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I have a 1950 3600 that has a front disk brake conversion and 14-bolt rear axle with drum brakes combined with a master cylinder and proportioning valve from a '90s vintage S10.

On my initial test drive I found a tendency for the rear brakes to lock up when braking so I need to install an adjustable proportioning valve.

I am looking for anyone's experience in selecting and mounting of an adjustable proportioning valve. I assume (which I hate to do) that I will need to remove the existing valve and plumb the adjustable valve into the line feeding the rear brakes but is there a way to avoid modification to the front brake line connections?

If this should be in a different area please feel free to move it.

Looking at your picture, it looks like you have two combination valves plumbed in. Combination valves include proportioning valves. The master cylinder looks like a "step bore" model.

I think that a straight bore master should be used with a single combination valve.

Here is a pic of how I set up my adjustable brake proportioning valve.

There are details above and below in the thread. Let me know if you want more pics/details.

Can we get a pic of the other side of your setup? I can only see one line from the MC to your valve in this pic.

There is only one valve but the master is a step bore. Based on my research I believe the master bores are 0.94" and 1.42". I had considered changing to a straight bore master, and may still change if needed. The calipers are 75mm bore and the rear wheel cylinders are 30 mm bore. Sorry for mixing metric and english units. I believe the C30 that the rear axle came from originally had a straight 1.312" bore master but I was considering a 1.250" straight bore master.

Fibonachu, thank you for the pictures of your valve arrangement.

I am attaching some better pictures of the existing setup.

I use an adjustable valve from Speedway Motors, it's mounted under the bed in the rear brake line. I went to a local shopping mall that closed and made a bunch of 30 to 50 mph stops adjusting the valve till it would stop quickly with out the rear coming around. It will still just start to slide the rear tires but it takes a full on the pedal stop to do it. You will notice when driving normal the pedal feels a little softer, but mine still stops when needed. I have a valve pretty much like yours just off the master cylinder, it came off a late '70s early '80s Chevy Malibu, figured the weight and portions were close to my truck. Not sure what it does, but suspect its more for residual pressure than for proportioning.

I posted this in another thread.


The book is still available. Page 50 talks about "fast fill" master cylinders. I have not seen a disk brake conversion kit that uses that type of m/c. The straight bore m/c seen in Fibonachu's post is typical.

Gord&Fran, thank you for the information about the book.

Joe H, thank you for your experience. I also do not know exactly what the existing valve is designed to do, I just know it has been generically referred to as a proportioning valve.

When evaluating options to change the master cylinder I tried to determine what GM trucks that had front disk and drum rears used but the results did not point to a clear preverence for straight bore or what bore size/sizes were used so I decided to start with what I have. If I can resolve the rear brake lock up issue with the addition of an adjustable proportioning valve I will probably go that route but if I need to change the master to a straight bore it would be better to do that at the same time.

Did the donor truck have rear wheel ABS? If so, then the existing valve was designed for that to kick in and control rear wheel skid. Allowing the rears to activate first and do more of the work (in a loaded truck situation) so the front brakes didn't burn up. However, it never seemed to work right and the rear shoes ended up glazed over, with faulty wheel cylinders, and burnt up front brakes with the attendant customer complaints of poor braking performance and a soft pedal.

It wasn't a great system, back in the '90s when I fixed rigs like that for a living, we had a lot of problems with full size GM trucks and their rear brakes either not working, or locking up too soon. With mushy pedal feel.

Going to a straight bore master cylinder will likely fix a lot of the problems, as you are returning the system to a state where it's designed to work without rear wheel ABS. You're also running the proportioning valve from an S10 which has puny rear brakes and essentially the brake balance of a car, while using a burly 14 bolt rear axle and its much larger brakes.

I would suggest the straight bore master cylinder and finding a prop valve from a 1 ton '90s truck, as long as the donor rig has rear drums. Then the valve will be in a system much closer to what it was designed for.

Tronman, thank you for the insight and recommendation. I understand that the donor S10 system is not really compatable with my system so your recommendations regarding a path forward make a lot of sense. I do not know but suspect that the donor S10 did have ABS as it was a later 90's vintage and I beleive that S10's of that era all had ABS.

I am not a fan of salvage yard brake parts but using a 90's vintage 1 ton truck as a model I should be able to determine what master I need to combine with an adjustable proportioning valve to dial in my brake system.

Looking at information that I can find it appears that the 1990's vintage Chevy 3500 used a 1.250 bore master cylinder so that is what I will plan to use. Once I source the master and proportioning valve (adjustable) I will tackle the replumbing of the brake lines to make it all fit and then work on getting the adjustable proportioning valve dialed in.

I noted that Fibonachu used a 4 port adjustable valve but I think a 2 port valve would work for my system. Any insight regarding the benefit of 4 over 2 port valves before I pull the trigger would be appreciated.

Mine is actually a 3 port and the benefit is purely convenience.

It has one rear output and 2 front outputs. Having that saves a Tee at the frame for the two front lines.

It actually came with a plug for the second front output. The two of them are connected internally.

I went with the Wilwood because it was on sale for less than the Speedway branded one when I was ordering.

Edit: Apparently they call mine a 4 port (which seems odd since it has 2 inputs and 3 outputs) and the Speedway one a 3 port (having 2 inputs and 2 outputs).

Maybe I don't know what "port" means in this context?

Fibonachu, do you think that the front lines being connected to the valve do anything other than pass through? My impression is that the 2 port versions only go in the line feeding the rear and the front(s) come directly from the master. Whereas the 4 port (sorry I don't understand the naming either) front lines are connected to the valve. I just don't know if the fronts being connected infleunces the way the valve adjusts the rear braking pressure.

I know Wildwood has both versions but I don't know if they created the 2 port version because of pricing pressure from competitors marketing 2 port valves mush cheaper.

There exist some versions that actually track the front and rear pressure and adjust the rear pressure based on the front.

The one I have does not. It just functions as the convenient Tee for the front lines.

There is an explainer from Wilwood on the physics behind how it works.

A "bias bar" would be the other option, but those are generally only used in racing applications.

Fibonachu, thank you for the additional information. I plan to order the 2 port version (probably the Speedway clone of the Wildwood valve) to save a few bucks.

Once I have the valve in hand I will pick up the replacement master cylinder and then configure the brake lines to make it work. I am hoping to get it done in time to get it adjusted prior to weather forcing me to put the truck away for the winter.

I installed a 1.250" straight bore master and an adjustable proportionig valve in the line for the rear brakes.

The adjustable valve is mounted to a bracket that I made from the original S10 prop valve mounting bracket with several areas removed and a mounting surface for the adjustable valve added. I used the lines that previously connected the master to the S10 prop valve to connect to the front brake line using a union and to the adjustable prop valve for the rear.

As a starting point I have the adjustable valve all the way out that I believe should provide minimum pressure to the rear brakes. I have tried a few tests and the driver's side rear tends to lock up first but it takes a pretty hard braking event for that to happen. I will continue to test and adjust as needed.

I have basically the same setup on my 49 3600. The huge difference I found was adding residual check valves -

I've deleted proportioning valves in other vehicles and could give or take the residual valves, but maybe because the MC is under the floor?? Who knows, but at least on my setup that made the difference between crappy brakes with poor feel and a tendency to lock up (and no, there was no air in the system - none); and brakes that functionally, I can't tell apart from anything else.

Also, (and apologies if you've already mentioned this) make absolutely sure that your MC pushrod is set correctly. I've installed several of these types of units and find that more often than not, the "universal" setups are shipped without that adjustment being correct. More than once I've had to machine a new pushrod to get that measurement right - the manufacturers don't seem to care.

Sounds like you're on the right track. One side of the rear locking up first is likely a simple adjustment issue, pretty common to drum brakes. Give the right side a couple more clicks and that will likely make both lock up at once. Assuming good wheel cylinders, linings and etc. of course.

I've had brand new wheel cylinders not be good, too.

Tronman, thank you for the insight. All of the rear brake parts are new and I suspect that you are correct about the adjustment being different between the two rear drum setups. I am not overly concerned about the side to side difference in the rear at this point as the shoes and pads are far from broken in (about 20 miles total to date).

Brake adjustment affects pedal height- - - -not how much force the shoes apply to the drum. If a brake adjustment is slightly loose, there will be NO braking at all until both sets of shoes are firmly against the drum. One wheel locking up is usually the result of a hydraulic or a mechanical restriction to movement of the shoes. A collapsed flex line to a front brake, or a sticky wheel cylinder in the rear can cause one set of shoes to start braking before the other. Another problem that's often overlooked is a step worn in a backing plate that allows the shoes on one side to hang up momentarily and fail to contact the drum. Don't overlook the nitpicking details, but don't try to repeal the laws of physics! Slight differences in drum diameter that affect how much of the brake lining actually contacts the drum can result in uneven braking while new linings are "wearing-in" however.
Jerry

Thank you Jerry. Once I get a few miles on to get all the "new" brake parts worn in I will evaluate the braking performance and inspect/adjust as needed.

One other thing to remember- - - -a proportioning valve doesn't alter the amount of force applied to a brake system- - - -just the timing of when it's applied. The total force is determined by things such as the diameter difference between the master cylinder and the wheel cylinders, leverage on the brake pedal linkage, and whether or not there's a power booster present. Most OEM proportioning valves delay application of the front brakes slightly to allow for better steering control in slippery conditions- - - -ice, snow, mud, etc. Nothing is more disconcerting than trying to steer with locked up front wheels while the rear wheels are still trying to drive the vehicle- - - - -been there, done that! Most people don't think to push in the clutch pedal in that situation, or put an automatic transmission in neutral.
Jerry

I think part of the problem with discussions regarding proportioning valves is that some valves, described as proportioning valves are really just pressure limiting valves (as I understand it). The information I have reviewed indicates that the valve I installed any others like it "control' the braking force by limiting the pressure that can be passed through to the brake (calipers or wheel cylinders). They do not change when the force is being applied just how much pressure can be carried through to the brakes. The following is from Wilwood Engineering and relates to a valve that is different than the one I used but has the same characteristics: Pressure at M/C vs Rear Brake Lines

Second the groove worn into the drum brake backing plate. I had to weld up both backing plates on the front of my bus because there were grooves worn into them, mine seemed to be more the bottom than top. I haven't yet driven it since I did all that because I'm still working on exhaust, fuel, tires, rear brakes. Seems like there's always something! However I was-after a concerted three decade search-able to find new front drums for it, and they were even a still produced part number! A still produced $440 a pop part number.. but at least I'll never have to worry about those drums again. I think the disc swap might have wound up being cheaper..

When searching for a part for a 60 year old vehicle, you're happy it's available at any price.

This is definitely true!