Front brakes lock up in panic stop

[gotyourgoat]

 

[WebMonkey]

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.

my 85 k20 super double xtra heavy duty (13" drums) has a functional load valve in the rear.
as the bed is loaded down, it opens the valve more and more allowing more stop juice to the drums.

so, in short, my rear drums don't lock up with no load.

good luck to the op, i think the front lock up is more normal than not.

 

[WebMonkey]

Sorry but I don't agree with any of this.

Maybe we are getting together with the front doing most of the work, where we are drastically differing is apparently the bias or brake balance. I waver on saying balance because they are not equal because of science. Weight inertia, **** like that.

Not being able to steer since your steering wheels are locked up is no way to drive.


Btw. 50 seconds --to-- 1:25.

xc_hide_links_from_guests_guests_error_hide_media

i patrolled the west german/czechoslovakian border in a cucv during the early 80s.
i, along with other drivers, got sent to the adac. german driving school.
they had a full skid pan, half skid pad (lane) etc.

in one of the classroom lectures, the instructor demonstrated front brake lockup vs rear brake lockup.
(one car had the front wheels glued stuck, opposite on the other)

he put one on a sloped board at the top and let it go.

the rear locked car ALWAYS spun around backwards halfway down the board.
the front locked car ALWAYS 'slid/skidded' straight.

so, i agree not being able to steer sucks but since we don't have ABS, skidding straight is better than spinning around.



lastly, the CUCV always locked the fronts.
the instructor stated that they should and would lock on pedal stomp.

then taught the stomp/lift-steer/stomp.
also known as operator actuated anti-lock brakes.

 

[Mossyman]

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.

Not sure if it’s the same but in my ‘84 C10 I had to do a quick stop attempt and the ass end went sideways on me. Turns out the brake line on the left front was froze up and wasn’t letting any fluid in.

 

[Goldie Driver]

Every non ABS vehicle I have owned would lock the rears, and sliding- well, other than straight- was not an issue unless the rears were not in close to equal adjustment.

Rear ant lock is worse IMHO as when you lose steering problems can arise.

Or, like one Ramcharger I had ( no ABS, just wonky brakes ) , really challenging in slippery situations.

Seems like a master cylinder fixed that, but been over 30 years ago so not 100%.

My 77 Suburban would make the rears howl when I wanted so I would expect Goldie to do the same with the same brake set up.

Of course, I would prefer to not have to panic stop and worry about it in the 1st place.

My line of thought.

 

[Grit dog]

If you can lock up the front brakes on dry pavement, I’d say you have a very good working brake system and nothing more.
But that said, either your rear brakes are pulling their weight or not. And IMO it’s pretty easy to feel a vehicle that is stopping only on front brakes.

Id go test the brakes. Do a couple hard stops, see if you can get the back brakes to lock up or not. Heat your brakes up some and see if the drums are hot.

 

[SirRobyn0]

my 85 k20 super double xtra heavy duty (13" drums) has a functional load valve in the rear.
as the bed is loaded down, it opens the valve more and more allowing more stop juice to the drums.

so, in short, my rear drums don't lock up with no load.

good luck to the op, i think the front lock up is more normal than not.

With all this talk back and forth I'd really like to know what the answer really is, I'm genuinely curious. I think in the end the only way we would know for sure would be if someone that was in design on these trucks, were to answer, otherwise it's just going to be what we "think" is right. If memory serves me I'm pretty sure @Keith Seymore was in brake design on these trucks. Maybe he'd be kind enough to tell us if one axle or the other is designed to lock up first in a panic stop.

 

[rt66paul]

Brakes are designed so that most of the stopping is from the front. ABS brakes were invented because trucks have extreme weight differences from not loaded, to fully loaded and misloaded(mis-distributed loads). Those of us who drove bobtail trucks or other trucks above 1 ton capacity from the 50s, 60s and 70s are fully aware of this. I can't imagine driving a truck or bus from the 20s or 30s with mechanical brakes - maybe that is why we never see them outside of museums.
Driving an empty truck or a full truck from this period was much different, you had to be aware of the limitations of the braking system. Add to that wear and old parts, and you could have a recipe for disaster. Be careful and do not overdrive your braking capacity.

 

[AuroraGirl]

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.

My car has 4 wheel abs but the system can only base it off the front wheels(if your front locks, your tears prob do too) so gm made the rear brakes limit in psi in a way that the
Fronts will always hit lock first under braking, so the abs can work and keep you from kicking out the rear.

well my shop never installed these bad boys when they did brake lines. So now my car dangerously will lock up the rear and cycle the abs on the front 2 wheeels but it can’t cycle the rear because the system can’t tell a non moving wheel from a wheel that lost traction. , so it thinks the fronts are only locked.

this has been annoying in winter

 

[Goldie Driver]

Remember the rule of weight transfer, too - when you brake, the nose of the vehicle pitches forward, and the weight distribution temporarily changes as well.

Thus, the rear becomes lighter and easier to lock up - non ABS.

The fronts have bigger brakes because they carry more of the braking load - assuming front discs.

Not sure on 4 wheel drum brakes if the fronts were larger diameter or thicker in width or they just did not think of it back in the day.

You know, like on the '65 Ford P/U I had in HS - my Dad's 1st new truck when he bought it.

Single reservoir master cylinder - no fail safe - and no power brakes, either.

Yee Hah !!

 

[Rusty Nail]

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.

I concur with the gentleman here and also suggest starting with adjustment in the rear.

It's so rare to hear squealing tires these days - they command a lot of attention. I think it's hilarious!

P.s OP:
40 year old brake springs are not-so-much these days. Prolly seen a few heat cycles?

Let me share a story...

I ruined some REALLY GOOD brakes one time after a fifty cent hardware part failed a year later.
One of the little ****** pieces in the hardware kit nobody ever bothers installing/replacing/buying/selling?
The part failed and cost me an expensive brake setup teaching me a lesson about running ancient brake parts. Hope this helps!

 

[C10MixMaster]

Remember the rule of weight transfer, too - when you brake, the nose of the vehicle pitches forward, and the weight distribution temporarily changes as well.

Thus, the rear becomes lighter and easier to lock up - non ABS.

The fronts have bigger brakes because they carry more of the braking load - assuming front discs.

Not sure on 4 wheel drum brakes if the fronts were larger diameter or thicker in width or they just did not think of it back in the day.

You know, like on the '65 Ford P/U I had in HS - my Dad's 1st new truck when he bought it.

Single reservoir master cylinder - no fail safe - and no power brakes, either.

Yee Hah !!

Even in the days of 4 wheel drums the front were bigger.

 

[SirRobyn0]

With all the ABS talk I'm going to add this.

I do not know if GM did this on the next gen of truck after the square, but my 92 Dodge pickup has rear wheel only ABS, to prevent rear wheel lock up when empty. This is one of the reasons I "think" that it is normal for the rear end of a empty pick up to lock up easier than the front under a panic stop. I'd image this would be less pronounced in a Blazer / Jimmy or Suburban but I "think" because they use the same basic braking system that it would be the same, but possibly less so because of the body of the vehicle.

 

[SirRobyn0]

Remember the rule of weight transfer, too - when you brake, the nose of the vehicle pitches forward, and the weight distribution temporarily changes as well.

Thus, the rear becomes lighter and easier to lock up - non ABS.

The fronts have bigger brakes because they carry more of the braking load - assuming front discs.

Not sure on 4 wheel drum brakes if the fronts were larger diameter or thicker in width or they just did not think of it back in the day.

You know, like on the '65 Ford P/U I had in HS - my Dad's 1st new truck when he bought it.

Single reservoir master cylinder - no fail safe - and no power brakes, either.

Yee Hah !!

At one time I had a 65 chev 1 ton on the farm, with drums all the way around, and yea you had to be much more aware of stopping limitations when loaded that's for sure. I do not remember if the fronts were bigger or not, but they were different part numbers. That truck was damn near impossible to lock any of the wheels up on, unless you really stood on the brakes and the truck was empty. But still that truck did a lot of work for use back in the 90s.

 

[swilson143]

Front lockup is PREFERRED. Nothing is wrong with your truck.

In a front skid scenario the truck skids straight. You can let off the brakes, reapply, and still be in control.

If the rear locks up, the ass end of your truck will want to come around on you and the truck will rotate even with no steering input. Much more dangerous. You can easily demo this by getting up to speed and locking your rear wheels with the parking brake inn a wet parking lot.

My 1991 suburban had an early GM ABS system. It was rear brakes only.