How risky is it to not use a proportioning valve on a power front disc brake conversion? Thanks

I can't say how risky it would be but I would not drive mine without the prop valve. Without the pop valve, your front brakes will be nearly all the stopping with the rears next to no stopping power. I just don't like the idea of hardly no rear brakes...


Kev

I agree. I remember back in the early 1990's a place I worked at had a 1986 Chevy Celebrity station wagon. Maybe it was a Cavalier Wagon. It's been so long I forget. We used that car for various work things. The proportioning valve went bad and the brakes locked up all the time. It was the rears which locked up even on a nice dry summer day. On rainy wet days it was dangerous.

I would put a proportioning valve in it. Forget about having it under the frame rail like an SS. That's a dumb place the factory used. I would place it right next to the Master Cylinder which lessens exposure to dirt & road grime. P-Valves under the frame rails back in the day would see dirt, road grime & road salt in harsh climates.

The bottom line is: please use a proportioning valve. You will be better off in doing so.

I am not sure on how dangerous either but I'm running the stock 4 wheel drum splitter and did a power disc/drum set up and have had no issues to date. I do run big and littles so maybe that helps ?

The proportioning valve keeps the rears from locking up too soon. If they do, the back end is liable to come around on you in a hard stop. The combo units also have a residual pressure valve to keep the rear wheel cylinders from retracting all the way for better response and pedal feel.

I can't say how risky it would be but I would not drive mine without the prop valve. Without the pop valve, your front brakes will be nearly all the stopping with the rears next to no stopping power. I just don't like the idea of hardly no rear brakes...


Kev

It's just the opposite. The prop valve reduces the pressure to the rear brakes and keeps them from doing most of the braking under normal easy braking. You will also want a metering valve to meter the fluid to the front discs and a residual valve to keep the rear wheel cylinders from emptying out when you release the pedal.

A proportioning valve limits fluid pressure to the rear brakes under hard braking to compensate for weight transfer and the servo action of rear drum brakes. A 4-wheel disc system will not require as much proportioning as a disc/drum setup assuming the caliper piston areas are in the correct front/rear ratio.

A metering valve limits fluid pressure to the front brakes in a disc/drum combination under light braking. A disc brake requires very little fluid pressure to make some friction while a drum brake must overcome the return springs. The metering valve holds off the front discs to allow the rear drums to make some brake first under light braking. This is not necessary in a disc/disc setup.

A residual pressure valve holds a few lbs of pressure in the line at all times. It only applies to drum brakes or when the master cylinder is mounted below the wheel cylinders/calipers. In a drum brake, when you release the pedal the fluid pressure immediately drops to zero but the return springs take a little time to fully return the shoes. This can allow the fluid pressure to drop below atmospheric for a bit which can let a little air to be ingested past the wheel cylinder cups, hence the residual pressure valve. In most factory cars, the residual pressure valve is located in the master cylinder outlet(s). This is the reason for changing the master cylinder that is otherwise identical when changing from drum to disc brakes.

As said above, run a proportioning valve especially in a disc/drum combination. An adjustable valve is best if you take the time and effort to adjust it. I learned this one the hard way. My first car (a Ford) would lock up the rear brakes before the front under hard braking after converting the front brakes from drum to disc. I was young and dumb enough to use the proportioning valve from the same boneyard car (identical to mine) that I got the disc brakes from. I spun the car in a panic stop in a corner. The car was totalled and I took a few stitches and was plenty sore for a while. I consider myself lucky as all it cost me was a car and a major chewing out from my father. Don't repeat my mistake-use a proportioning valve!

It's just the opposite. The prop valve reduces the pressure to the rear brakes and keeps them from doing most of the braking under normal easy braking. You will also want a metering valve to meter the fluid to the front discs and a residual valve to keep the rear wheel cylinders from emptying out when you release the pedal.

Yep...
I should have said no front braking power. I have tried the basic pop and the adjustable and once dialed in, its great. If you play with it, you can use it almost like a line lock where the front brakes are holding while the rears have no braking. Just remember the settings (turns) and write it down for ease of making the correct change back and forth.

Kev

http://www.racerpartswholesale.com/images/901000.gif

I used a lever style on our road racer and put it right next to the driver's seat. There are 7 different settings you can select from while you're flying down the road.

WOW!!! Lots of good info. Thanks Guys. I will be putting in a valve.

To add to what red67camaro said, there are two types of residual pressure valves, 2lb for discs, and 10lb for drums.

Hey just to add this in I have a brand new SSBC proportioning valve I bought for a project but I sold car. If anyone needs I would like to sell for $25 . Hickory NC. CLICK > SSB-A0707 (http://store.summitracing.com/partdetail.asp?autofilter=1&part=SSB%2DA0707&N=700+%2D48578+115&autoview=sku)

Good look RAYZZ:secret:

I finished installing the power brakes yesterday and had to give it a try. Although I have not installed the valve yet, it brakes well. However, only the left rear tire is locking up. Any ideas?

Bleed, Bleed, Bleed !

You will be fine after bleeding all the air out . BTW there is a POP valve in the Classifieds here on TC for $25.00..


Kev

I will try bleeding it some more. Thanks

I finished the installation of the proportioning valve today and it works great. Thanks for all of the input.