Aflac

I'm havin a problem with my brakes. The light is on, the fluid level is good but the pedal hits the floor just as i get stopping power. I take the vacuum line off the booster and it works fine( brakes are tight) I'm thinking that I have a booster headding south, Forum gurus, i'm throwing my self at your mercy here... what do you think the problem is??

Greg

I think you have a pressure (fluid) problem Loren. Fast check for the booster, with key off pump the pedal 3 or 4 times to lose all vacuum from the booster, then with your foot still pressed on the booster start the car, if you feel the pedal drop slightly on start-up your booster is probably fine. A bad booster won't trip your brake light. Even if the fluid is full in the MC you could have uneven pressure at the calipers. Try power bleeding the system before you do anything else. Make sure you get all SIX bleeders. Has the car been sitting idle for a long period?

timbo79

Boosters rarely fail. Did you bleed in the correct order? I replaced mc, all calipers and put new lines in. I had the same problem. Got the car raised, used a spray bottle with water to clean everyhting and found a tiny leak in the front caliper pison seal that only "leaked under full pressure. Problem was a faulty caliper. Returned the caliper and got a new one and NO problem.

Also check your bleed. If you are using your old lines or old calipers, a full bleed could also dislodge old gunk and cause calipers to leak. Check arounf=d the inside of the rims (after cleaning with water) for any "drops" This could be indicitave of a leak. You can also check the threads in the archives as almost everyone has the same problem at some time or another

zwede

I agree. Most likely air in the lines. If not air probably a bad MC.

That said, I once had a booster that did the weirdest thing. I hit the brakes and they would seem to work OK, although not great (mushy). Once the car stopped I would keep my foot on the brake and the pedal would slowly sink until it hit the floor. No warning light.

Sounds like air in the lines or a bad MC, right? Well after messing with it for weeks with no luck I replaced the rebuilt booster with a brand new GM unit and the brakes have worked fine ever since! Note that when I replaced the booster I didn't open the lines so no bleeding. Only thing that changed was the booster.

And lastly a tip: I highly recommend a pressure bleeder for Vettes. Our brakes are really hard to bleed and a pressure bleeder makes it so much easier. They cost about $60 online and make bleeding trivial.

Aflac

the car did sit for a while, and last labor day when i drove it, the brakes were kinda spongy. i didnt really see any significant loss of fluid for that matter, either. i did disconnect the vacuum line from the booster and started it up. the pedal was back to hard again i know that is the equivalent of making it manual brakes but the light didnt go out either. i am definitely gonna look at bleeding before i change the booster...

Rockn-Roll

I would suspect the master cylinder. I'd say your booster is fine from the description of the symptoms. The pedal WILL 'feel' firm if the booster is not connected...but the sad news is that the pedal won't be applying much force to push fluid through the lines to get stopping power. If you drive around 25mph and slam on the brakes do all wheels lock up, or just the rear? The m/c is designed to apply all the fluid pressure to the rear system in case of an m/c inner seal failure. If the rear is the only ones that lock up then it's an m/c failure.

I recommend removing the master cylinder and bench testing it. Pour some brake fluid into it and push the piston in...if you get a good stream of fluid out both outlets then try to partially cover them one at a time and make sure there's good pressure. I suspect that the front outlet will just have little or no pressure. If you can completely stop the fluid from coming out of the front outlet by just holding a finger over the hole then the m/c is shot.

Be sure to bench bleed the new one to make sure it's working OK, then clean it up and paint it to prevent rust.

Here's my brake help URL:

http://67.116.80.82/Vette/BrakeHelp/BrakeHelp.html



[Modified by Rockn-Roll, 1:53 PM 5/11/2003]

79corvette

The instuctions you have posted on your website state to start bleeding on the RRI, my service manual says the LRI. Which is correct? And I'm having the exact same problem with my brakes, which were working fine a few days ago, I hate them.

Aflac

i did the 25 mph test. When i stomp on the pedal, it does apply all 4 brakes and it's a firm, smooth stop. I'm gonna try to bleed the brakes this week and see if it helps. Seeing i havent found anything in my manual and the other person in this thread brought up the sequence of bleeding, what is the order to bleed? And by the way, you guys are an awesome wealth of information! Thanks a lot!

Greg

Sorry but this information is SO INCORRECT as to be completely absurd. The master cylinder will apply pressure to both front and rear systems without prejudice. The only way it would apply all pressure to the rears only is if the integrity of the front system is compromised or vice versa. If only the rears lock up the MC is unlikely the cause, but more likely a fault in the front calipers.
The reason I asked if the car had been sitting for an extended period is that the calipers are a VERY COMMON failure after a long period of no use. I've probably replaced 200 Corvette calipers in 27 years(veteran auto mechanic) as opposed to maybe 20 master cylinders.
Try bleeding the system but don't be surprized if the problem reappears soon. You can replace all 4 calipers from a vendor like Muskegon Brake for about $240.
Don't want to dis anyone else's input but I'm an ASE Master Certified Tech and hate to see someone get the wrong info.

Rockn-Roll

The basic rules for hydraulic line bleeding is to start with the bleeder furthest from the master cylinder. In our cases it will be the RR; however, the caliper itself is not a line, but a slave cylinder, and all slave cylinders should be bleed from the closest bleeder first if there is more than one...or bled at the same time if possible...if not then they should be bled twice...inner then outer then inner again and outer again...because some air bubbles may leak between them in between bleeding.

The M/C is comprised of a single shaft with three seals...one at the end which draws fluid from the front reservoir and pushes fluid out through the front outlet, one in the middle (between the front and rear reservoirs) and one at the other end for the rear system. This creates two syringes which draw fluid from their respective reservoirs and pushes it out their respective outlets.

All of the 5 m/c's that I have replaced had the middle seal damaged in such a way that which effectively turned it into a single syringe with a baffle. Because of this...and because the proportioning valve uses the pressure from the two systems to balance the pressure between the front and back lines there will be an imbalance of pressure which results in the proportioning valve triggering the brake warning lamp and directing all of the pressure to the rear system. The front of the car receives more weight on it under breaking, so the rear wheels will lock up instead of normal braking as when the proportioning valve is working properly and can equalize the pressure. This is by design so if there is a problem with the braking system the front wheels won't lock up and prevent the car from being steerable.

Haven't you ever wondered why the front and rear systems have seperate reservoirs? Have you ever wondered why the rear system reservoir is toward the back of the master cylinder while the front system reservoir is toward the front of the master cylinder? Now you know. If it was the reverse then the front wheels would be locking up if the m/c is starting to fail.




[Modified by Rockn-Roll, 12:29 PM 5/12/2003]

Greg

Sorry dude, don't want to get into a pissing match here but, you're just wrong. On many cars the rear resevoir feeds the front brakes and the front resevoir feeds the rear brakes. Now YOU know. If your brakes are functioning properly and you lock them up, the fronts will skid first and most, as they have the majority of stopping power. Why would any designer want to reverse this braking procedure in the event of a failure? If you don't think this is correct, try this: run your car down the road about 40 miles an hour and pull the E-brake on, you will experience some VERY out of control stopping/spinning. You make no sense, I'm not guessing, I didn't just read a book on this yesterday. You should be careful not to steer people in the wrong direction on sytems you clearly aren't as familiar with as you'd like to be. Someone just might follow your ill-informed advice. I wouldn't disagree with you when it comes to computer programming. It's your field, I'd assume you were some kind of authority.

dath

The BRAKE light is supposed to come on if you have an imbalance in pressure between the front and rear systems when you apply the brakes. What you are describing sounds quite a lot like you have air introduced into either the front or rear system, or both. This can be caused for a variety of reasons, but the calipers seem to be the #1 suspect when this happens (from my limited experience). I'd look for any sign of a leak around the pistons on all of your calipers. It is possible to get air into the system even if liquid isn't making it out, so don't rule out a bad caliper if you cannot find any sign of a leak. If your rotors have runout (are out of true), this can cause a pulsating of the pistons and somehow introduce air into your system. This can be at least partially eliminated by removing the springs from behind the pistons of your calipers according to several folks on the forum.

I'd start with the calipers and check your runout. If you think you need to replace one, my tendency is to go ahead and do all four. Vette Brakes and Zip both sell kits for the brake systems for fairly reasonable prices. Go with the SS Sleaved calipers.

Rockn-Roll

Hey, If you don't agree then check the Bureau of Automotive Repair (BAR) in your State. They have a Brake Inspection and Repair (BIR) manual that you can buy, or visit your local library and ask for it. I'm assuming that the book you read was not the BAR BIR...if it is, then please quote me some of that information cause I don't have mine with me, and not able to quote from it. I will admit it has been 15 years since I've looked at it, so I'm not going to bet my life that EVERYTHING I said should be taken for gospel. But, I do stake my life on my brakes and I base everything I know on the BAR BIR and my 3 years experience as a professional fleet mechanic.

Do what you want with your own vehicle...for now...at some point in time the people that consult official brake books will be voting on legeslation to require any mechanic working on brake systems to pass the BAR test. I would not want this to happen, but the more I hear about "gravity bleeding" and people like yourself that thinks the front brakes locking up before the back because they "have more stopping power" is how it should be then the more I lean toward voting yes on such a proposition.

By the way, if your front wheels are locking up on you before your rear then you may have a stuck/clogged proportioning valve, or possibly clogs in your rear lines. Locking front brakes are a hazard as it prevents you from steering under hard breaking conditions. If the front brakes locked up first then those of us who race would be flying off the solo courses on each turn instead of gently sliding the back end around to side track around the apex. Setting up brakes takes some skill as well as good equipment...I've been in several panic stop situations where having my front wheels available for steering saved my skin, as well as stroking my heart rate as I slid around turn number 20 toward the finish line.

Greg

3 years experience vs. 27 years experience........ you decide.

peace dude :cheers:

Rockn-Roll

Sorry Aflac, Greg (and others) are welcome to disagree, but when they just outright say I'm wrong about something I feel the need to explain myself before you or others become confused. Ideally, I should have a copy of the BAR BIRM myself and will have to get it back from my friend that I loaned it to. Perhaps I am wrong on some points, but only consulting the BIRM will convince me that I am. But let's get back to your problem.

Sounds like the master cylinder is working properly, as well as the proportioning valve, booster, all 4 wheel cylinders, etc...or you would encounter a pull to the left or right if only one of the calipers was leaking under pressure. It's possible that you could be getting air back into the lines...so that after awhile the brakes will become spongy. Bleed the brakes the way I indicated and see if any of the lines have air in them. If so, then check the line connections...loosen then tighten them again, then bleed the system completely. If you get air in the lines again then you could try some permatex part B pliable non-hardening to make sure they seal good; but, after that you will need to pull the calipers...and replace them. You could also try to replace the flexible lines and see if that's where your problem is...they will wear after awhile.

The furthest caliper from the master cylinder is first...then work your way toward the m/c. RightRearInner, RROuter, LeftRI, LRO, RFront, LF

Yes, and oddly enough I even appreciate Greg for tweaking my butt into getting my BAR BIR back, cause I really should consult it again before regurgitating my brain soup.

Greg

I feel all warm and cuddly again :grouphug:

Cheers, Steve :cheers:

zwede

Aaaaww... It was just getting good. :blueangel:

Chris O'

Bad booster = hard pedal

Easy check for MC - pump the pedal then clamp all four rubber lines. If it is hard then it is not the MC. If it is soft you eliminate the calipers as a problem.

If the pedal is hard, then get someone to release each line while holding the pedal down. Remember the pedal travel - foot off the pedal and then reclamp the line.

Do this for all 4 lines and you will usually get 1 line that travels further. If they are all the same then rebleed the brakes.

:cheers:

Rockn-Roll

Don't worry...it's that time of season for over heating problems...might be time for that tried and true discussion about running without a thermostat! It always gets people @#$%^&*! :reddevil

cheers Greg :cheers:

dath

I've read about doing this before myself, I'm just wondering (not to try to usurp the thread or anything) is this not an abusive thing to do to the poor rubber hoses? I always thought it seemed a bit rough on the hoses myself since they aren't really designed to be squashed like that... Wouldn't it fatigue the clamped area?