Learn Me Brake Proportioning Valves

For factory applications that had both disc/drum and drum/drum brake options what is the difference in the proportioning valves used? Does the disc valve start reducing pressure to the rear sooner because of greater weight transfer with the better front brakes?

If this is the case could you leave the stock drum proportioning valve in place and add an adjustable proportioning valve in the line to the rear to fine tune? What about changing from drum/drum to disc/disc? Would the stock drum proportioning valve be ok in this application?

As a specific example - '68 El Camino going from drum/drum to 4th gen Camaro front brakes with stock rears, maybe upgrading to 4th gen Camaro rear brakes at a later date. What proportioning valve(s) would be best?

Paging @AngryCorvair and any other brake experts

Drums are self-energizing, so IIRC they need less line pressure. I would expect a disc/drum system to have more relative front pressure than a drum/drum setup.

I would suggest that an adjustable valve would probably suit you best so you can fine-tune as you continue to develop the car.

In reply to Racingsnake :

Keith Tanner is pretty much my go-to guy when i have brake balance questions.  :-)

His answer is exactly right.

In a brake system with no prop valve, the front and rear pressures would be equal. This would almost inevitably result in rear tires skidding well before the fronts on dry pavement, and we would call this a rear-biased system. Of course we could have huge front brakes and tiny rear brakes and greasy front tires and sticky rear tires and maybe even without a prop we could have a front-biased vehicle, but i digress).

Prop valves are described by their split point, and by their slope. The split point is the value at which the rear pressure diverges from the front pressure, and the slope is the percentage of rear pressure rise relative to front pressure, that occurs above the split point.  So a "100 / 0.2" valve begins limiting rear pressure at 100 psi, and the rise rate of the rear pressure above 100 psi would be 20% of the front pressure, *above* 100 psi. Then, when the front is at 200 psi, the rear would be at 120 psi. When the front is at 300 psi, the rear would be at 140 psi, etc.

As Keith stated, because of self-energizing nature of drum brakes, they require less pressure to create the same amount of brake torque than a disc brake of similar package size (for example, to fit inside a 15" wheel).

So a car with a drum/drum system might have a split point of 500 psi and a slope of 0.4, the same car with a disc/drum system would need to begin limiting rear pressure rise much earlier, and might have a split point of 250 psi and a slope of 0.25.

If you convert a drum/drum car to front discs, and you keep the drum/drum proportioning valve, your brake system output will be shifted toward the rear brakes doing more work relative to the fronts than they used to, when you're pushing hard enough to be above the split point.

Alternately, if you installed a disc/drum prop valve on a drum/drum car, your brake system output will be shifted toward the front brakes doing more work relative to the rears, when you're pushing hard enough to be above the split point.

And much like Forrest, Forrest Gump, that's all I got to say about that.

Woohoo! Passed the test. I've reworded my post about relative front/rear pressures to make it a little more clear, I blanked on a better term than "bias" when I wrote it.

I once had to recover a busted up Jeep from Moab Rim after someone "upgraded" the rear drums to discs and didn't do anything to the proportioning. It had a mechanical failure that led to the vehicle relying on the rear brakes and someone got a helicopter ride to the Level 1 trauma center.

For those who like graphs and actual examples, but nothing to do with drums: https://flyinmiata.com/pages/tech-stock-brake-proportioning-valves

If you like the talkies, here's my coworker Mike playing with green goo. Again, no drums, just prop valve prattling.

Regarding adjustable prop valves, the adjustment is in the split point.  The slope remains constant.  Here's a link to a great technical description from the Wilwood website

and here's a graph showing a bunch of different knee points, each with the same slope above the knee (source: the Wilwood website linked above).  Note this is for a lever style adjustable valve with multiple discrete settings. A knob style offers infinite adjustment of knee point above some minimum value.

In reply to Keith Tanner :

and i have edited my first post to reflect the content of your edited first post. :-)

as long as you don't refer to pedal force as pedal pressure, we'll get along just fine.

In reply to AngryCorvair (Forum Supporter) :

Well, not TODAY

I totally skipped over the OP's questions. 

Racingsnake said:

For factory applications that had both disc/drum and drum/drum brake options what is the difference in the proportioning valves used? Does the disc valve start reducing pressure to the rear sooner because of greater weight transfer with the better front brakes?

Disc/drum valve does start reducing pressure to the rear sooner (ie lower knee point), but not because of "greater weight transfer with better front brakes".  It's actually the opposite.  Disc/drum valve starts reducing rear pressure earlier because discs make *less* brake force compared to drums of similar size. So we limit rear pressure earlier to keep the rears from doing too much work and locking before the fronts.

If this is the case could you leave the stock drum proportioning valve in place and add an adjustable proportioning valve in the line to the rear to fine tune?

the stock valve is a flow-through for the front circuit so no difference there.  you could keep the stock drum/drum valve and put an adjustable downstream of it in the rear circuit.  in this case you'd have two knee points and two slopes, assuming you set the adjustable knee higher than the fixed knee.

What about changing from drum/drum to disc/disc? Would the stock drum proportioning valve be ok in this application?

going from drum/drum to disc/disc, *IF* the front and rear drums were configured exactly the same (drum diameter, drum width, lining arc lengths, wheel cylinder diameter), *AND IF* the replacement discs were configured exactly the same front as rear, the answer is still NO!

Imagine disc torque output as a function of pressure is half that of drum. Consider a drum/drum car with knee point 500 psi, and let's say that neatly coincides with 0.5G decel.

Now, replace those drums with discs and you're reaching your knee point at only 0.25G. Above this value you're reducing rear output relative to front, so above 0.25G you're going to be using more front brake relative to rear than when the car was drum/drum. IE car is now more front-biased.

EDIT: why does this matter?  Because you'll wear out your front pads faster than if the valve was properly tuned for disc/disc. Also because it will take more pedal force to reach any value greater than 0.25G than it would have taken if you had a valve properly tuned for disc/disc. 

As a specific example - '68 El Camino going from drum/drum to 4th gen Camaro front brakes with stock rears, maybe upgrading to 4th gen Camaro rear brakes at a later date. What proportioning valve(s) would be best?

Paging @AngryCorvair and any other brake experts

OK, by way of example i did pretty much answer that one with my first reply. Yay me.  In this case i would ditch the OE valve and install an adjustable in the rear.

There is also the factor that the combination valve has more to it than just a proportioning valve.  It may have a residual pressure valve in it to hold a small amount of pressure to the wheel cylinders to keep the cup seals tight against the bores. (Ford used to incorporate this into the master cylinder)  There may also be a pressure hold valve that prevents the front brakes from getting pressure until the drum rear brakes get 50psi or so line pressure, this is so the rear brakes get enough pressure to overcome the shoe return springs first, so all four brakes apply at roughly the same time.

You definitely do not want either of those in a four wheel disk system.

Practically speaking, a stock disk/drum combo valve for an A-body should be close enough for your purposes when you do the front brakes.  Brake piston dimensions have not changed much over the years.  When you do the rear disk conversion, the one for a disk 4th-gen should be fine.  IIRC they did not apply the rear brakes very much in an attempt to prevent axle hop with the torque arm rear suspension, so this should translate close 'nuff for a rear-light El Camino.

That probably sounds horrifying to the brake engineers in the house   But practically speaking, no matter what they did on the assembly line, when you order a parts store combination valve they don't have different part numbers for 2 door or 4 door or wagon or "ute" or big block or six cylinder or 9 inch rear drum or 11 inch or...  Just one size fits all.  And there hasn't been a huge issue with it.

And then there is the Extra Fun with drum brakes with how nonlinear they are, the self energizing effect means they apply much differently at different speeds. Or weather conditions.  So the valves are probably heavily biased to not be dangerous at 80mph at the expense of being front heavy at parking lot speeds or when it is wet outside.

To me, who off-roads in a truck with four drum brakes, the Extra Fun is that the mechanism that makes them self-energizing going forward makes them self-defeating in reverse. Trying to hold yourself on a slope in a single line system with no assist...okay, I should really upgrade those brakes.  

Oh! Oh! Oh!

I actually have a short video on these for my students in their brake unit!

Not quite as engineeringly in-depth as this conversation is going, but I hope it makes you giggle:

Keith Tanner said:

To me, who off-roads in a truck with four drum brakes, the Extra Fun is that the mechanism that makes them self-energizing going forward makes them self-defeating in reverse. Trying to hold yourself on a slope in a single line system with no assist...okay, I should really upgrade those brakes.  

The nice thing about starting with a '68 is that it already has a dual stage brake system.

I'd converted many single stage vehicles over the years, usually there are kits available or a newer version of the car, but I'd had to roll my own for some things, like an early 50s Oldsmobile that I ended up having to mount the combo valve in the left inner fender because, for some annoying reason, all the common GM master cylinders that would bolt up have their brake lines exit to the left, and the master cylinder was practically grazing the inner fender, so the only neat way to route the lines from the master was through a pair of 1/2" holes through the fender.

Thanks guys, all very informative! Just need to get on and do the swap now

In reply to AngryCorvair (Forum Supporter) :

I have been shopping adjustable valves. They seem to come in two flavors, Tilton and everyone else. Tilton is 2/3 reduction, so 150+ / .33. Wilwood and everyone else seem to be 57% reduction, so 150+ / .43. Are there .2 ratio and .25 ratio, et. al that non OEM folks can get access to the numbers on? Have you seen an adjustable with a ratio value at .25 or less?

Second question: I have been doing a bunch of calcs (made an excel spreadsheet) for specifying the brake system in my Rabbit. I am not lowering it, so I am guessing that is going to put my CG in the 22-24" range (WAG, based on reading that many rally cars are in the 18" range, lower than me, with more fancy steps to keep weight low). Wheel base is 94.5, Front weight bias will be a little over 55% but not all the way to 60%, yada yada yada. Being shortish and highish, the rear end will get very light, and eventually the actual required rear brake pressure for threshold will be decreasing at 0.9 g's or so for 24" CG. Best I can do is make my applied pressure intersect the threshold line at somewhere higher than 1.05 and just never run awesome tires and/or cut out more rear brake so that I can move the intersection to 1.2 or so.  I am trying to make sure the fronts lock first at any deceleration greater than using the brakes to help rotate. Things look a little better at 22" CG height. I am looking at something along the lines of the gray dots using a Tilton. The orange dots represent what would happen if I used a Wilwood set at 175, with a downstream Tilton set at 305.  Getting my CG down to 22" is most likely the right answer, but just because I am curious, is there any particular danger in running two valves in series?

This is all relevant to my interests.  I switched out to a disc rear axle on my S10, but haven't messed with the proportioning valve yet.  If I understand correctly from the above posts, that mostly means I've reduced rear braking power for now?

The swap was not to get discs, they just happened to be what was on it and I didn't feel like tearing everything apart to stick with drum brakes.

matthewmcl said:

In reply to AngryCorvair (Forum Supporter) :

I have been shopping adjustable valves. They seem to come in two flavors, Tilton and everyone else. Tilton is 2/3 reduction, so 150+ / .33. Wilwood and everyone else seem to be 57% reduction, so 150+ / .43. Are there .2 ratio and .25 ratio, et. al that non OEM folks can get access to the numbers on? Have you seen an adjustable with a ratio value at .25 or less?

i'm only aware of the .33 and the .43, i used those other numbers in my examples because they made the math easier. ;-)

Second question: ... snip cool details showing much good thought ... just because I am curious, is there any particular danger in running two valves in series?

in terms of failure modes, you'd have two more fittings to leak, and one more component to suffer internal failures.  when all components are working as intended, there's really no additional risk.

eastsideTim said:

This is all relevant to my interests.  I switched out to a disc rear axle on my S10, but haven't messed with the proportioning valve yet.  If I understand correctly from the above posts, that mostly means I've reduced rear braking power for now?

correct

Pete (l33t FS) said:  That probably sounds horrifying to the brake engineers in the house  

thank you for your concern. 

AngryCorvair (Forum Supporter) said:

eastsideTim said:

This is all relevant to my interests.  I switched out to a disc rear axle on my S10, but haven't messed with the proportioning valve yet.  If I understand correctly from the above posts, that mostly means I've reduced rear braking power for now?

correct

I may also be removing the non-functioning rear wheel abs system over the winter.  Would it make sense at that point to pull the stock proportioning valve and install an adjustable one on the rear lines? 

Pete. (l33t FS) said:

Practically speaking, a stock disk/drum combo valve for an A-body should be close enough for your purposes when you do the front brakes.  Brake piston dimensions have not changed much over the years.  When you do the rear disk conversion, the one for a disk 4th-gen should be fine.  IIRC they did not apply the rear brakes very much in an attempt to prevent axle hop with the torque arm rear suspension, so this should translate close 'nuff for a rear-light El Camino.

I think in the S10 world there are also C3 corvette master cylinders some people use when doing a rear disc conversion, including one that drag racers use for manual brake conversions.

eastsideTim said:

AngryCorvair (Forum Supporter) said:

eastsideTim said:

This is all relevant to my interests.  I switched out to a disc rear axle on my S10, but haven't messed with the proportioning valve yet.  If I understand correctly from the above posts, that mostly means I've reduced rear braking power for now?

correct

I may also be removing the non-functioning rear wheel abs system over the winter.  Would it make sense at that point to pull the stock proportioning valve and install an adjustable one on the rear lines? 

Yes

eastsideTim said:

Pete. (l33t FS) said:

Practically speaking, a stock disk/drum combo valve for an A-body should be close enough for your purposes when you do the front brakes.  Brake piston dimensions have not changed much over the years.  When you do the rear disk conversion, the one for a disk 4th-gen should be fine.  IIRC they did not apply the rear brakes very much in an attempt to prevent axle hop with the torque arm rear suspension, so this should translate close 'nuff for a rear-light El Camino.

I think in the S10 world there are also C3 corvette master cylinders some people use when doing a rear disc conversion, including one that drag racers use for manual brake conversions.

I know the offroader guys loved the Corvette masters.  I think this may be a volume issue, the C3 master has equal sized brake fluid reservoirs and presumably an equal amount of potential stroke for the front and rear.  Dedicated disk/drum masters don't.

Probably an issue if you are using 2" bore calipers on the rear.