Rear brakes lockup

[cudajim]

I have a 69 Barracuda fastback and have always had a problem with the rear brakes locking up sending the car into a spin. I updated the drum system to 72 front disc (small bolt pattern) and changed the master cylinder and proportioning valve at the same time. All parts came off a donor 72 Valiant but the master was no good so it's a rebuild. I have the cheapie stock rear end and drum brakes and all brakes are new and installed correctly. I bled the system several times. The car never stops well but is slightly better than the old drum brakes. I also kept the power booster from the drum brake system. Any ideas on how to keep em from locking up?

 

[dusterny]

That could be a huge problem in a panic stop. I would try a adj. prop valve. MP ,Jegs and Summit all sell them. You could balance your system that way. About 70 percent of your stoping power should be on the front.

 

[7demon2]

i am so glad you posted this! i am going to search to see if there is an existing thread about this and if there isn't i will make one. your problem is exactly what i am experiencing. i have a 1971 dart swinger. it was a 4 wheel drum brake car. i upgraded it to a lbp front disc setup and the rear was kept as a drum but it was upgraded from 10x2' to 10x 2 1/2' rears. well i have a bad rear brake lock up problem. it stops greak from 20 to 25 mph but above that it wants to swap ends on me. i was thinking a new portion valve might fix it until i read the article here :http://68cuda.com/disk.html. towards the bottom it says
"BRAKING IMBALANCE
Having fat front rotors is a treat. I can now almost out brake most cars on the road. However there is a common problem with this setup that is easily remedied. The back brakes tend to lock up before the front brakes which is not good (fishtail in the rain). Solutions below:

1) My 8 3/4 from a Fury cop car with 11" drums uses a 15/16" rear wheel brake cylinder. Look closely at your rear end as you may have the same size, if you are unsure change it anyway as cylinders are $9.00 or so and why not put new ones in your car. If you swap out to a 7/8" or 13/16" version (old mopar P.N.# 2530136) the rear brake action will be lessened. These wheel cylinders were supposedly used on vans and trucks in the 80's."

a light went on with me then. i also found this article as well from mopar action: http://www.moparaction.com/tech/archive/disc-main.html

further down it says:

"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).

The second variable is caliper piston size. The slider-type calipers used on 1973-‘75 A-bodies used a smaller piston: 2.60" as opposed to the more common 2.75". Using these can be helpful if your car has a “reverse” proportioning problem and tends to lock the fronts first, usually only a problem with no-option, slant-six cars or lightened drag cars.

well i am going to do this when i get the chance and see if it fixes my problem. again i am going to post this in a new thread. if this is common knowledge to everyone else here well....i guess i missed it, but at least i know now!

 

[dusterny]

I had a 70 Duster 340 car brand new from factory that did the same thing. Wet roads and Goodyaer polyglass tires wow It would switch ends in a heartbeat. Let us know how you make out. Go to a wet parking lot and jump on your brakes hard and see what you got.

 

[cudajim]

I already have an adjustable proportioning valve which did little to fix the problem. The rears locked up even before I changed to front discs so I'm thinking there is something wrong with the rear shoe setup.

 

[MVRCorp]

My first thought was leaking rear wheel cylinders but it sounds like you changed them. Did you put new spring kits on the rear? Are the drums up to min. tolerances? As a last desperate effort you could eliminate the self adjusters and manually set them up. That sure beats the car switching ends on hard braking.

 

[cudajim]

I replaced everything in the brake system including shoes, slave cylinders, springs etc, etc. It's all stock stuff for a 69 Barracuda and it has the crappy rear end in it. The front brakes are dual piston type with a small bolt pattern. The only thing I did not check are the drums to min tolerance but they have been turned.

 

[pntastar69]

Were the brake hoses replaced? The hose may be collapsing inside when braking and not allowing the fluid to return to the m/c when the pedal is released.

 

[cudajim]

I think the front hoses are new but not sure about the rear one. I have 6 cars so I don't always remember what was done to each. 4 of the cars are Mopars! The others are a Lexus and a Toyota.... no Chebies here and certainly no Fords. I grew up in Detroit where all my buddies had Mopars!