vice grips near the end of the hoses. Clamp off near the frame and test, then clamp off near the other end and compare results. If they are the same, then the hose is very likely still good. I have not seen a flex hose cause a low pedal in an automobile. I have seen it happen on a motorcycle though, Then again, a motorcycle generally has nothing but flex hoses between the master and they calipers.

Dirtautoguy Enthusiast

Dirtautoguy

Posted
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ok. this is what I have done. we bled all 4 brakes again for good measure. we checked all the hard ware and made sure everything was tight and leak free.

 

it didnt change anything. I clamped the rubber lines one at a time and think there maybe something to that. I have replced the passenger side rubber line but not the driver or the rear rubber ones. with the line clamped off my wife said the pedal didnt feel any different. however I could feel the line swell. with my vice grips just touching the outer hose the driver side deffinatley swelled more than the newer passenger side. and the rear was about the same as the driver side so there is something to be said there that I would probably bennefit from replacing the lines.

 

also is it normal for the line to move when the brake pedal is pressed?? once again the rear and driver side moved more than the newer passenger side. just for kicks I made sure I wasnt losing vaccuum or anything at the booster. and the check valve is working as well.

 

the last thing I found is that if you press down on the master cylinder a little you can hear air come out is that normal?? also I am not sure if my friend spilled a little fluid when topping the mc off but where the reservoir meets the master cylinder was wet like they may have been leaking???? I really hope freakin not its new!! not rebuilt!!

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

ok. this is what I have done. we bled all 4 brakes again for good measure. we checked all the hard ware and made sure everything was tight and leak free.

 

it didnt change anything. I clamped the rubber lines one at a time and think there maybe something to that. I have replced the passenger side rubber line but not the driver or the rear rubber ones. with the line clamped off my wife said the pedal didnt feel any different. however I could feel the line swell. with my vice grips just touching the outer hose the driver side deffinatley swelled more than the newer passenger side. and the rear was about the same as the driver side so there is something to be said there that I would probably bennefit from replacing the lines.

 

also is it normal for the line to move when the brake pedal is pressed?? once again the rear and driver side moved more than the newer passenger side. just for kicks I made sure I wasnt losing vaccuum or anything at the booster. and the check valve is working as well.

 

the last thing I found is that if you press down on the master cylinder a little you can hear air come out is that normal?? also I am not sure if my friend spilled a little fluid when topping the mc off but where the reservoir meets the master cylinder was wet like they may have been leaking???? I really hope freakin not its new!! not rebuilt!!

 

When the line swells up it will attempt to shorten up as well. This is due to the braided sheath under the rubber coating. If the sheathing expands, it has to shorten up at the same time. Take a look at your shoe laces. If you take the lace and push it towards itself, you will see it expand in diameter at the same time. Or if you pull the lace, it gets thinner. Same action on the flex lines.

 

Brake fluid is water soluble, so, flush off the master cylinder and booster face to remove any errant brake fluid, then dry it off as best you can, compressed air would be handy for this, make sure you have removed all the brake fluid from the surface before using compressed air. You could also remove the bolts holding the master cylinder onto the booster and take a rag to clean up any fluid in there as well. Drive the truck for a bit and see if the fluid comes back.

 

The air sound you are hearing could be normal. Is the engine running when you hear this? Does the noise only happen as the pedal moves, and stops when the pedal stops moving?

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