Brake sytem - adjustable proportioning valve ??

[rodrun35]

I have just completed plumbing the brakes on my 35 pickup.  It is a front disc / rear drum system with an under the floor corvette power booster / master. The shop that sold me the booster told me that I didn't need a 2 lb. residual valve up front because the master has it already built in.  The rear line has a 10 lb residual valve and adjustable proportioning valve.  In order to test the system, where do I start in terms of the adjustable proportioning valve.  The knob on the proportioning valve has arrows to turn to "increase" or "decrease".  To start out, should I turn the knob all the way in the increase direction or all the way in the decrease direction or somewhere in between?
I read some of the tips on the Master Power brake site but am still confused.

Thanks for your thoughts.

[DocsMachine]

Start with it all the way out. The proportioning valve is just a needle-and-seat restriction; it doesn't limit the overall pressure that goes to the brakes, it can only slow the rate of pressure increase.

On mine, a Behr (WilWood copy) "increase" means "increase proportioning"- add time to the pressure curve, which simply means increase the restriction.

Setting it can only be done seat-of-the-pants; Find a quiet street or open, empty back corner of a parking lot and try hard braking runs, and experiment with the diffeetn settings. Easiest thing to do is try one run wide open (on the valve, that is) and another full closed. (Most P-valves only add about 40%.)

Now, I might be off on this, but I think you might have some problems with the setup because the 'Vette master cylinder is a disc/disc unit. Drums need less pressure and less volume to actuate (smaller cylinder and the self-energizing nature of drum brakes) so it's entirely possible you'll find that the rears are doing all the work, even at full proportioning.

Doc.

[rodrun35]

Doc,

Thanks for the input.

Let me make sure that I have this correct.  When you say start with the valve all the way out you mean - turned as far as you can go in the "decrease" direction.  Correct?  If the brakes lock up right away then "increase" the proportioning which will slow the rate at which the brakes lock up.  Correct?

regarding the master:  Hopefully, I have the correct power master.  The shop I bought it from specializes in street rod building and this is what they use on all their builds.  So, I assumed it to be correct.   It could be that the master is simple a Corvette "style" master.  I guess I'll find out.

[DocsMachine]

The valve is just a needle-and-seat; knob all the way out is less restriction, knob all the way in is more restriction, just like a carb idle-mixture adjustment.

The more restriction, the longer it takes that pair of brakes to come up to line pressure. If you try a panic stop with the knob wide open, and only the rears lock up, try it again with it full closed.

If you're lucky, you'll find a "happy medium" in between somewhere. Also, don't bother playing with it until the brakes are well adjusted and broken in. If you find out, for example, the fronts aren't being held in place by the residual valve, the extra puck travel will mess with the feel, and therefore how you set the P-valve.

Doc.

[PeterR]

The valve is just a needle-and-seat; knob all the way out is less restriction, knob all the way in is more restriction, just like a carb idle-mixture adjustment.

The more restriction, the longer it takes that pair of brakes to come up to line pressure. Doc.

As Docsmachine has explained a line restriction device can only produce a pressure reduction while fluid is moving, so can only affect the rate of rise, not the front/rear balance with a steady pedal effort.

Pressure reducing valves are a totally different device that permit the steady state front/rear balance to be adjusted.   This type of valve passes the full pressure to the rear circuit until a preset pressure is reached.   When this "cracking point" is reached the valve then passes through a portion of any additional input pressure.    Most OEM and after market valves pass through 50% of the input pressure above cracking point.

To demonstrate the action of these valves assume for the moment the cracking point has been set to 500psi and the input pressure is progressively increased from zero to 1000psi.

Up to 500psi the output pressure is exactly the same as the input
Beyond 500psi, 50% of the additional pressure is allowed through.

Pressure in.. 0 100 200 300 400 500 600 700 800 900 1000
Pressure out 0 100 200 300 400 500 550 600 650 700 750
Actual ratio___ 1.0_1.0_1.0_1.0_1.0 092 0.96 0.81 0.78 0.75

If the adjustment is wound right up to give a cracking point close to 1000psi, then the output pressure will be the same as the input right up to that value.

Pressure in.. 0 100 200 300 400 500 600 700 800 900 1000
Pressure out 0 100 200 300 400 500 600 700 800 900 950
Actual ratio___ 1.0_1.0_1.0_1.0_1.0_1.0_1.0_1.0_1.0_0.95

If the adjustment is wound down so the cracking point is at zero, then the 50% pass through operates over the full input range.

Pressure in.. 0 100 200 300 400 500 600 700 800 900 1000
Pressure out 0 ..50 100 150 200 250 300 350 400 450 500
Actual ratio____0.5_0.5_0.5_0.5_0.5_0.5_0.5_0.5_0.5_0.5

This is the basis of the advertising specifications for these valves as being adjustable over the range of 50% to 95%.

The effect of these valves is quite different from the action of a dual cylinder with balance bar which allows the front/rear ratio to be adjusted, but maintains a consistent ratio over the full pressure range.