Rear brakes locking first

[NateDN10]

First, the back-story:
Last summer I replaced the front rotors and pads on my Cobalt (rear drums, no ABS). Then I bled front and rear.

This winter, I notice that in the snow, the rear wheels lock up under moderate braking, causing the rear to slide out. This is even with 120 lb. of sand in the trunk.

Only thing I can think of is that the fronts need to be bled better, that some air is causing hydraulic pressure to be lower there. Is that plausible?

 

[hounddog]

No.Rear drums will lock.Disc don't.If air in the lines pedal would go to the floor unless pumped.Sounds normal to me.

 

[UNC017]

Proportioning valve not calibrated right, or sticking and not working?

 

[bepperb]

What is the condition of the tires? Their depth will certainly be a factor.

But no, you don't want your rears locking before your fronts. Perhaps putting the better tires in back will remedy this?

 

[Eddie]

Did you replace the pads in front with the same characteristics as the originals? If you used a harder pad that requires more pressure, that could also be a reason.

 

[rpn453]

Check the tires first. At worst, the rears should be almost as good as the fronts.

I doubt your brake pedal would feel firm if you had any air in the system. That shouldn't have happened if you kept the reservoir full. You could re-bleed to be sure though.

Did you keep the front rotors clean of oil and grease during installation?

What is the friction rating on the pad? They are usually marked, though you likely won't be able to see the markings without removing them. You could probably check a new one from wherever you bought them. Maybe you have EE or FE pads where FF pads would be more suitable.

It could also be something going on inside the drum. I once had a drum that would occasionally lock up and not release easily, due to severe grooving inside the drum.

Until you remedy the problem, do any significant braking in a straight line!

 

[mechtech2]

This is one of the situations where ABS and a lobotomized driver can far surpass an expert driver with standard brakes.
Locking up brakes in snow/ice is very easy. It is almost impossible to control them as well as a micro computer can.
So consider this normal. Your brakes may be in perfect condition.

 

[rpn453]

Originally Posted By: mechtech2
So consider this normal. Your brakes may be in perfect condition.


I disagree. The rear brakes should never lock up first on a reasonably consistent surface, especially on a slippery surface where there isn't much weight transfer.

 

[unDummy]

Any rear brake can lock up, drum or disk.

They are probably poorly adjusted and usually 1 rear will lockup 1st causing the symptoms. So, there could also be a mechanical issue with either side. Check your parking brake and brake releasing springs.

Re-bleed the whole systems just in case some air got into the front.

Get better front pads.

Rear lockup is easier to control for the poorly skilled driver and is how most non-abs cars are set up. Its the nature of the beast.

 

[NateDN10]

Tires are good all around. They're winter tires with less than 15000mi on them.

I'm not sure about the front brake ratings. I went to Autozone and got the cheap ones! So that could have something to do with it.

I'll probably try re-bleeding the fronts first. The pedal is a little bit squishy, and bleeding the brakes is cheap and easy. If that doesn't do it I'll look into getting new front pads.

 

[hounddog]

What you described is normal.You can look all you want but you won't accomplish anything.Does it stop good on dry payment?

 

[NateDN10]

Originally Posted By: hounddog
What you described is normal.You can look all you want but you won't accomplish anything.Does it stop good on dry payment?

It does seem to stop fine on dry pavement.

Why would that be normal, though? With the sand, weight distribution is 56.5/43.5 F/R, which means front brakes would have to be about 30% stronger than rears to account for the difference, which doesn't seem like much to me.

Although now that I think about it, I can remember two instances in previous winters, before the brake job, where the rears locked first as well.

 

[UNC017]

Originally Posted By: rpn453
Originally Posted By: mechtech2
So consider this normal. Your brakes may be in perfect condition.


I disagree. The rear brakes should never lock up first on a reasonably consistent surface, especially on a slippery surface where there isn't much weight transfer.



Well, I'd DARN sure rather have the rears lock up before the fronts, if one has to lock before the other! Every non-ABS car I've ever had would lock a rear *just* before a front. If the fronts lock first, you a) lose all remaining control, and b) leave a LOT of un-used braking ability on the table.

The main thing to remedy is either end locking *significantly* sooner than the other. It sounds like the rears are locking noticeably too soon in this case, which is why I'd suspect a proportioning valve issue. I've had a factory car with a prop valve that locked the rears too soon, and just replacing the valve cured the issue. Granted, this was a 1973 vehicle, but some things haven't actually changed much... like nonABS disk/drum brake systems. There still has to be a residual pressure valve(*) and proportioning valve to get it to work at all.

(*) I didn't mention suspecting the RPValve because if it's bad enough to cause premature lock up, the drum brakes will drag all the time.

 

[SteveSRT8]

It is absolutely not normal braking performance for any late model vehicle!

You do know about leading and trailing shoes, right? This can be as simple as reversing them when you changed them out. Most drums have a 'self servo' action designed to reduce effort. Check this first.

After that it becomes tricky. Any time you change pads/shoes the braking characteristics can change dramatically.

Does anyone know if the Cobalt has dynamic brake proportioning (electronic)??? And why does the OP claim no ABS???

 

[NateDN10]

Originally Posted By: SteveSRT8
It is absolutely not normal braking performance for any late model vehicle!

You do know about leading and trailing shoes, right? This can be as simple as reversing them when you changed them out. Most drums have a 'self servo' action designed to reduce effort. Check this first.

After that it becomes tricky. Any time you change pads/shoes the braking characteristics can change dramatically.

Does anyone know if the Cobalt has dynamic brake proportioning (electronic)??? And why does the OP claim no ABS???

I have the base model. Stick shift, no power windows or locks. No ABS.

Also, I didn't do any service on the rears except to bleed them. Just changed the front pads & rotors.

 

[NateDN10]

Would this be the proportioning valve? If so, it's definitely not electronic.

 

[hounddog]

One of the reasons for ABS and on solid rear axels like on trucks which is called RWAL is to STOP the issue of locking rears.It was a big problem on trucks for decades.You got a bare baboon Colbalt.No ABS.Drum brakes can and will lock.Disc brakes are not designed to lock.In 25 years running auto repair shops I have seen a half dozen or so proportioning valves replaced.They didn't fix squat.Wasted money.Your chasing a non issue on snow/ice and even a wet road.Glazed shoes and drums can ENHANCE the issue you are describing but even in good working order your Colbalt will LOCK THE REARS.Before ABS and four wheel disc where do you think all the SKID marks you saw on the payment came from? It was REAR DRUM brakes locking.Now if you want to spend some money and do it right buy FACTORY front pads,deglaze the rotors.Then buy some FACTORY rear shoes and deglaze the drums.If it will make you feel better replace the proportioning valve.Now you have put the brake system to like FACTORY NEW.I bet it will STILL LOCK the rears on ice,snow and even on wet.It may be a little less touchy but it will still do as you described.

 

[SteveSRT8]

Good point on the proportioning valve swap. We did that once on a GM van that wore out the front brakes prematurely. It didn't do squat!

Whenever you change the pads/shoes you change the braking characteristics of the vehicle. Not always for the better.

 

[mechtech2]

To add to my first post:
Drum brakes are self energizing . They are hard to modulate in some circumstances.. This is why it makes them a second choice compared to discs for road racing and snow/ice conditions. It is a knife edge of control at or near break away conditions.

 

[NateDN10]

Thanks for the info.