Rear brake locking up - Chevelle Tech
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  #1  
Old Dec 1st, 12, 3:05 PM
gzwhite gzwhite is offline
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Default Rear brake locking up

Just wondering what would cause the rear brake to lock up during a panic stop situation.

In the summer I was driving and another car caught my eye so I looked over, look back to where I was going and needed to slow down quickly, hit the brake maybe harder than necessary and the rear wheels locked up. Is this normal - no not looking away form what I should be looking at but the brake operation.

Also the front distribution and switch assembly is there any active mechanisms inside? Replacing the brake lines and was wondering if I should replace this or not. Just getting a tired of spending money.

Thanks!
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  #2  
Old Dec 2nd, 12, 12:18 PM
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Denny
 
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Default Re: Rear brake locking up

First, on any car, the front wheel should lock up first during braking for vehicle stability.

To achieve that, the brake system must be balanced. Here are a list of modifications that will increase the rear bias (hence more chance of rear wheel locking up first):
  • Increased rear rotor or drum diameter
  • Increased rear brake lining coefficient of friction
  • Increased rear caliper piston or wheel cylinder diameter(s)
  • Increased front tire diameter
  • Decreased front rotor or drum diameter
  • Decreased front brake lining coefficient of friction
  • Decreased front caliper piston or wheel cylinder diameter(s)
  • Decreased rear tire diameter
  • Higher center of gravity (i.e. raised vehicle)
  • Less weight on rear axle (i.e. unloaded)
  • More weight on front axle
  • More sticky tires (higher deceleration limit)
Of course, if your front brakes are not in working order, the rear might also lock first, so you should look for these on your front brakes:
  • Air in hydraulic system.
  • Brake shoes need adjusting (drums).
  • Excessively worn brake linings.
  • Faulty master cylinder.
  • Leaks or insufficient fluid.
  • Linings glazed.
  • Incorrect brake lining.
  • Blocked metering valve (disc/drum set-up).
On 1970 & up Chevelle, the proportion valve might be defective, allowing more line pressure than needed to the rear brakes. Starting in 1971, the proportion and metering valves are incorporated in the combination valve with the distribution block.

That being said, I suspect that pre-1970 Chevelle disc/drum are not well balanced by original design and that rear wheel lock up might be «normal» on these cars. I'm saying that because 1969 & 1970 have the exact same brake systems, except for the addition of the rear prop valve in 1970, which role is to reduce rear line pressure when pressing hard on the brake pedal. This would imply that there was too much rear pressure before, hence possible rear wheel lock up. Maybe it's only a problem with big block engines (More weight on front axle) and that's why engineers did not bother with it at first (because the prop valve was used on the Camaro/Firebird in 67-69, so they obviously knew about the concept).
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  #3  
Old Dec 2nd, 12, 12:55 PM
JWagner JWagner is offline
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Default Re: Rear brake locking up

Drums that are not round will lock up during a panic stop.
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  #4  
Old Dec 2nd, 12, 3:56 PM
Bryan59EC Bryan59EC is offline
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Default Re: Rear brake locking up

A seeping wheel cylinder.

Some brake fluid may have contaminated the brake linings and this will cause a lock-up.
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  #5  
Old Dec 2nd, 12, 4:40 PM
gzwhite gzwhite is offline
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Default Re: Rear brake locking up

Thanks for all the input. In review It appears leaks are the cause.

There was a leak by the front distribution valve, mounted on the frame, and the right rear wheel cylinder is showing signs of weeping, but the dampness was limited to the backing plate. Also the master cylinder fluid was low.

The car was purchased a year ago and had all the original brake and fuel lines - as the car is on cinder blocks the plan at this point is replace all the lines, front to rear and distribution valve. The master cylinder was replaced/new in June. In place are the AC Delco rear cylinders and would like to rebuild them if possible - any suggestions on how?
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  #6  
Old Dec 2nd, 12, 4:44 PM
oktunes oktunes is offline
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Matt
 
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Default Re: Rear brake locking up

It's not safe to put a car on cinder blocks. If you use blocks at all they should be concrete.
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  #7  
Old Dec 2nd, 12, 11:42 PM
TXCR13 TXCR13 is offline
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Default Re: Rear brake locking up

Be sure to replace your brake hoses, front and rear. They can fail internally and trap pressure, causing the brakes to lock, especially the rears.
Rebuild kits, including both rubber goods and metal, are available for the wheel cylinders.Depending upon their condition, the wheel cylinders can be honed and the cups replaced. Adding parts and labor to rebuild, it probably will be around the same price to just replace with new cylinders. I have rebuilt some in the past, with mixed results.
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  #8  
Old Dec 3rd, 12, 5:04 PM
gzwhite gzwhite is offline
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Default Re: Rear brake locking up

I read theposts about honing the rear wheel cylinders and would like to go this route as they are GM parts. where dwould one pick up the honing tool and any rebuild recomendations/instructions. All rubber liens are being replaced as well - thanks for sharring.

As for the cinder blocks - if they support a house what is 4klbs distributed across 8 blocks stacked 2 each? backed up by jack stands just in case! Just asking cause I sure dont need issue......
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  #9  
Old Dec 3rd, 12, 5:11 PM
Philip Philip is offline
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Default Re: Rear brake locking up

Quote:
As for the cinder blocks - if they support a house what is 4klbs distributed across 8 blocks stacked 2 each? backed up by jack stands just in case! Just asking cause I sure dont need issue......
Have a photo of this??
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  #10  
Old Dec 3rd, 12, 7:44 PM
861_ford 861_ford is offline
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Default Re: Rear brake locking up

in a panic stop, the rears will lock up as will the fronts.
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  #11  
Old Dec 4th, 12, 7:58 PM
gzwhite gzwhite is offline
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Default Re: Rear brake locking up

The frame on blocks.
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  #12  
Old Dec 5th, 12, 8:15 AM
starkie starkie is offline
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Default Re: Rear brake locking up

Check the e brake cables one may be seized,
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  #13  
Old Dec 5th, 12, 9:18 AM
sbvelleman sbvelleman is offline
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Default Re: Rear brake locking up

Quote:
I read theposts about honing the rear wheel cylinders and would like to go this route as they are GM parts. where dwould one pick up the honing tool and any rebuild recomendations/instructions. All rubber liens are being replaced as well - thanks for sharring.
Brake cylinder hones are easily obtainable at an auto parts store for around $15 to $20 . You should only have to make a few passes with the hone using brake fluid as a lubricant to restore the bore. If your wheel cylinders are badly pitted / scored they can be saved by sleeving them with brass ( about $50 / cyl. ) or stainless steel ( about $80 / cyl. ). Two places that come to mind are White Post Restorations or Apple Hydraulics but there are others.
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  #14  
Old Dec 6th, 12, 1:09 PM
OLDED OLDED is offline
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Default Re: Rear brake locking up

For my safety and braking performance point of view and from experience, unless the car is going to be a perfect restoration, numbers deal, I would rather use a new US made aftermarket wheel cylinder than a semi smooth, honed and oversize rebuilt 403 year old wheel cylinder. Many times the ID gets too big when the honing requires cleaning up a few deep pits in the bore. FWIW, Ed
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  #15  
Old Dec 6th, 12, 5:10 PM
gzwhite gzwhite is offline
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Default Re: Rear brake locking up

Quote:
Originally Posted by OLDED View Post
For my safety and braking performance point of view and from experience, unless the car is going to be a perfect restoration, numbers deal, I would rather use a new US made aftermarket wheel cylinder than a semi smooth, honed and oversize rebuilt 403 year old wheel cylinder. Many times the ID gets too big when the honing requires cleaning up a few deep pits in the bore. FWIW, Ed
I agree with you and will probably put new cylinders on and rework the original ones, and keep them in stock for the time it is made into that perfect restoration -JK.

I do need to try the rebuild just to say I did it!
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