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Author Topic: 68 Power disk brakes problem  (Read 2591 times)
monaroman
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« on: November 05, 2010, 03:54:02 PM »



Hello,

Sadly i have a issue with my brakes. Disk brake front and drum on the back.

The facts. I had a specialist replace all bushes on the steering and suspension.

They removed the calipers off the disk.

The issue they face is very soft pedal once the hose is connected to manifold and hydraulic line connected to the cyclinder.

They have removed the hose from manifold and get a hard pedal (hydraulic brake only).

I had no problems with my braking system untill now.

I was told of the issue late Friday afternoon so i have not had the chance to have a look at the problem.

Could the booster (power brake cyclinder) be faulty?

Could the hose to the manifold have a fracture in it?

Could the check valve be faulty?

Cheers guys.
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Ed Bertrand
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« Reply #1 on: November 05, 2010, 10:40:17 PM »

It sounds like you probably have air in the lines so they'll need to bleed the brakes.

If you still have a spongy pedal after bleeding, then you have an air leak. Since the only thing they did was remove the calipers, I suspect it's a seal that's torn just enough to let air in, but not allow fluid to escape, so it may be very difficult to find.

If you need to have your calipers rebuilt, I highly recommend you go with a company that will o-ring the pistons. This will aleviate a lot of problems typical of the 4 piston calipers.

Ed
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monaroman
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« Reply #2 on: November 06, 2010, 02:17:49 AM »

Hi Ed,

Thank you for your reply.

I did ask the lad if he had bleed the brakes. He had not tried that and was going to try bleeding the brakes first thing Monday.

Yeah im hoping its something as simple as a little air!!!

If it is a seal were would you recomend trying first?

Once again thank you for your suggestion.

Cheers.

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Gramps69Z
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« Reply #3 on: November 06, 2010, 09:42:50 AM »

As Ed stated, sounds like air got into the system, and not a booster problem.  Were the calipers removed from the car or just taken off the rotors?  Was the master cylinder bled?
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Captain John Wykoff
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dutch
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« Reply #4 on: November 06, 2010, 11:03:25 AM »

When they took the calipers off did they just hang them by the rubber lines rather than tie them up to take the weight off?
If the calipers hung with full weight on the flex lines for any amount of time and especially if the lines aren't in good condition, they could have weakened them to the point where they are bulging when the brakes are applied and giving you that soft feel to some degree...
Its a long shot, I know - but if everything else fails I'm told it may be worth a look - Randy
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Ed Bertrand
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« Reply #5 on: November 06, 2010, 12:44:25 PM »

Since he's going to bleed them, I recommend you go ahead and replace the rubber lines. These should be replaced every 5 to 10 years anyway, and since they expand internally, it's difficult to know when they've gone bad. These are cheap insurance, so I replace mine every time I change pads (about every 5 years).

If you still can't get the hard pedal feel after bleeding, then yes, you'll probably have to have the calipers rebuilt. Especially if they've never been rebuillt before! Original pistons were made out of aluminum, and after 45 years they're usually shot (pitted). The bores will also be pitted and will require sleeving. There are two ways to go on this, stainless or brass, and I personally recommend stainless. It's much harder than brass and will last a lifetime.

I personally use VETTE BRAKES, but there are other companies that do the work just as well. You can also purchase new or rebuilt calipers from places like RICK'S, CLASSIC INDUSTRIES, etc..., so there are several options available to you. If you want to keep your original calipers because of the date codes, then have them rebuilt. If you don't care about the date codes, then go with exchange (or new) calipers and you'll be up and running within a few days.

Ed
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monaroman
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« Reply #6 on: November 06, 2010, 04:57:12 PM »


Hi fellas,

The calipers were taken off the rotor's and the weight taken off them. They were not hanging by the hoses. From what i could gather on Friday the master was NOT bleed.

The history on the calipers is a little hard. I can see new pads and they have been painted. Untill they are pulled apart hard to tell condition.

Im thinking the best approach would be to start at the easy fix and work back from there.

Once again thank you for your input it all helps.

Cheers.

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L22K19
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« Reply #7 on: November 11, 2010, 06:52:49 AM »

Monaroman sounds like an Australian?? 

You stated that you are able to get a solid pedal with the vacuum hose disconnected. If this is the case then it's doubtful you have a seal problem with the calipers or air in the lines. Both of these issues will give you a spongy pedal regardless of wether the booster is connected or not. If you had a brake hose that was damaged internally, you will usually get a solid pedal all of the time as the hose is either stopping fluid going to or from the caliper. If it is stopping fluid going to the caliper then you will have a solid pedal and no (or significant reduced ) brake on that particular side. If it is stopping fluid coming back you will find the brakes sticking on, on that particular side. If it was damaged and bulging, then again you would have a soft pedal all of the time.

Get a vaccum gauge and check how much manifold vacuum you have. For it to work nicely you want around 22 - 24" of vacuum. . If the vaccum is very poor then you will get a odd sense of the pedal being what appears to be spongy. If you are concerned about the vacuum line, it's worth about $7.00 for a new length nad the check valve is very common and is available at any auto parts store

If you are an Aussie, call me on 0408 CAMARO and I'll discuss the issues.


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Ed Bertrand
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« Reply #8 on: November 11, 2010, 10:31:01 AM »

Quote
You stated that you are able to get a solid pedal with the vacuum hose disconnected. If this is the case then it's doubtful you have a seal problem with the calipers or air in the lines. Both of these issues will give you a spongy pedal regardless of wether the booster is connected or not.

That's what I used to think too, but the same thing just happened to me!

I noticed metal shavings inside my front wheels a few weeks ago, and when I examined the brakes, I noted that even though the tops of the pads looked almost new, the bottoms of the pads had actually worn down so much that the metal backing was digging into the rotors. When I pulled everything apart, I noticed that the pistons in the calipers were "cocked", which allowed the pads to wear unevenly. So I bought new rotors, bearing, pads, etc... and put it all back together again, making sure to gently push the pistons back into their bores before I reinstalled. after that I blead the brakes, and everything appeard to be fine. However, as soon as I started up the car (and the booster kicked in), the pedal went all the way to the floor and I couldn't get a good feel. I shut off the car and blead them once more and they felt fine again. Until I started the car up and they went to the floor again! I went through this routine several times and although I could get a nice hard pedal each time I blead them, I could never get the brakes to work when the booster kicked in.

So I pulled the calipers and sent them back to Vette Brakes for evaluation, and they found a few bad O-ring seals and "rubber residue" behind the pistons. They also found one sleeve that was scored (probably because of the "cocked" pistons). They said that the worn O-rings were probably causing the calipers to suck in air, but they weren't bad enough to leak fluid out. Now, I have no idea why the booster exagerated the problem so I'm not positive the calipers were the issue, but it seems to add up because the master cylinder and booster are only a few months old, and everything was working fine before I installed the new pads.

I haven't gotten the calipers back yet (they'll be here early next week), so I won't be able to let you know what happens until they arrive and I put everything back together again, so stay tuned...

Ed
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L22K19
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« Reply #9 on: November 14, 2010, 05:48:18 AM »

Ed,

While we are waiting for Monaroman to come back in,

Having used around 50 sets of this on Historic race cars, I also went accross to using O Ring style seals 10 years ago but I found a number of issues with them. The O ring's have significatly reduced seal retraction properties, and if they got cocked, they would actually stay that way until physically corrected,and would also acutually leak under high pressure even though the seals were in perfect condition. They always left the rubber residue you have and found all of these  to be inherant design problems  of using a O ring in this application. I ended up replacing every one of the O ring stlye pistons back to lip type pistons and seals as a warranty and safety concern. For all but severe race conditions, the piston design with the original style lip seals are by far the best combination as they are a true hydralic seal. In severe race applications we converted the caliper to have seals in the bores ( like 95% of calipers on the market) and made new heat treated, centreless ground, stainless pistons. Once we did this, we NEVER EVER had a problem of ANY description.

Regards
Alastair
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Ed Bertrand
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« Reply #10 on: November 14, 2010, 10:37:21 PM »

Alastair, thanks for that information. The real reason I went the O-ring route was because of problems I've experienced over the years on other 67-68 Camaros, and 60's/70's Corvettes I've owned, with "air pumping". This is air leaking past the seals due to excessive rotor lateral runout. A problem with these "fixed caliper" brake systems. John Hinckley told me that this "air pumping" can occur with as little as .003" runout! What happens is that the pistons in the bores literally "vibrate" in and out at such a high rate, that the seals allow air to seep in, compromising the system. The O-ring solution is supposedly something that helps minimize this.

I can't explain it as good as he can, but basically you have to make sure your rotors are true to .003" or you could have problems. And you have to have them turned as an "assembly" (hub and rotor), or you won't get accurate results. Not so much an issue on the Camaro one piece rotors, but difficult on the original Camaro/Corvette two piece "hub and rotor" assemblies.

John, care to add to what I've said? (and don't be afraid to contradict me if I've explained it wrong!!)

Ed
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JohnZ
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« Reply #11 on: November 15, 2010, 11:36:45 AM »

John, care to add to what I've said? (and don't be afraid to contradict me if I've explained it wrong!!)

Ed


"Air-pumping" due to excessive rotor lateral runout has always been a chronic problem on '65-'82 Corvettes, especially on the rears, where lateral runout is further aggravated by axial wear/slop in the non-adjustable (without complete teardown) rear spindle bearings. The O-ring pistons/seals are less susceptible to "air-pumping" than the OEM lip seals in normal street use, but I have no history on the O-ring design when used under race conditions.
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'69 Z/28
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L22K19
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« Reply #12 on: November 17, 2010, 04:58:01 AM »

I looked back through all of my notes on these calipers last night and found some information on all of the different "brands"of stainless sleeved calipers I have used. Some of them "professionally" resleeved and some new sleeved calipers ,I found that 'some of the sleeves had upto 0.008" ovality and 0.004" taper on them. While this dosen't seem a lot it affects the preload on the seals my nearly 15% and further aggrivates all of the issues we have collectively identified here.

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Ed Bertrand
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« Reply #13 on: November 17, 2010, 11:53:57 AM »

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I found that 'some of the sleeves had upto 0.008" ovality and 0.004" taper on them

I wonder what the original specs were. John, any idea on this one?

Ed
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JohnZ
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« Reply #14 on: November 18, 2010, 04:43:31 PM »

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I found that 'some of the sleeves had upto 0.008" ovality and 0.004" taper on them

I wonder what the original specs were. John, any idea on this one?

Ed


Dunno, Ed - I don't have the caliper drawing. Lone Star Caliper or Vette Brakes would probably know - they're the two largest fixed caliper rebuilders.
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'69 Z/28
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