Rear brake bias

[RichardJ]

The rear bias isn't fixed, it's PROPORTIONAL. That may just be why they call it a proportioning valve.
The pressure point where the proportioning begins is fixed for combination valves(the proportion section) and stand alone proportion valves.
Different valves can have a different start point and they can also have a different slope or rate of proportioning.

In the first chart you can see that up to about 600 psi, the pressure front and rear is the same. Above that point, the greater the pressure is, the greater the difference between the front/rear pressure.

600 psi is relatively light pressure. For manual brakes, 75 lb from your foot, a 6:1 brake pedal arm leverage and a 1" MC bore, produces 573 psi brake fluid pressure.

The second chart shows a change in start point for load-sensing or height-sensing proportioning valves. Note the rate of proportioning stays the same.

Proportioning valves have nothing to do with auto-adjust brake shoe adjusters. All that is required for self-adjusters to work is for the shoes to be pushed out against the drum, when the drum is turning backwards.

Dave, you may have a bad proportioning valve(combo valve), but I would first find out what your front brakes are doing. Find a hard packed dirt road or a hard area with a dusting of sand on it and see what it takes to lock up the front brakes.

[jeffahart]

I had quite a time with my brakes after putting a disc axle conversion on the front. I originally took the spring and rubber check out of the RPV on the front pot. It braked fine with no rear lockup. The brakes were a bit touchy but braked fine. Then the master went out. So I decided to do it with the correct disc/drum master and dist block/prop valve. I tried every combination and could not stop the rears from locking. I did not want to go with an adjustable, so I bought a new dual pot drum drum master and removed the check from the front RPV. Again I had great brakes no rear wheel lock, just a good old fashion four wheel lockup.

Now, I put a 225lb winch on the nose, guess what, I get a rear wheel lock under heavy braking again. What a pisser! I guess it was never that close to correct if it's that finicky!

Now I'm seriously considering hydro boost before I take anymore stabs at it. It really bugs me, the whole rear wheel lock up thing! And, we should be able to correct without adjustable props. That's what I'm thinking anyway.

[Keith Seymore]

Originally Posted by RichardJ

The rear bias isn't fixed, it's PROPORTIONAL. That may just be why they call it a proportioning valve.
The pressure point where the proportioning begins is fixed for combination valves(the proportion section) and stand alone proportion valves.
Different valves can have a different start point and they can also have a different slope or rate of proportioning.

In the first chart you can see that up to about 600 psi, the pressure front and rear is the same. Above that point, the greater the pressure is, the greater the difference between the front/rear pressure.

600 psi is relatively light pressure. For manual brakes, 75 lb from your foot, a 6:1 brake pedal arm leverage and a 1" MC bore, produces 573 psi brake fluid pressure.

The second chart shows a change in start point for load-sensing or height-sensing proportioning valves. Note the rate of proportioning stays the same.

Proportioning valves have nothing to do with auto-adjust brake shoe adjusters. All that is required for self-adjusters to work is for the shoes to be pushed out against the drum, when the drum is turning backwards.

Dave, you may have a bad proportioning valve(combo valve), but I would first find out what your front brakes are doing. Find a hard packed dirt road or a hard area with a dusting of sand on it and see what it takes to lock up the front brakes.

Excellent post -

Originally Posted by davepl

Under hard braking, the rear brakes of my '72 (from a brake perspective) Longhorn will lock up.

Does the combo valve on these trucks do ANY kind of magic, or is it a fixed bias all the time that sends X percent to the rear?

I wasn't sure if they have a slider valve that manages deceleration or something fancy I've never heard of, or if it's just "always send X to the back".

The characteristics of the combo/prop valve are defined as "meter", "split" and "slope".

Meter is the amount of holdoff of the front disc, to allow the rear drums to close up the running clearance.

Split refers to the pressure point at which the front and rear pressure begin to deviate.

Slope defines the amount of proportioning that takes place in the rear system from the split point onward.

K