79 Vette Brake Problems; spongy pedal

[lobes290]

My brake pedal is extremly spongy, but when i bleed the brakes i firms up for about 70 or so miles then it gets soft again, i have replaced the master cylinder and all pads i also have blead them about 5 times. Does anyone know what the problem is , please help thanks. Email= Lobes290@hotmail.com

thanks alot

 

[1979toy]

Turn the rotors. The rotors get slightly out of round and bump the pads back after a while causing a spongy feel to the brakes since the pads have to move quite a ways to the rotor. I didn't belive it either until a friend talked me in to trying to have the rotors turned before we replaced the wheel bearings.
If that doesn't fix it then you'll have to replace the wheel bearings.

 

[cruzer99]

Pressure bleed them.

 

[TWINRAY]

How old are the caliper seals?

 

[Bob Chadwick]

You need to find the source of your leak. A common problem is air being introduced at the caliper seals due to out of round rotors or wheel bearings out of spec. This can cause pulsing of the rotor, which will introduce air.

 

[lobes290]

im not sure how old the seals are but i was told by the man that i bought it from, that all the calipers were recently rebuilt, its a arizona car so its possible that they could be bad again, also i have blead them many times and im pretty sure air isnt comming out of the system, but the pedal gets stiff.

 

[RonV87]

I had this problem with my 78, its a "well know" problem, called cavitation. The rotors can be just slightly out of round and it causes the pistons in the brake calipers to go in and out. The seals around the pistons allow air to get in behind them on the in stroke and in with the fluid. Air compresses when you push on the brake pedal and that causes the spongy feel. The only real solution is to replace the calipers with those that have O rings as seals and are designed to keep air from getting in. Vette Brake Products sells them ( iIthink its VBP) but others do also. Check the mail order catalogs. I replaced all 4 of mine and that fixed the problem for good. Of course you have to bleed the brakes after putting on the new calipers. And if your rotors are out of round turn them down or replace them. The calipers I bought were no more expensive than "regular" calipers. Just make sure they have the O rings and are not stock or stock rebuilt ones. Make sure you get stainless steel sleeved calipers also.

 

[GTR1999]

There have been recent posts here on rear brakes and rear bearings. They're related,bad bearings and/or rotor runout will give you the problem you have. Check the other posts and you'll see what was said to solve this common C2 & C3 problem. You can have new ss calipers and still have the problem,I've seen so called ss calipers that were junk once opend up for inspection, rebuilt M/C's also are known for spongy pedal because they aren't always clean in the bore. Were the rotors turned recently?
Rather then go on what the last guy said was odne I would start from the beginning and check the runout and bearing end-play you want both to be under .003" if find more you might have found one or more of your problems.
Good luck

 

[JohnZ]

You have excessive lateral runout in the rotors, which is causing classic "air-pumping" in the calipers; new or rebuilt calipers won't fix it. If lateral runout is over about .004" (difference between the highest and lowest reading for one revolution), you need to have the rotors turned ON THE CAR. If it's the rears, also check the axial play in the spindle bearings, as that makes it worse.

 

[lobes290]

Thanks for all the feedback it is really helping me.

 

[Hib Halverson]

The 65-82 Corvette disc brake system can be a service nighmare. The "air pumping" discussed above as "cavitation" is the number one problem.

Because the calipers are fixed and the sealing sysetm (the inner piston seal) can only tolerate about .010-in. movement before they suck air, the system is very sensitive to brake disc runout and lack of parallelism. In the rear, this problem can be compounded by axle shaft flange run-out and rear bearings with excessive play.

If the car in question has not been retrofitted with stainless-steel sleeved brake calipers, corrosion of the caliper bores and the brake pistons can also be an issue.

There are some articles on the Idaho Corvette page that discuss servicing 65-82 brakes in great detail. See:
http://www.idavette.net/hib/BBfHInet12.htm
http://www.idavette.net/hib/BBfHInet13.htm

 

[JohnZ]

Hib - Article #12 is outstanding - very comprehensive piece of work! Should be required reading for all '65-'82 Corvette owners (with a quiz at the end)

 

[Dave L.]

John Z - Very informative article. What is your opinion on the recomendation of flushing the brake system every 18 months? I flushed my system 3 years ago when I purchased my car. To date I have had no leaking calipers or any other brake problems. Should I leave well enough alone or flush on some sort of bi-yearly scedule? What type of pressure bleeder do you recommend for my home use,I have an air compressor. Thanks in advance John.

Dave

BTW.. I read your monthly articles in Corvette Enthusiast Magazine and find them very informative.

 

[80convertible]

While your servicing your brake system I would also check the rubber hoses. Not related to your problem, however, old hoses can swell causing slight spongy feel due to bulging. I checked my front hoses one day and found a huge bulge in one of the hoses. As an fyi, if you are on a budget...Autozone sells stainless sleeved calipers for $55.00 each with lifetime warranty. Also, check for any signs of a leak around all your calipers. I wonder if a seal could have a pinhole leak.

 

[JohnZ]

I recommend flushing/bleeding the system every two years; suck the old fluid out of the master cylinder, clean the bottom of the reservoirs, fill with fresh fluid, and bleed until you see fresh fluid at all four wheels.

Using a compressor for pressure-bleeding can be tricky unless you have precise air regulation at low pressure. I prefer using a self-contained pressure bleeder:

http://www.motiveproducts.com/

This has its own pump, pressure gauge, plenty of fluid capacity so you don't run the master cylinder dry in the middle of the process, and makes C2/C3 bleeding a breeze.

 

[Dave L.]

Thanks JohnZ!

John - Thanks greatly. I just ordered that unit you recommended. I will check my shop manual for proper bleeding sequence and do it every 2 years as suggested.


Dave

 

[Hib Halverson]

Hib - Article #12 is outstanding - very comprehensive piece of work!

Thanks.

Somebody asked about how often one should change the brake fluid. Like John Z, I do it about once every 24 mo. or so, unless I run the car hard and boil the fluid. Then I change it immediately.

 

[Dave L.]

Thanks....

JohnZ & Hib- Thanks for the great advice and lesson on brake bleeding. I purchased the pressure type bleeder from Motive products as John suggested. It took only about an hour or so to do the complete job. Very easy to do. Got some nasty looking fluid out of there!
For anyone else who may purchase the Motive bleeder I have some advice. They give you some j-hooks and small chains to hold the pressure cap on the master cylinder. Do yourself a favor and toss them out. Get (2) 6" c-clamps and place a block of wood on top of the pressure cap on the master cylinder. Use the c-clamps to retain the pressure cap...works much,much better,trust me. Thanks again fellas!!

Dave

 

[lobes290]

I took it to a vet place, they said that i have runout and shimmed the rear rotors i have put about 150 miles and still a firm pedal but im still a little skeptical.

Thanks for all the help

 

[KenSny]

Check air in brake lines, then rotor runout, then calipers. I would check runout with the rotors on the car. That's not the way most shops would do it but it is the true indication - consider that the factory checked runout with them riveted to the spindle.