Front brakes lock up in panic stop

[SDJunkMan]

Some idiot pulled out in front of me in my CUCV Blazer yesterday and I had to slam on the brakes, the fronts locked up, it got a little squirley, but luckily I was able to stop before hitting them. I'm wondering if this is normal for a Blazer due to the short wheelbase, or if I have a bad proportionng valve or something. Brakes seem fine in normal driving conditions and it doesn't seem like my pickup has a problem with the fronts locking that bad up in a panic stop.

 

[Goldie Driver]

I don't believe it is normal, and have had similar issues with Goldie in wet areas and know it is not normal for a Sub.

I'd make sure the rears are adjusted properly as a starting point.

Beyond that, not sure. Could be a bad prop valve, but don't know how to test one.

Maybe more knowledgeable folks will chime in.

 

[82sbshortbed]

I remember a guy starting a thread talking about his brakes would not lock up but, he wanted them to. Lol

I think in these trucks if you stomp hard enough (like an emergency)they will lock up because they don't have antilock brakes. But, I'm about 5 beers deep and what do I know. Lol

 

[SDJunkMan]

Yeah, this doesn't seem right, seems like it locks up too easy, but I thought it might be due too the short wheelbase. I had a few hundred pounds of stuff in the back, so there should have been enough weight on the rear.

 

[Dutch Rutter]

I too had to do an emergency stop today. My rears locked. That is also what happened with my old c20. I'd be thinking rear drums need some adjustment.

I'll tell you though, nothing gets people moving like hearing a pair of 305 tw tires screeching to a hault.

 

[C10MixMaster]

The reason ABS was invented was to prevent wheels locking on panic stops. Our truck are old school if the wheels lock during excessively hard stopping you have to lift and reapply (or learn the sweet spot where the tires are making black marks /hazing but not locking). Also the fronts should always lock first, if the ass end locks first it will pass you up.

 

[Camar068]

I don't believe it is normal, and have had similar issues with Goldie in wet areas and know it is not normal for a Sub.

I'd make sure the rears are adjusted properly as a starting point.

Beyond that, not sure. Could be a bad prop valve, but don't know how to test one.

Maybe more knowledgeable folks will chime in.

How often should they be adjusted? Mine do the same. I adjust them in reverse prolly once a quarter. This is one reason I've thought of going disc in the rear.

 

[SirRobyn0]

The reason ABS was invented was to prevent wheels locking on panic stops. Our truck are old school if the wheels lock during excessively hard stopping you have to lift and reapply (or learn the sweet spot where the tires are making black marks /hazing but not locking). Also the fronts should always lock first, if the ass end locks first it will pass you up.

I agree with this but... In C/K20 & C/K30's because of the larger rear drums, if unloaded it's likely the rears will lock first due to relatively little weight in rear compared to stopping power in the rear. In a non-ABS vehicle one axle will always lock before the other in a panic stop. However it is not all that uncommon in an old rig for the rears to quit working or not working well and the driver is never aware until a panic stop when the fronts seem to lock to easy. Out of adjust rear brake shoes, or bad proportioning valve being the most common, also, obviously worn out tires will make it easier to lock. Was the OPs brake lock a normal lock or was it to easy to lock, it's hard to tell on a forum, but you can do the following things. Check rear brake adjustment, if out of adjustment fix, check to make sure the shoes aren't worn out and that the wheel cylinders and axle seals aren't leaking. If the rear brakes check out ok, replace proportioning valve. About the only test you can run on the valve is to open a rear bleeder and have an assistant step on the brake pedal. You should get a nice strong shot of fluid out of the bleeder if opened up enough. If nothing comes out or only a trickle the valve is bad. It's not all inclusive test, you can get a good shot of fluid and still have a bad valve but if you get only a trickle or nothing then you know the valve is bad.

 

[SirRobyn0]

How often should they be adjusted? Mine do the same. I adjust them in reverse prolly once a quarter. This is one reason I've thought of going disc in the rear.

If the automatic adjusters are working properly they really shouldn't need to be manually adjusted ever. Personally I do not think the reverse backing trick really does much, but it couldn't hurt anything either.

 

[RecklessWOT]

My '87 1/2t longbed used to lock up like crazy if I hit the pedal too hard. I always assumed that was normal because hey that's why ABS was invented and all (and think how often you'll trigger the ABS in a more modern car just driving a little hard, figured that would just be locked up if it weren't for ABS). I grew up mostly driving light little *** cars and they all do it. I always just trained myself for how hard I could hit the pedal before the fronts would lock, and in a panic just need to pump the brakes. When I got my Suburban I noticed no matter how hard I STOMPED on the brakes they pretty much would never lock up unless I was on sand or something. I know obviously locking them up is not the desired result, but I always worried that maybe my system wasn't working to its full potential. Not like you want to lock the brakes, but if the truck CAN'T lock it's own brakes even under a very hard stop, how did I know that it was even squeezing the rotors as tight as is should/could, and maybe I was operating with reduced braking power? I feel like some slight amount of brake lock is to be expected in a very hard stop, but should not be excessive to the point where you can easily lose control

 

[RecklessWOT]

The reason ABS was invented was to prevent wheels locking on panic stops. Our truck are old school if the wheels lock during excessively hard stopping you have to lift and reapply (or learn the sweet spot where the tires are making black marks /hazing but not locking). Also the fronts should always lock first, if the ass end locks first it will pass you up.

kind of my thoughts exactly

 

[Doppleganger]

My truck was like this when I first got it. Turned out to be the flex lines collapsing.

 

[gotyourgoat]

 

[C10MixMaster]

Gonna have to disagree with your police work there Lou.

If anything is locking up it should be the rears first. But just before the fronts and they shouldn't be locking up except under hard braking. The fronts locking up first means you can't steer. Can't imagine not wanting to be able to steer the vehicle I'm propelling down the road. Neither should lock up way before the other unless it's a race car and the bias is set up for maximum efficiency/control on that particular track.

Vehicles have proportioning valves and smaller braking systems on the rear for a reason.



Abs was first invented for planes, helping pilots brake and to not have to skid the tires to maintain threshold braking. Plane is able to stop more efficiently and in more conditions without popping the tires prematurely.

Not sure who lou is?
I didn't design them but they are designed to lock up in front first. If you fronts lock first you will continue in a strait line , if the rears lock there is a good chance the rear will go sideways . neither is a good situation but going down the road sideways is worse. You even brought up the point of front brakes are larger, if they wanted the rears to lock first the rears wouldn't be smaller.

 

[SDJunkMan]

Thanks for everyones comments. It's not the tires, they are good. I'll check the adjustment on the rears, but I'm guessing it's the proportioning valve, so will pull a line and see if I get fluid when applying brakes.

I'll tell you though, nothing gets people moving like hearing a pair of 305 tw tires screeching to a hault.

That's the truth, better than a horn!

I understand that modern ABS systems are designed to avoid this, but I have been driving pre ABS trucks for a lotta years and this one just seems to lock up too easy. Just wasn't sure if it was just a Blazer thing with the short wheelbase, since most of my trucks have been long beds.