Adjustable Brake Proportioning valve

[Auggie56]

Just what does a PV actually do? I have rear brake lockup under heavy braking. Does it reduce volume or pressure? I bought one off EB and now, even adjusting the valve up and down no fluid is getting past that valve. To me pressure reduction would be what I'm looking for, as volume will still have the same pressure in brake system.

Any suggestions as to a valve you have experience with?

 

[jimbosride]

Me thinks you have a valve issue. Jack up car and close the valve. apply brakes and see if the rear will spin? Then open the valve until it starts to grab.

Take the car out and adjust until the back wheels brake where you want them set. if the car has a factory PV for disc brakes adding an adjustable will not work right.

4 wheel drum brakes have no PV.

But I maybe wrong on this?

 

[WileE]

It limits the pressure that goes to the rear brakes.

 

[HALIFAXHOPS]

I always thought it limited the volume.

 

[R/T Boy 67]

At low pressure it does nothing. When the pressure is high enough to overcome the spring then it reduces pressure to the rear brakes at a set ratio, which is designed into the valve. Adjusting the spring changes the point at which the pressure reduction starts. If no fluid passes through the valve when bleeding then the valve is faulty, assuming that fluid is entering the valve, and there is not a problem upstream.

 

[Sixpactogo]

Just what does a PV actually do? I have rear brake lockup under heavy braking. Does it reduce volume or pressure? I bought one off EB and now, even adjusting the valve up and down no fluid is getting past that valve. To me pressure reduction would be what I'm looking for, as volume will still have the same pressure in brake system.

Any suggestions as to a valve you have experience with?

That is exactly what the PV does. Drum brakes require about 400 psi to lock them up. Disc brakes require closer to 900 psi. The PV reduces the pressure to the rear by about half. If you have the factory style unit, there is a plunger in the valve that will move to block the flow of fluid if there is a failure in either the front or rear brakes. Yours has blocked the flow to the rear. You need to remove the switch and slide that plunger back to the center. (if that makes any sense to you
You said you have an adjustable PV. I am not familiar with them so your issue may be different but something is blocking fluid to the rear brakes.

 

[Auggie56]

That is exactly what the PV does. Drum brakes require about 400 psi to lock them up. Disc brakes require closer to 900 psi. The PV reduces the pressure to the rear by about half. If you have the factory style unit, there is a plunger in the valve that will move to block the flow of fluid if there is a failure in either the front or rear brakes. Yours has blocked the flow to the rear. You need to remove the switch and slide that plunger back to the center. (if that makes any sense to you
You said you have an adjustable PV. I am not familiar with them so your issue may be different but something is blocking fluid to the rear brakes.

Yes, that switch, but the brake warning light should be on, if it's off-center. But I will do as you suggested. Thanks.

 

[Auggie56]

Many thanks everyone for the input.

 

[R/T Boy 67]

To answer the question, volume or pressure, it is actually both. Pressure and volume are always linked together. When the piston slides over it blocks flow to the rears, so the pressure stays the same until the pressure increases in the front circuit which pushes the piston over and flow resumes. The piston has two different diameters, which is how the pressure differential happens.

 

[gkent]

The above explains it well except that an aftermarket prop valve is just a needle and seat - no piston. That said, if you adjust it with an empty tank your brakes will react differently with a full tank.

The prop valve works like a tap in a water line - it restricts the flow.
Pressure is determined by volume and the size of the orifice.
The prop valve reduces the flow to the rear - allowing the pressure to the front to build faster than the rear. The rear would eventually achieve the same pressure but at a slower rate.

 

[RemCharger]

The above explains it well except that an aftermarket prop valve is just a needle and seat - no piston. That said, if you adjust it with an empty tank your brakes will react differently with a full tank.

The prop valve works like a tap in a water line - it restricts the flow.
Pressure is determined by volume and the size of the orifice.
The prop valve reduces the flow to the rear - allowing the pressure to the front to build faster than the rear. The rear would eventually achieve the same pressure but at a slower rate.

This is what I thought until I investigated. Maybe some have a needle and seat, but some are a spring and piston that " kicks in" at the adjusted rate. As previously stated by R/T boy in post 5

 

[gkent]

Whether there is a spring and piston or a needle and seat is neither here nor there, either one will be changing the orifice size to control flow. I think the piston style is more of a factory type unit where are lines go into one block and if either end of the system fails then the piston closes of the failed end altogether. That style would be pointless where a prop valve is plumbed into just the rear system.