Adjustable Proportioning Valve

[TriodeLuvr]

My Speedway purple proportioning valve is leaking where the adjustment shaft enters the body. How important is it in our cars for this to be an adjustable valve? I have discs up front, drums on the rear, and a dual master cylinder. I think there's also a residual valve in the system, maybe two. What other info about the system would I need in order to know whether an adjustable proportioning valve is needed? Or is it just a foregone conclusion that it's a necessity?

Jack

 

[choppedtop]

I would think you need it. Best way to find out is a fast stop test on wet asphalt or grave. Open it all the way and do a fast stop. If the rear tires slide before the front, you need it. You may be able to replace the oring on the shaft.
Lee

 

[benT]

Oh man, don't tell me that. We just bought a new purple speedway prop valve for our current T project!

 

[2old2fast]

I think I'd call speedway & see if that valve is seviceable.
dave

 

[TriodeLuvr]

Ben, you don't need to worry about the valve you bought. My car was built about ten years ago. Lots of things need updating and maintenance.

Dave, that's a good idea. I'll take a pic of the valve and send it to Speedway. This isn't exactly the same as the current offering, which is one reason I hate to replace it (different mounting holes and maybe lines). This must be a very slow leak, there wasn't any fluid on the garage floor when I got home from work today. Drove the car around for awhile, everything working fine including that new axle.

Jack

 

[PotvinGuy]

We all have a story. I have a prop valve in my bucket, and I did testing, with the thing adjusted from one extreme to the other. Couldn't tell any change. But I have discs all around; not quite the same as your setup. FYI, I also bought two residual pressure valves, but they're still hanging on the garage pegboard. The car's been running for 20 years with no change in the pedal and no leaks, so I think I'll just let it be.

 

[PotvinGuy]

I don't want to get off on a rant here (of course I do), but we build our buckets with tires and brakes intended for much heavier vehicles which have to perform in all kinds of nasty weather. But our little one-ton wonders typically cruise on dry streets and keep a safe distance from other vehicles. My point (I will get to it) is that we have plenty of reserve braking power.

My first bucket had rear drums and no front brakes (motorcycle tires and Hallcraft wires; wish I had kept them). And it was just fine around town.

 

[fordsbyjay]

I don't have one on my car and have no problems.

 

[spayne]

I have the same valve with the same problem slow leak from adjustment shaft also, I too have adjusted to both extremes with no difference now I think I know why. Please keep us all updated on what you find for replacing the o ring or whatever it is the manufacturer says thank you.

 

[TriodeLuvr]

No repair parts are available from Speedway, so I've decided to just order a new valve to save time. I don't know how necessary it is in my particular installation, but this way I only have to install it and bleed the brakes once. The car gets back on the road quickly and it's done. I'm keeping my fingers crossed the new style will bolt in without having to fab a bracket or new lines.

Jack

 

[T-Test]

Try the other brand. Just cost a little more. And I really think one is necessary but not the residuals. Make sure rear brake shoes are adjusted properly and the proportional valve will work as intended.

 

[TriodeLuvr]

Right, I don't understand why residuals are necessary either. Isn't the master cylinder supposed to hold pressure in the lines?

Jack

 

[T-Test]

Right, I don't understand why residuals are necessary either. Isn't the master cylinder supposed to hold pressure in the lines?


No. There is no pressure in the system until you mash the brake pedal. The residuals hold 2/10 lbs of pressure on the wheel cyls. and calipers so you will not have so much pedal travel when you first apply the brakes because of the volume of fluid to fill them. Residuals are SUPPOSED to keep fluid from draining back to the master cyl because it is lower than the calipers and or wheel cyls. It is a CLOSED system, so no air can get into the system, hince no residuals are needed which means 4 more fittings to leak. Others may disagree but this is what I know to be fact. Try to get an answer from FatPat. He is the expert on brake systems.

 

[choppedtop]

Since my master cylinder is lower than are the slaves, I had to add residual valves. Without them after just a little while, I would have to at least double pump the pedal to get the correct pedal. Kinda hard to do in a panic situation. Now the brake pressure/pedal stay up.
Lee

 

[TriodeLuvr]

OK, so the fluid "falls" toward the master cylinder, causing the pistons/wheel cylinders to retract. Does this mean the only thing holding the pads/shoes next to the braking surfaces in a "normal" installation is the weight of the brake fluid? Really, I always thought the master cylinder maintained a residual pressure for this purpose. Isn't that the reason so much effort is required to fully collapse a piston when new pads are installed?

Jack

 

[choppedtop]

That is the way that I understand it. Gravity lets the fluid drain back to the master cylinder. Unless, it has built in residual valves, which some do. Mine do not. When the pressure is off the cylinder, it leaves an open passage for fluid to return, which in turn would release the brakes. So, any slave cylinder or line that is above the master, would seek its own level. Someone please correct me if that is wrong, but that is what I am told, and seems to be true on my 2 cars with under floor masters.
Lee

 

[fordsbyjay]

The fluid allows the piston/pads to retract. This inturn increases pedal travel before the pads make contact with the rotor. Who wants a pedalthat goes half way to the floor before the brakes start working?

 

[fordsbyjay]

Choppedtop I would think the fluid would flow back till the m/cyl was full. Pad thickness and how much fluid is in the m/cyl would effect the required pedal travel.

 

[choppedtop]

Choppedtop I would think the fluid would flow back till the m/cyl was full. Pad thickness and how much fluid is in the m/cyl would effect the required pedal travel.

Yep, I agree on that. Then you have to displace all the fluid in the lines when you press the pedal, before slaves can operate. I really doubt that I needed a residual valve on the front calipers. They are a bit lowers than the master. The rear slaves are for sure higher.
Lee

 

[PotvinGuy]

Theoretically in a full system fluid won't drain back from the brakes into the master cylinder regardless of the relative heights of the two; there is no way for air to get into the brakes to allow the fluid to drain. But 1) if there is any leak at the brakes, fluid can slowly drain to a lower master cylinder (and you may see leaking fluid at the brake and the pedal will eventually have extra travel due to the compressible air in the brake), and 2) if there is any air trapped in the brakes or lines (because the system wasn't properly bled) you will get a spongy pedal.

FYI, I noticed when we filled and bled my system initially, the bled fluid had lots of small air bubbles in it. After a few months of spongy pedal we bled the system again until the fluid ran clear. I think the bubbles happened when I filled the master cylinder too fast in a hurry to pump it all up and hit the road.

My MC is under the floor and lower than the brakes, but once it was done right it has been good for 20 years.