Mopar Muscle Association UK

my 68 charger has reconditioned original factory bendix 4 pot calipers up front and standard drums on the back. under heavy braking the back always locks up first. is there a restriction devise on the rear brake hydraulics that could be faulty?, or is there an after market thingy which i could fit to stop so much pressure at the back cylinders?

There's a fixed proportioning valve on all the factory setups, I'd guess your new calipers need more fluid than stock, causing the locking up asa knock on effect. I bet Wil can get you adjustable proportioning valves that would suit

According to Don he sees this all the time, when cars are converted from drum to disc on the front a lot of people don't realise the sytems are totally different.

on a drum brake car the proportioning is done by using bigger drums on the front and the line pressure is 50/50.

On a disc car the proportioning valve does this, apparently a lot of owners in the states convert to disc on the front but don't fit the valve.

I know this ain't your problem, if your car was disc all along then it will have the valve.

Now here's a question... I seem to remember a long time ago reading somewhere that in the disc setup there's also a shuttle valve that delays the rears slightly on heavy braking, i'm not 100% on this but maybe Alex would know ?????

If you need an adjustable valve for the rear line then i'll see what I can do, would need more info on the front calipers though

front calipers are the original stock 4 pot bendix units. ssbc rebuilt them for me last year, if there is a proportioning valve, perhaps it is seized?

Perhaps it is seized or certainly not working correctly, check it out first as if it's all stock stuff then the rears shouldn't lock first

Ma Mopar had a "one size fits all" type of policy when it came to these valves.
OK they had more than one size but many models used the same size valve in their 340 cars as their 440 cars for example.

Also fitting wider tyres than stock on the back effects the braking force of the back wheels.

I personally think EVERYONE should use an adjustable unit and you should test your car to make sure it is doing what it is supposed to be doing. It might save you from a nasty accident one day.

Took this froma Mopar action tech site the full deal is here

http://www.moparaction.com/Tech/archive/disc-main.html

note below there's a number for an adjustable valve

"Next, you'll need a rear brake proportioning valve. The valve may be of the stock non-adjustable variety (which, in some years, was built into the brake tee assembly.) Ideally, though, for ease of installation and optimization of front/rear braking balance, the Mopar Performance/Wilwood "add-on" adjustable valve, Mopar #P5249088, is highly recommended, and, therefore, was used in our swap. The standard, nonadjustable valve was, of necessity, a compromise. The same valve was used on wagons and 2-door coupes, slant sixes and Hemicars, meaning that some cars rolled out of the assembly plant with less-than-ideal proportioning, characterized by a tendency toward early rear-wheel lockup.

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)."

I used a Summit Racing own branded valve. It is just a re-labled Wilwood unit but much cheaper.

thanks for the advise, i assume the valve fits in line-to the rear brakes and does it come with fittings? and roughly how much?
thanks

Looking at the Summit one (only a few cents cheaper than the Wilwood these days by the look of it) you do not get the fittings for the brake line.

http://store.summitracing.com/default.a ... 07&NeXID=5