Rear Brakes locking up issue

[mr. fix it]

ok so last night I had a near miss and discovered that my rear brakes like to lock up in a panic mode.

I have converted from a single manual pot M/C to a P/B dual pot drum brakes all around.
- P/B pedal assembly bought from Murray Park 66 Polara and installed.
I used a 1967 spec dual Drum brake set up with P/B.

All 1967 spec C body parts.

Previous threads have shown that a proportioning valve is not required but simply plumb the front brakes into the distribution block, block off the 3rd open port then connect the rear brakes to the line running to the back of the car.

Lastly the M/C
The rear pot feeds the front brakes
The front pot feed the rear brakes.

System feels decent brake pedal with no pulling or grabbing.

The Front brakes are all B/N parts le3s the backing plates and spindles

Rear brakes are brand used drums slightly oversized

All new Wheel cylinders, all new steel & flex lines less two front lines which are like brand new and push fluid with no restrictions

Anyone following my work has witnessed the new parts being thrown at this car.

I have checked the adjustment of the brakes a number of times so I am confident that adjustments are not the cause.

Finally I was speaking with another Mopar local retired shop owner and he told me that regardless of being a drum or disc setup, if you run a dual M/C you need a proportioning valve.

What has everyone here done that has done the same thing as I have?

 

[70bigblockdodge]

By distribution block I'm guessing you are talking about the 3 way split left over from single pot. If not and you used a later model switch block and rears are not running through it that is most likely your problem. If done with 3 way from original I would say your shoes are not set in to drums( arc is not matching drum ) or your proportioning is way out and you may need a valve. Are the rear wheel cylinders a large diameter for some reason?

 

[mr. fix it]

I have only the splitter from the original setup.

I have adjusted the shoes a few times but will recheck the adjustment.
Thanks.

 

[commando1]

Refresh my memory of a pic of the type of block the lines from the booster go to.

 

[70bigblockdodge]

Had one large line from single to a T or 4 way, he removed the line to rear brakes and plumbed straight into front pot on dual M/C, rear pot plumbed to T or 4way with rear port plugged(front brakes). Should be fine, has to be in the self energizing and the arcs not matching. If it is not a low speed grab you may be due for a adjustable proportioning valve, which is not a bad idea anyway. You can also see if you can get smaller piston dia. wheel cylinders. Fronts need to lock first or you will come around.

 

[mr. fix it]

Had one large line from single to a T or 4 way, he removed the line to rear brakes and plumbed straight into front pot on dual M/C, rear pot plumbed to T or 4way with rear port plugged(front brakes). Should be fine, has to be in the self energizing and the arcs not matching. If it is not a low speed grab you may be due for a adjustable proportioning valve, which is not a bad idea anyway. You can also see if you can get smaller piston dia. wheel cylinders. Fronts need to lock first or you will come around.

Thankfully this did not happen but I sure suffered from a severe pucker affect during the panic stop...

 

[mr. fix it]

Had one large line from single to a T or 4 way, he removed the line to rear brakes and plumbed straight into front pot on dual M/C, rear pot plumbed to T or 4way with rear port plugged(front brakes). Should be fine, has to be in the self energizing and the arcs not matching. If it is not a low speed grab you may be due for a adjustable proportioning valve, which is not a bad idea anyway. You can also see if you can get smaller piston dia. wheel cylinders. Fronts need to lock first or you will come around.

The braking system has seemed to be disappointing even last year upgrading to P/B and dual M/C but I didn't notice this issue other than not having the firm pedal like a modern car has but this may have been due to still having 1966 style brake drum setup.

At least I didn't have to use 2 fett to stand on the brakes afterwards
I tried it again and got the same results today but will go back to the drawing board if needed and add the proportioning valve PDQ if I can't get it to stop stopping the way it is stopping...

 

[Big_John]

Finally I was speaking with another Mopar local retired shop owner and he told me that regardless of being a drum or disc setup, if you run a dual M/C you need a proportioning valve.

I think he's confused about the distribution block. For some reason, even usually knowledgeable people think that distribution block is a proportioning valve. All the drum brake cars never had a proportioning valve and a lot of the disc/drum cars don't have one either.

A proportioning valve is a good idea, but I have never even heard of the need for one on a drum brake car.

Are the brake shoes installed correctly? Long shoe towards the rear? That's a problem that happens a lot.

 

[mr. fix it]

I think he's confused about the distribution block. For some reason, even usually knowledgeable people think that distribution block is a proportioning valve. All the drum brake cars never had a proportioning valve and a lot of the disc/drum cars don't have one either.

A proportioning valve is a good idea, but I have never even heard of the need for one on a drum brake car.

Are the brake shoes installed correctly? Long shoe towards the rear? That's a problem that happens a lot.

yes sir, the shoes are correctly installed. I learned that lesson back in 1978 with my first car! LOL!!!

Actually Short on the one side and the long on the other side...

Distribution blocks offer no proportioning or balancing of the front to back brakes...

Again the old timer I spoke with showed my his Belvedere wagon parked right beside me last night and he insists I need one and as soon as possible.

 

[66/440]

Refresh my memory of a pic of the type of block the lines from the booster go to.

Multiple styles of distribution blocks with and without proportioning valve. Unit on right has proportioning valve built in.

 

[mr. fix it]

are you sure the one on the right isn't a proportioning valve block?

 

[saylor]

+1 i was told that one is a combo block... at least the one on my 68 fury power drum (stock) was a combo block..

try not to die - for real

- saylor

 

[70bigblockdodge]

yes sir, the shoes are correctly installed. I learned that lesson back in 1978 with my first car! LOL!!!

Actually Short on the one side and the long on the other side...

Distribution blocks offer no proportioning or balancing of the front to back brakes...

Again the old timer I spoke with showed my his Belvedere wagon parked right beside me last night and he insists I need one and as soon as possible.

Proportioning valve is probably your best bet and not big$ so why the hell not.
Put both short linings on the right rear of a open rear, easier to overcome and do smokey burnouts

 

[66/440]

are you sure the one on the right isn't a proportioning valve block?

The one on the right DOES contain a proportioning valve. Sorry for not labeling all the photos

 

[mr. fix it]

The one on the right DOES contain a proportioning valve. Sorry for not labeling all the photos

no problem, I didn't want others being confused and jumping in all over your reply.

 

[mr. fix it]

The one on the right DOES contain a proportioning valve. Sorry for not labeling all the photos

no problem, I didn't want others being confused and jumping in all over your reply.

Proportioning valve is probably your best bet and not big$ so why the hell not.
Put both short linings on the right rear of a open rear, easier to overcome and do smokey burnouts

With friends like you who needs enemies right??? LOL!!!!
is that a poor man's line lock?

 

[mr. fix it]

The one on the right DOES contain a proportioning valve. Sorry for not labeling all the photos

no problem, I didn't want others being confused and jumping in all over your reply.

Proportioning valve is probably your best bet and not big$ so why the hell not.
Put both short linings on the right rear of a open rear, easier to overcome and do smokey burnouts

With friends like you who needs enemies right??? LOL!!!!
is that a poor man's line lock?
And here I was thinkin' I would grease the rear shoes up!

 

[66/440]

I have read that when you do the type of conversion that you did(with no proportioning valve) the diameter of the rear wheel cylinders need to be changed so that they apply slightly less pressure to the rear drums so that you do not experience the lockup you described. I can not remember if you need a larger or smaller wheel cylinder. I will look around and see if I can find some more info.
But a proportioning valve would solve the problem.

 

[Big_John]

I have read that when you do the type of conversion that you did(with no proportioning valve) the diameter of the rear wheel cylinders need to be changed so that they apply slightly less pressure to the rear drums so that you do not experience the lockup you described. I can not remember if you need a larger or smaller wheel cylinder. I will look around and see if I can find some more info.
But a proportioning valve would solve the problem.

I've only heard of needing to do that with disc brake conversions.

 

[70bigblockdodge]

With friends like you who needs enemies right???

Always thinking.lol