Rear brake bias
 

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".

Re: Rear brake bias
 

Its a fixed valve.
If you are driving it empty all the time (as we know you will be) an adjstable prop valve to the rears makes a lot of sense. That way you can dial it way down for cruising or crank the rears back in for say an Airstream trailer.

Disclaimer:
Just remember if you do a downstream valve it will still reduce the rears even in the event of a front brake failure.

Re: Rear brake bias
 

I have one of the load-sensing prop. valves in the back of my '85 C3500, and I drive it unloaded a lot. It seems that, with that valve reducing rear braking effort, even a good hard back-up-and-slam-on-the-brakes won't adjust the rear drums and I have to do them manually or eventually put undue stress on the fronts, increasing the chances of warping them - which has happened twice in the last 40,000 miles. It's disconcerting to have your rear brakes locking up, but in part it's just the nature of the weight distribution on these trucks and I'd a rather have them stay adjusted and be prepared to baby it under adverse driving conditions.

Re: Rear brake bias
 

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.

Re: Rear brake bias
 

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.

Re: Rear brake bias
 

Why would you go to a hydroboost? Sounds like if you're locking them up the amount of assist is already fine?

I've had a car without hydroboost to which I added it and later removed it. It wasn't a big change either direction... because the vacuum booster worked. Hard to beat "it works".

Unless you've got a big cam or something, that's different.

Re: Rear brake bias
 

I'll probably never have a cam in this vehicle that would make the vacuum an issue. I just want the stopping power. I will move to 35s on my next tire purchase. I ran 35s on this truck years ago fine with drum drum and trailers, sand, gravel bricks. It stopped fine, but only fine. I want the braking power, that's it.

I don't mind doing the mod, I want to reroute my rear lines and clean things up a bit. So, the hydro is under serious consideration right now.

I may not get around to it, but the rear wheel lock up is going to be solved one way or another without an adjustable prop.

I thought I had it. I stop great even with the tail dragging on hard braking. It just bugs me. It's become a pet peeve at this point.

Re: Rear brake bias
 

>>rubber check out of the RPV on the front pot.<<

RPV, is that supposed to be an acronym for residual pressure valve? Not a common acronym since it is usually simply referred to as a residual valve.
Front pot? Is that street slang for reservoir?

There is no functional difference between a disc/drum MC and a drum/drum MC. MC is a common acronym for master cylinder.
Disc/drum masters often have a larger reservoir for the front calipers, but in no way effects its operation. A 1" disc/drum master works the same as a 1" drum/drum or a 1" disc/disc.

There is a difference between a MC used for power brakes and one used for manual brakes. The manual MC has a deep hole in the rear of the piston for the brake rod. The power MC has a shallow hole for the short rod on the front of the brake booster.

CPP and others make a universal MC and supply a plug to allow the manual MC to be used with a power booster.

Some guys foolishly use a power MC for manual brakes and hope the rubber dust boot doesn't rot away and let the rod drop down, leaving them with no brakes.

The metering valve on the end of the combination valve must have the pin either pushed in or pulled out when breeding the brakes. If the metering valve is not pushed/or pulled when bleeding the brakes you will have poor or no front brakes. You may not realize the fronts are not working until you have a problem with the rears locking up.


Sorry, but made up words and uncommon acronyms hinder communication.

Re: Rear brake bias
 

Excellent post -

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

Re: Rear brake bias
 

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

True, but a residual valve and/or cup spreaders and well adjusted shoes take care of much of that.
Some vehicles are prone to nose diving at parking lot speeds with very light brake pressure. The metering valve or hold-off valve, holds back the pressure to the calipers and allows the rear drums to lightly drag.

Re: Rear brake bias
 

Yes to all, MC is MC RPV is RPV ect ect ect. Not made up just not wanting to type it out. But I can see you are quiet smart... I think everyone knows what I was saying. Yes, pot is reservoir, is a pot not, that's what we call them sometimes. But I cannot spell reservior. see.

Tedious, like a 5th grade school master.

Re: Rear brake bias
 

RichardJ I like that post man...Bringing science to the forum.. nice.