Rear drum brake to disc - is a prop valve needed?

[fairlaniac]

I have a Granada front disc brake set up on my Fairlane. Been working great for years so no need to change it. I installed a ’89 Ranger manual disc/drum master cylinder with the upgrade. I recently picked up some aftermarket rear disc brakes. The current distribution block is a late 60’s Torino unit. I bought new from Dearborn about three years ago.

My question, do I need an adjustable proportioning valve as I replace my rear drums with discs? I’m getting confusing results with my internet searches.

As I understand the prop valve only adjusts the rears (if needed), correct?

Thanks!

[machoneman]

Too many variables (and there are a lot!) to say that one doesn't need an adjustable prop valve. And yes, the valve only adjusts the rear brake line pressure. Generally, turning the knob CCW lessens rear pressure while going the other way, clockwise, increases rear line pressure.

You could try it w/o that valve and test same in a big parking lot or side street. The goal is to see if the rears lock up before the fronts, not a good thing. If they do after multiple tests, then adding that valve is pretty much mandatory. I added one with the engine out of my '70 Mach one as doing so with the engine installed can be a bitch. Check too if your master cylinder has residual pressure valves as they are there solely for drum brakes. Removing them (it's a check valve and spring inside the bores of the master's front/rear lines) is required.   

https://info.mpbrakes.com/faqs

https://www.speedwaymotors.com/the-toolbox/residual-vs-proportioning-valves-and-what-your-brake-system-needs/31069

[6667fan]

Doug, you should probably plan to put a bias valve into your system. If it turns out you don’t need it you can leave it in the non restricted position. It’s not a big deal to plumb it in. I have a Wilwood unit that dimensionally looks like the Speedway version that is in the link Bob shared.
You are sticking with your Ranger mc?

[Falcon67]

FWIW & LOL - I never got past the expense and the extra hassle of using read disks on my race car.  The 10x2 drums back there have been pretty happy for the last 20+ years.  I keep looking every so often, then get kinda "meh' about the whole thing. 

If I did, I would for sure place an adjustable prop valve in the rear line. The fronts do easy 70+% of the work so I'd think you'd need to delay and reduce the rear line pressure to keep the back end from grabbing up before the fronts.  Granted disks need more line pressure to engage, so it would also depend on the calipers used on both ends.  The standard swap - "granada" - uses the large 2.60 single piston front unit. 

[Stangman]

Is it true the smaller the bore in the master the more pressure. Example I have a 1 inch bore and if I wanted more pressure I would get a 7/8 bore?

[garyv]

I put Wilwood disc brakes on the rear of my 66 Fairlane a few years back and I did put a proportioning valve in the system and
did have to adjust the rear pressure to keep them from locking up first. 



garyv

[Falcon67]

Is it true the smaller the bore in the master the more pressure. Example I have a 1 inch bore and if I wanted more pressure I would get a 7/8 bore?

Yes.  Small bore = higher line pressure with more pedal movement (lower volume).  IIRC the common stocker cast iron disk/drum manual like I have on the Falcon is a 15/16 bore unit.  Have read a lot of posts where people swapped in 1 1/8" units and ended up with a higher/stiff pedal and less braking action. 

My dragster uses a bitty 7/8" bore unit to pressure the two rear 4 piston Strange calipers. 

[Keith Stevens]

Yes, ford used a "Proporting valve" AKA residual valve in the rear from the factory. It holds 10 PSI to the rear brakes.
https://www.npdlink.com/product/proportioning-valve-disc-brake/186693?backurl=search%2Fproducts%3Fsearch_terms%3Dbrake%252Bproportioning%252Bvalve%26top_parent%3D200001%26year%3D1967&year=1967

[fairlaniac]

Thanks guys, a lot of great information. It looks like I'll pick up a Wilwood proportioning valve and a 2 psi residual valve and do some plumbing. I currently do not run any aftermarket residual valves. Would a valve for each end be appropriate or just to the rear?

Thanks,

[machoneman]

Thanks guys, a lot of great information. It looks like I'll pick up a Wilwood proportioning valve and a 2 psi residual valve and do some plumbing. I currently do not run any aftermarket residual valves. Would a valve for each end be appropriate or just to the rear?

Thanks,

No residual pressure valves needed nor wanted. See Summit's notes. Btw, other brake suppliers say the same.

https://help.summitracing.com/app/answers/detail/a_id/5297/~/what-is-a-residual-valve%3F#:~:text=Most%20disc%20brake%20systems%20do,the%20fluid%20from%20flowing%20backwards.

[pbf777]

Too many variables (and there are a lot!) to say that one doesn't need an adjustable prop valve.

     This is the correct answer, and I will add that in my opinion the application of such valves is an admission of poor engineering choices in one's brake system.  In other words, if one requires a proportioning valve plumbed in the rear hydraulic circuit, that has solely the function of retardation of the braking effect (also be aware that some units provide an effect even if adjusted to their least effect position) , then that would mean one bought to much in the way of brakes for the rear, that is as compared to the fronts anyway, look cool, but just not right; this including even in road course cars where this is often used to allow adjustment to maintain the bias balance (front to back) during the race as say the fronts falter during the race. And there are installation instances where the proportioning valve is installed to limit the braking value on the front (say drag car with big fat sticky slicks on the rear and hard little skinnies on the front, particularly with fair amounts of engine set-back; also even metering left to right as practiced in roundy-round, so as brake application aids in pulling the car into the turn.   

     I believe the residual valve first appeared as a solution to aid in damping the resultant effect of one not following the required maintenance procedure of adjusting the resting position of the shoes of drum systems (residual valves rarely have a purpose in disc systems) and due to material losses the shoes present excessive travel distance in operation. But is often an unfortunate requirement with instances of the master cylinder being mounted lower than the wheel cylinders or calipers to control gravity drain-back; so technically with the exception of a few say "Super-Cars" sports machines where windshield is mounted lower than the gigantic multi piston calipers, or other racing cars including of course some dragsters, but in other examples it is only another admission of poor execution; or this being a result of just buying to much 'stuff' if only for 'just in case' it's needed.   

     Scott.

     

[Falcon67]

I was thinking "no residual valves" in a disk system.  You don't want the pads riding the disks.  here agrees, except for the special condition in a typical street rod where the <C is lower than the calipers

https://www.speedwaymotors.com/the-toolbox/residual-vs-proportioning-valves-and-what-your-brake-system-needs/31069

I will say that my dragster MC is well lower than the rear calipers but the system is so short and simple that no residual valve seems to be needed.  Pedal, MC, line, T, flex line to each caliper. 

https://www.strangeengineering.net/ufaqs/when-do-i-need-a-residual-pressure-valve/

Strange suggests that a 2 lb unit would not be out of the question for the dragster.

> my opinion the application of such valves is an admission of poor engineering choices in one's brake system.
All the factory disk/drum systems use a prop valve that also doubles as a safety switch on the dual master systems.  I use a factory prop valve on my race car because Falcon/Fairlane/Maverick/Comet/Granada/etc are all in a similar weight class and used very similar pieces in the systems - same calipers, all the 10x2 rear drums used pretty much the same wheel cylinder bore, etc, etc.  I don't see how you'd not have a prop valve in a street car because under braking, especially heavy braking, the CG is going to move a lot of weight off the rear.  one reason why the fronts do 70% of the work. 

[pbf777]

All the factory disk/drum systems use a prop valve.

     Previous statement:

   

   
    I believe the residual valve first appeared as a solution to aid in damping the resultant effect of one not following the required maintenance procedure of adjusting the resting position of the shoes of drum systems (residual valves rarely have a purpose in disc systems) and due to material losses the shoes present excessive travel distance in operation.       

     And I will add that in mixed systems (disc/drum) there is also the fact that a greater volume of fluid (hence pedal travel) is initially required to offset the above statement, this before a higher pressure is applied the calipers initiating their function, and this is often another responsibility of the distribution valve/metering block assembly, which may or may not actually consist of a true proportioning valve function though often right or wrong referred to as such.   

 


 

  I don't see how you'd not have a prop valve in a street car because under braking, .................the fronts do 70% of the work. 

     Previous statement:

     ............if one requires a proportioning valve plumbed in the rear hydraulic circuit, that has solely the function of retardation of the braking effect, then that would mean one bought to much in the way of brakes for the rear,...........     

     In other words, just mount the proper capability braking system (less than the front of the proper sum  ) and one wouldn't need a proportioning valve for a correction solution. 




     And yep...........

     

Too many variables (and there are a lot!)

     And that makes you............ the new engineer!   

     Scott.

[Nightmist66]

Last time I had the car down for maintenance it received Wilwood discs all around. I used an '87(I think) Ranger m/c and did plumb in a Wilwood adjustable prop. valve. I looked in the master to verify there was no residual valve installed. I left the prop. valve set as-received and upon first test drives found the front brakes to be doing what seemed to be 90-95 percent of the braking. I would adjust a little and try again. I think I had a good couple turns to get a comfortable setting. I would recommend installing the adjustable prop. valve from the beginning just in case you would need it, and in my case I did. I have my line loc solenoid plumbed directly to the inlet of the prop. valve. If you need any pictures of the install, just holler.

[machoneman]

All the factory disk/drum systems use a prop valve.

     Previous statement:

   

   
    I believe the residual valve first appeared as a solution to aid in damping the resultant effect of one not following the required maintenance procedure of adjusting the resting position of the shoes of drum systems (residual valves rarely have a purpose in disc systems) and due to material losses the shoes present excessive travel distance in operation.       

     And I will add that in mixed systems (disc/drum) there is also the fact that a greater volume of fluid (hence pedal travel) is initially required to offset the above statement, this before a higher pressure is applied the calipers initiating their function, and this is often another responsibility of the distribution valve/metering block assembly, which may or may not actually consist of a true proportioning valve function though often right or wrong referred to as such.   

 


 

  I don't see how you'd not have a prop valve in a street car because under braking, .................the fronts do 70% of the work. 

     Previous statement:

     ............if one requires a proportioning valve plumbed in the rear hydraulic circuit, that has solely the function of retardation of the braking effect, then that would mean one bought to much in the way of brakes for the rear,...........     

     In other words, just mount the proper capability braking system (less than the front of the proper sum  ) and one wouldn't need a proportioning valve for a correction solution. 




     And yep...........

     

Too many variables (and there are a lot!)

     And that makes you............ the new engineer!   

     Scott.

Why thanks Scott!