Rear wheel lock up during hard braking

[Paul G]

This clown turns left right in front of me the other day. I hit the brakes pretty hard and the rear wheels locked up. This happened before, but since I rarely hit the brakes that hard it never really mattered. Those 295/50's I have on the back put out a pretty loud bark. But, is there a way to apply more braking force to the front wheels, or less to the rear?

[John_Kunkel]

Are the brakes stock? Discs? Drums all around?

If it is stock and has discs on the front you might have a faulty proportioning valve. The purpose of the prop valve is to cut back the pressure to the rears on heavy braking.

[Paul G]

Are the brakes stock? Discs? Drums all around?

If it is stock and has discs on the front you might have a faulty proportioning valve. The purpose of the prop valve is to cut back the pressure to the rears on heavy braking.

They are stock front disc rear drum. Any way to test or adjust the proportioning valve?

[resq302]

I had this problem once with my 69 Charger.  Turns out the rear axle seal was leaking and contaminating the brake shoes.  Suprisingly enough, the gear oil on the brake shoe lining made it catch and lock up as opposed to lubricating it and not making it catch.  Might want to pull the drum and see what you have back there.  I have also heard that sometimes the automatic brake adjuster sometimes jumps and keeps adjusting the brakes when it shouldnt be.

[six-tee-nine]

 

Greasy brake pads make your brakes lock very easy

[bordin34]

I think it came like that from the factory. My 74 does that also. I also read a review from 1974 and it said that on every hard brake thing they did the rears locked up and they even spun around in one of the times.

[Ghoste]

The factory proportioning valves were an approximate at best.  One valve was used in a wide variety of dissimilar applications.  Weight and wheelbase are two very important factors is setting up the brake proportioning and a valve that is calibrated for a fully loaded Newport station wagon would be completely wrong for a stripped down Belvedere with a smallblock for example.

[Paul G]

I rebuilt the rear brakes last summer. No leaks. Shoes and springs are new. I recall having this problem with several other old Mopars in the past. Is there anything I can do to adjust the brake force to the rear?

[Ghoste]

Remove the factory unit and put in an adjustable proportioning valve.

[Paul G]

You mean like this one?

http://www.piratejack.net/web-catalog/pages/APV1_jpg.htm

[Ghoste]

Yeah, that looks like it should work.

[Nacho-RT74]

really, stock pieces are enough.

about different prop valves to different bodies.. is true but not a BIG difference... I think You could just change the prop valve spring inside valve to get better rear/front brakes relationship. however dunno if sof or harder spring what would it be the result. is Just an idea.

As I stated on the other thread, once I got similar experience and was the emergency brake link inside the assembly somehow stuck against the brake shoe.

[sean68charger]

Remove the factory unit and put in an adjustable proportioning valve.

yep. i had the same problem when going to wider rear tyres-this is probably why the rear breakes away first as your changing the factory settings, fitting the adjustable valve is the way to go

[Paul G]

Remove the factory unit and put in an adjustable proportioning valve.

yep. i had the same problem when going to wider rear tyres-this is probably why the rear breakes away first as your changing the factory settings, fitting the adjustable valve is the way to go

How did you install it? Any pics?

[daytonalo]

EASY ONE , RESET PROP VALVE, PUSH IN THE BRAS PIN ON VALVE . HAD THE PROBLEM YEARS AGO !

[sean68charger]

Remove the factory unit and put in an adjustable proportioning valve.

yep. i had the same problem when going to wider rear tyres-this is probably why the rear breakes away first as your changing the factory settings, fitting the adjustable valve is the way to go

How did you install it? Any pics?

installed instead of the original valve with the knob facing downwards, screwed to the chassis rail. its easy to adjust then. will try to get some pics for ya

[Steve P.]

Before you do anything else, pull both rear drums and check for rear end  and brake fluid leaks. Both will make your brakes grab. 

Also keep in mind that the wider you go with tires the less PSI you have across the tire width. In other words the wider the tire the less friction pressure you are putting to the ground.

[T16]

I,ve got this problem on my 72 to.. but only on the pass. side. That wheel is the first to lock up 

[Steve P.]

I,ve got this problem on my 72 to.. but only on the pass. side. That wheel is the first to lock up 

Very often you will find that a wheel cylinder is rusty and NOT working. Let's say the pass. side is rusted and not moving. This will make all the fluid going to the rear brakes go only to the Drivers side since the pass. side cannot move. In this case the drivers side will lock up.

Allllllways change both front or rear calipers, hoses and or wheel cylinders in pairs. Alllllways bleed the shit out of the system while doing so..

[T16]

I,ve got this problem on my 72 to.. but only on the pass. side. That wheel is the first to lock up 

Very often you will find that a wheel cylinder is rusty and NOT working. Let's say the pass. side is rusted and not moving. This will make all the fluid going to the rear brakes go only to the Drivers side since the pass. side cannot move. In this case the drivers side will lock up.

Allllllways change both front or rear calipers, hoses and or wheel cylinders in pairs. Alllllways bleed the shit out of the system while doing so..

Wheel cylinders are new.. front pistons rebuilt.
New fluid and bleeded.. Only thing left to do is new rear shoes and maybe resurface the drums (one got som nasty grooves)

[Steve P.]

T-16,,, the rear wheel cylinders are fed evenly by a (Y) block. You need to check the lines from the (Y) to the cylinders for a kink or flat spot. This is just part of any good brake job.

Were the cylinders leaking when you changed them out?? Any time you leak fluid or gear oil onto your shoes I suggest changing the shoes and cutting the drums. I know many just clean the shoes up and get away with it, but if the fluid has soaked in you cannot get all of it cleaned off. Cleaned up shoes are not worth a collision to me. New every time..

If your drums do not measure close to the same after turning them I also feel they should be replaced. A thin drum can be heated quickly and cause all kinds of grabbing and pulling. Again, we are not talking about a throw away pile of crap. We are talking about a prized Mopar.. Not to mension your life or someone Else's..

A simple rule I live by.  If it doesn't start I won't get killed in it. If it doesn't stop I easily could..

[Charger-Bodie]

T-16,,, the rear wheel cylinders are fed evenly by a (Y) block. You need to check the lines from the (Y) to the cylinders for a kink or flat spot. This is just part of any good brake job.

Were the cylinders leaking when you changed them out?? Any time you leak fluid or gear oil onto your shoes I suggest changing the shoes and cutting the drums. I know many just clean the shoes up and get away with it, but if the fluid has soaked in you cannot get all of it cleaned off. Cleaned up shoes are not worth a collision to me. New every time..

If your drums do not measure close to the same after turning them I also feel they should be replaced. A thin drum can be heated quickly and cause all kinds of grabbing and pulling. Again, we are not talking about a throw away pile of crap. We are talking about a prized Mopar.. Not to mension your life or someone Else's..

A simple rule I live by.  If it doesn't start I won't get killed in it. If it doesn't stop I easily could..

well said !!

[resq302]

Something else to think about..... are your rear shoes equally adjusted?  If they are not, one might be grabbing before the other as it might be closer to the drum versus the other side.

[is_it_EVER_done?]

I agree with the others that the first things to check are brake lining contamination, adjustment, and add an adjustable proportioning valve, but understand that under hard braking, drums will always lock up at some point. This is why new cars have rear disks instead of drums.

The design of drums will cause them to "cam" lock at some pressure point no matter how tuned/adjusted the system is. This is usually not a problem, but with enough brake pressure, they will lock no matter how adjusted/balanced everything is, and do so without warning or change in modulation of the brake pedal.

Up until the point of lock-up, they will brake as well as disks if everything is in good condition, adjusted, and not over heated, but you need to make sure everything is in good working order.

[T16]

Come to think about it.. I had a leak from the axle seal at that side..

Soo i guess problem solved 

[Steve P.]

Rear brake "camming" is a design flaw, for lack of better words. It is caused by racking of the shoes during braking.  The better the condition of your system and the better adjusted, the more control you have over full on camming or lock up.  I came across a quick video that will better explain. Most people don't really understand how a drum brake system works on cars and light trucks. Follow the link to get a better understanding.

http://auto.howstuffworks.com/drum-brake1.htm



Now that you have sen it in action you can understand why having your shoes adjusted up right is so important. Also why having drums with plenty of meat is so important.  The farther the shoes must travel to make contact the harder the camming action will be.

Another common problem I came across when finding brake problems on customer cars was improperly installed shoes.   Yes, for those of you that don't know it, there are (2) sets of shoes that come in every box of replacement shoes. ( Set 1) is the PRIMARY shoes. These shoes ALLLLLLWAYS go toward the FRONT of the car.  (Set 2) is the secondary which go to the rear. This is very important as putting them on wrong can make for some terrible braking control.

I have seen 2 primaries on the same side. Primaries put on toward the rear and mixed. Meaning a primary on the front on one side and on the rear of the other... All of these will cause your rear brakes to act crazy.

Like I said before, your brakes are the most important part of your car. Check and double check that you are installing them the right way and with top of the line parts. The payback is your life and others. 

[Steve P.]

Come to think about it.. I had a leak from the axle seal at that side..

Soo i guess problem solved 

NOPE...  Problem not solved until you have no leaks and your braking surfaces are excellent.

Now get to work.. 

[NYCMille]

Rear wheel lock up can be reduced by putting in knew rear wheel cylinders that have a small diameter this will reduce the braking force to the rear wheels.

[BrianShaughnessy]

Rear wheel lock up can be reduced by putting in knew rear wheel cylinders that have a small diameter this will reduce the braking force to the rear wheels.

My 7/8" bore wheel cylinders work much better than the 15/16" I had.   This is from ye olde Disc-o-tech article.

There are other ways to alter front/rear balance. One way, which reduces rear lockup, is to use a 7/8-inch rear wheel cylinder in place of the standard 15/16-inch unit. Try Raybestos part number WC37236 for this. (Original application: late '70-early '80s light-duty Dodge trucks w/10-in. rear drums).