Do I really need a rear proportioning valve on 4 wheel disk

[mquinn66]

I just did a 4 wheel disk brake conversion on a 66 charger. I am doubting if I really need an adjustable proportioning valve for the rear line. I am using the bendix style booster kit with dual master cylinder that is from inline tube. I have read that i just need to remove the residual valve from the rear line on the master cylinder. Cost of adding a valve is not the issue, i just would rather keep it clean and simple if i dont need it.

 

[Don Frelier]

You could just test it without it, see if the rears lock up too easily.
If they do then add it later, doesn't seem like a big deal to do it afterwards.
But a note of caution, you could put flat spots in your rear tires.

 

[steve from staten island]

What does the company that supplied conversion say? If any doubt I'd call Dr Differential

 

[mquinn66]

What does the company that supplied conversion say? If any doubt I'd call Dr Differential

I havent called inline tube because I assume they are more of a brake line specialty company.

 

[mquinn66]

You could just test it without it, see if the rears lock up to easily.
If they do then add it later, doesn't seem like a big deal to do it afterwards.
But a note of caution, you could put flat spots in your rear tires.

Good point, I think i will test and add it only if needed. Thanks

 

[Dreamcatcher]

Because of weight front verses rear you really need less brake pressure in the rear. Should be at least 60/40 or even 70/30. When the rear brakes harder than the front the back end will pass the front if a hard brake circumstance arises.

 

[Don Frelier]

Just an FYI, but I have SSBC discs on the front of mine.
Manual 1970 vintage master cylinder with stock rear drums.
Mine stops real well, but I found out the hard way that the rears definitely lock up on a panic stop.
Yes it was tricky to keep the car straight and not hit the guard rail.
A close one...

 

[Hey-O]

I've done many conversations on chevys and found it to not be a problem, but they do hit different on hard braking.

 

[themechanic]

There is a reason there are separate MCs for disc/drum and 4-wheel disc braking systems. So, I question the wisdom in using a disc/drum brake MC on a 4-wheel disc system even without the rear brake residual valve.

Mine are from Dr Diff. He said his system was designed to use a MS for disc/disc and does not need an adjustable prop valve due to the ratio of front braking surface to rear.

If you have the same amount of braking surface in the rear as you do in the front then you may need a prop valve.

 

[Kern Dog]

On a 4 wheel DRUM car, the wheel cylinder sizes dictate the proportioning. 4 wheel DRUM cars used no proportioning valve.
On Disc/Drum cars, the disc calipers needed a much higher volume of fluid to operate properly so a proportioning valve was needed.
A 4 wheel disc setup, the caliper sizes are supposed to dictate the proportioning the same way that a 4 wheel drum system does. NO proportioning valve is needed unless the rear calipers are too large.
My '70 Charger has a 2.75" single piston front caliper and a 1.5" single piston rear. I have no proportioning valve and the car stops straight with no bad habits.

 

[themechanic]

My 4-wheel kit from Dr Diff includes the following:

Front disc brake kit uses a Mopar 11.75" disc and single piston caliper (piston diameter unknown).

Rear disc brake kit uses a 10.7" disc and a Mustang Cobra caliper (piston diameter unknown).

Cass told me the front to rear ratio is matched to eliminate the need for an adjustable proportioning valve and can just use the drum brake distribution block.

 

[JoePole]

Best way to do a braking test is on wet pavement. Find yourself a large lot with PLENTY of room and NO hazards (I used a college lot with school on holiday). Run your car up to a pretty good speed (35-40) and panic stop. Fronts should lock up just before the rears if functioning properly. This will test worst case conditions.
FWIW.....I added a summit prop valve mounted on the frame about equal with the front of the drivers side door on my B-Body.

 

[Kern Dog]

My 4-wheel kit from Dr Diff includes the following:

Front disc brake kit uses a Mopar 11.75" disc and single piston caliper (piston diameter unknown).

Rear disc brake kit uses a 10.7" disc and a Mustang Cobra caliper (piston diameter unknown).

Cass told me the front to rear ratio is matched to eliminate the need for an adjustable proportioning valve and can just use the drum brake distribution block.

This is essentially the same setup I have. Dr Diffs Mustang caliper is 1.5". The front could be either the 73-early 76 A body 2.6" caliper or the late 76-89 A, FMJR 2.75" caliper.