How to troubleshoot a soft brakes? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-06-2007, 02:00 PM   #1
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Default How to troubleshoot a soft brakes?

Well I was doing an autocross last weekend, and something didn't feel right.
I had changed my pads on Friday to my race pads, but after Saturday they didn't feel right. The pedal was really mushy and went down about 3/4 before I'd get any feel. Needless to say Sunday'* runs were not good. I went to take the pads off, to discover my calipers were stuck on. So I purchased new calipers and installed them. Now the race pads (after two days of autocross) were woren uneven. I installed them back on, and did a few "test" stops. The brake felt better, but not 100%. We bled all the lines and no major improvement.

When the car is OFF, and I press then pedal 2-3 times it becomes REALLY firm.
When the car ON (no engine), the pedal is really soft. When it goes "grab" is holds, it doesn't "fall" to the floor. Do you guys think it'* a bad BMS?
Or is it something else? Is there a test I can do?

Some people have suggested using duct tape and vice grips to carefully pinch off the brake lines to all four wheels and then to see if the pedal remains firm when the car is ON.

I'd like to get my pedal back so that it'* nice and stiff, with very little travel.
Any suggestions?

Thanks
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Old 07-06-2007, 03:25 PM   #2
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Have you ever replaced any of your brake lines? Especially the ones to the front?
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Old 07-06-2007, 03:43 PM   #3
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Default Re: How to troubleshoot a soft brakes?

Quote:
Originally Posted by Merlin 91/97

Some people have suggested using duct tape and vice grips to carefully pinch off the brake lines to all four wheels and then to see if the pedal remains firm when the car is ON.
Yipes! There is a bad idea. If the flexible hoses are not already delaminated internally (possible from the symptoms you describe. i.e., stuck calipers) they surely will be after the vise-grip treatment.

The uneven wear and sticking suggest a problem with either the flexible hoses, the caliper pins, or the caliper piston. Soft pedal suggests air in the lines. Remedy for air is bleeding.

Bad master cylinder usually presents as pedal that bleeds down toward floor under constant pressure or a pedal that rises as it is pumped. Usually these symptoms occur with no external leaks.

Adjusting rear brakes will bring pedal up higher as less fluid is needed to move shoes out to contact the drums.

If you routinely change from race to street pads and do so without opening the caliper bleeder, you have likely forced the dirty fluid that accumulates in the caliper back through the seals in the master cylinder each time. This often results in a leaking master cylinder. If you do open the caliper bleeder and are not careful how you do it, you may introduce air into the front brake lines requiring bleeding to restore a firm pedal.
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Old 07-06-2007, 05:22 PM   #4
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Quote:
Originally Posted by BillBoost37
Have you ever replaced any of your brake lines? Especially the ones to the front?
Not recently.... but I was thinking about stainless steel lines.
Thoughts?
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Old 07-10-2007, 09:44 AM   #5
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I've been pricing replacing my brake master cylinder. While the pedal doesn't go to the floor when held, if I pump it once or twice it does stiffen a bit.

GM wants $350 for a new one.
Local repair shop wants $240.
Where I normally take my car wants $110 (for a refurb)
Canadian Tire wants $120 but will give me $30 for my core, this is most likely a refurb too.
A refurb one should be ok... right?


All places are saying anywhere from $100-$200 in labour.

How hard is it to replace the bmc.... is it something that can be done in a driveway with a few cold beer? Or is the risk of screwing it up too much.
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Old 07-10-2007, 09:46 AM   #6
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the master cyl takes like 15 minutes to change... ok well maybe 30 minutes if that includes a beer b4 you start...
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Old 07-10-2007, 09:57 AM   #7
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Try bleeding your brakes front and rear first. If that doesn't solve the problem, then you probably need a new master cylinder. I have used remanufactured master cylinders without problem, especially when there is a significant difference in the price between reman and new. Instructions will come with the master cylinder on how to bench bleed before installation. After installation, you will need to bleed the brakes. Many omit the bleeding step, thinking that they have not introduced any (or much) air into the brake lines. But the fact is whenever the brake lines are opened, the brakes should be bled to eliminate the possibility of a spongy pedal.

Here are some bleeding instructions:
Quote:
Originally Posted by bill buttermore
OK, The idea is to remove any air bubbles in the brake lines from the master cylinder to each wheel cylinder (rear wheels) or caliper (front wheels).

These directions presume you have a good master cylinder or a new one that has been bench-bled.

You will probably need to remove the rear wheels to gain access to the wheel cylinder bleeders. You may be able to get at the front bleeders on the calipers by turning the wheels all the way left or right. I find bleeding is a lot easier with all the road wheels removed and the car up on stands. Never get under a car supported only by a jack!

Start by filling the master cylinder reservoir and replacing the cap. Don't let brake fluid be exposed to the air - it will absorb moisture from the atmosphere which can cause troubles in the brake lines. What you want is to have a helper pump the pedal a couple of times (press down - release; press down - release) then press down and hold. While he is maintaining pressure on the pedal, you open the bleeder valve on the wheel cylinder or caliper and relieve the pressure by releasing fluid from the bleeder valve. The brake pedal will suddenly go down, at first nearly all the way to the floor. The helper must not allow the pedal to come up while the bleeder valve is open or air will be sucked in through the bleeder valve. At the wheel, when the fluid flow stops, close the bleeder screw and instruct your helper to release the pedal, then "pump and hold" again. After a few times, depending on how much air is in the lines and where the air is located, you will notice as you open the bleeder valve, that the fluid flow is interrupted by escaping air. It kind of makes a "spitting" sound. That is good - that'* the air you are trying to remove. Keep repeating the procedure until the stream of fluid is solid - no air - and clean in appearance.

While you are bleeding each wheel, after about five or six squirts, check the level of fluid in the master cylinder reservoir. If you let the reservoir go empty, you will introduce air into the lines and you will have to bleed the master cylinder and all the brakes again.

I like to place a box wrench on the bleeder screw nut in a position that allows me to both open and close the valve (hopefully) without repositioning the wrench. I then attach a length of flexible vinyl tubing (maybe 3/16" inside diameter) that fits tightly over the end of the nipple on the end of the bleeder. I run the hose into a plastic pan to catch the fluid that comes out. Be careful, brake fluid removes paint. Also, don't use more than a couple of foot-pounds of torque to close these little bleeder screws - they are not solid and they can break off very easily.

There are a couple of inexpensive one-man methods. You can use a vacuum pump to suck out the air from each line, but the problem with that is that you have to open the bleeder screw to open the line and when you do, you usually end up sucking some air through the threads on the bleeder screw making it difficult to know when you have all the air out. An older method is to take the hose described above and put it into a (clean and dry) coke bottle with the end of the hose submerged under a couple of inches of brake fluid. This is not a bad idea, even with a helper, cause if your helper screws up and releases the pedal when the valve is still open, it will suck mostly fluid in from the bottle instead of air. The problem with this method is the same as with the vacuum pump. That is, on the release stroke, if the valve is open, air can be sucked into the system through the threads on the bleeder screw.

Be careful if your bleeder screws are corroded. They will break off in the cylinder or the caliper with surprisingly little force applied, and then you have a real piece of work on your hands. A good plan is to squirt around the bleeder screw (not into it) with a good penetrating oil like power blaster, kroil, or sea foam deep creep (WD-40 is not very good for this). Remove the cap from the screw and give it a couple of taps square on the end of the screw with a small ball-pein hammer, and squirt again to encourage the oil to penetrate the threads. Best if you can let it sit overnight, but give the oil at least 30 minutes to work before you try to open the bleeders.

If your bleeders don't have the little rubber caps on them, make sure the passage is clear to let the fluid out. Sometimes you have to clean them out with a small drill bit.

The traditional method is to start at the wheel farthest from the master cylinder, usually the right rear, and work to the shortest run, i.e., the left front. As you work your way around, the pedal should become firmer and progressively higher from the floor. When you are done, if you have removed all the air, the pedal should be firm, there should be no "sponginess," or softness when depressed.

Good luck!
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Old 07-10-2007, 01:33 PM   #8
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Thanks guys, as soon as I find a free evening to do this I will.
I'll let you guys know how I make out.
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Old 07-13-2007, 09:10 AM   #9
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My new brake master cylinder and new Brembo rotors arrived yesterday.
I'm going to try to get them both installed tonight so I can take the car out to the track tomorrow.
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Old 07-16-2007, 05:56 PM   #10
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Ok, we replaced the brake master cylinder on Friday night.
Installed new Brembo rotors and Satisfied GS3 Race pads....

Saturday was track day..... WOW!... the car has never stopped that well in it'* life!
I highly recommend the GS3 pads $160 CDN and Brembo rotors $70 CDN. Some performance DOT3 brake fluid and I was ready to stop!

However, after my lapping day I checked my brake fluid level and I noticed it was down a far bit. Not low enough to trip the light, but close. I topped it up, and took the car for a quick drive. I checked the brake master cylinder and all four wheels, no leaks. Is this normal?

EDIT: Nevermind, found that one of the lines to the right rear drum was leaking. It'* being fixed as we speak.
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