Bad rear lockup--prop valve or smaller cylinders?

Posted By: gd9704

Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 06:09 PM

Okay, my other project for spring is to reduce the tendency of the Demon's rear tires to lock up under hard braking.

I bought the smaller wheel cylinder's from E-booger and was about to go that route--but my cousin who's much more auto-savvy than me told me that a proportioning valve on the rear brakes hard line would be a better way to go.....

What say you?

Thanks
Greg

Posted By: bboogieart

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 06:14 PM

Rick Ehrenberg knows his stuff
nothing against your cuz.
there should already be a proportioning valve in place.

Posted By: John_Kunkel

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 06:17 PM

Either way will work but changing cylinder size is a guessing game while the adjustable valve takes the guess work out of it.

Posted By: gd9704

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 06:25 PM

Quote:

Either way will work but changing cylinder size is a guessing game while the adjustable valve takes the guess work out of it.

True....and I would have the ability to fine tune it as well....

Thanks, John.

Posted By: gd9704

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 06:40 PM

Quote:

Rick Ehrenberg knows his stuff
nothing against your cuz.
there should already be a proportioning valve in place.

I know Ehrenberg is the man.

The factory proportioning valve is in place.

The brakes work very well--but panic stops are very hairy--now with my wife and 2 year old in the car I worry about having to hit the brakes hard on the freeway......

My cousin was wrenching on these cars all through the 70s and 80s before they became "collector" cars. I respect his opinion on these things.

I like to get a number of opinions before I decide to screw something up!

Posted By: SCATPK

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 07:34 PM

I just installed disc brakes on my 66 dart and used an adjustable prop valve to eliminate rear wheel lock up. This is the way to go as you can tune it for your specific car and its suspension and tires. After fine tuning mine it is amazing how good it stops.

Posted By: CrAlt

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 07:59 PM

Have you pulled the drums and looked at the brakes?

My dart started having a rear lockup problem... Then i pulled the drums and found that the leaky axle seals softened up the brake shoes and made them grabby.

Posted By: MarkD

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 09:25 PM

I put front disc rear drum on my Dart with an adjustable prop valve. I still need to go with the smaller wheel cylinders. The proportioning valve is adjusted out. That's why E man suggests the smaller wheel cylinders.

Posted By: GODSCOUNTRY340

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 10:44 PM

In '68 &'69 Mopar used a delay valve to cure your problem. It was located just behind the proportioning valve. (pic). These were installed on High Performance vehicles. In '70 Chrysler redesigned the proportioning valve so this wasn't needed. It delayed the pressure to the rear brakes so they wouldn't lock-up first.

Attached picture 4316226-DSC04267.JPG

Posted By: GODSCOUNTRY340

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/30/08 10:46 PM

Here's another pic in relation to the distribution block.

Attached picture 4316233-DSC04266.JPG

Posted By: Rick_Ehrenberg

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/31/08 02:31 AM

You really need to get the proportioning close w/o a valve (by juggling wheel cyl / caliper piston diameters, etc), then use the valve for the last 10% or so. Here's why:

The valve doesn't "cut in" - begin to work - until there a few hundred PSI of line pressure. So in rain, etc., it doesn't do anything. And, if the rear wheels are locked, the car will - MUST - spin!

Rick

Posted By: 5spdcuda

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/31/08 03:32 AM

It took both to get my Cuda to stop to my satisfaction. Weight distribution and the relative grip of the front and rear tires all play a part in getting it right.

Posted By: gd9704

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/31/08 12:19 PM

Quote:

You really need to get the proportioning close w/o a valve (by juggling wheel cyl / caliper piston diameters, etc), then use the valve for the last 10% or so. Here's why:

The valve doesn't "cut in" - begin to work - until there a few hundred PSI of line pressure. So in rain, etc., it doesn't do anything. And, if the rear wheels are locked, the car will - MUST - spin!

Rick

Wow, thanks for the personalized response, Rick!

I guess I was hoping to avoid taking the rear brakes apart....but I guess this will be an educational experience!

Posted By: UCUDANT

Re: Bad rear lockup--prop valve or smaller cylinders? - 03/29/09 03:42 PM

Rear brakes are fairly simple. Spring tools will help make the job a snap. Do one side at a time so you have an assembled example. Check to ensure your drums are not heavily grooved, glazed or out of spec.