Proportion Valve with Crown vic front end

[70shortwide]

Hey guys,
I have had disc brakes for a long time... Never changed the proportion valve. now that I have the cv front end, it seems like my braking isnt as good as it was with the dentside discs. I am thinking its because of not having a proportion valve... Are the dual piston calipers putting a spotlight on the fact that I just have the drum/drum distribution block?

I know I should have done the proportion valve back when I first did disc brakes, but they worked well with the dentside brakes, and just slightly worse with these brakes. I wouldnt call it unsafe but I have a little more pedal travel than I want, and it takes a little more effort to push than Id prefer.

I am thinking about using a proportion valve out of a 73 f250, other threads say not to use them because the 250s have dual piston calipers, but I have dual piston calipers now, and I figure its worth a shot... Can someone explain why this wouldnt work? anybody have a better suggestion? I prefer to stay with a block (valve) that wont require me to change my brake lines, the ones that are in there are new, from before I realized that the new brakes would act different than my old brakes...

[70shortwide]

Turns out, through that whole post I was talking about distribution blocks...

Ive been reading up on this, Im thinking I will just get an adjustable proportion valve. does anyone know if I can install it right by the master cylinder (before the distribution block)? Is there any reason to install it downstream of the distribution block? Will this help with my pedal travel (I think my travel problem may be because Im operating the rear brakes first, instead of the same time)?



If I can put it right next to the master cylinder it will save me a fair amount of effort, and since I dont really have a good place to work on this kind of stuff Id much prefer leaning over the fender than lieing on my back getting brake fluid in my eyes...

[HIO Silver]

A prop valve meters flow to the rear brakes. It's a matter of installing it in the rear brake line circuit. IIRC, the rear circuit through the dist. block is just a pass-thru. You do have a disk/drum MC, right?

[70shortwide]

yea, i put the disc mc in years ago... is there an easy way to double check that it is for disc brakes?

[elgemcdlf]

What do you have for rear brakes?

[HIO Silver]

yea, i put the disc mc in years ago... is there an easy way to double check that it is for disc brakes?

Yes.. the easiest way is to do a visual comparison. It takes more fluid to move disc pistons so the large MC bowl is for the front, the smaller reservoir is for the rear. You could measure how much each reservoir holds but that is waaaaaay too much work.

[ultraranger]

A stock pressure differential valve is designed for an all wheel drum brake system. A stock combination valve is designed for discs front/drums rear.

If you've converted from all wheel drums to disc's in the front, the existing stock drum/drum pressure differential valve isn't going to correctly control the brake functions, regardless if an adjustable proportioning valve is added (inline to the rear brakes, AFTER the pressure differential valve).

Unless you have a set of pressure gauges to be able to read the line pressures, you really have no way of knowing what pressure setting you are setting the manually adjustable proportioning valve to. Even if you did install a gauge set, what pressure rating is the correct pressure point to set the rear brakes to? Various things would have to be known to calculate what this figure would need to be; MC bore diameter, wheel cylinder size, drum brake diameter, friction surfaces, shoe compound coefficient , center of gravity, weight shift of the vehicle, etc.

If your vehicle is never involved in a panic stop, the factory combination valve's proportioning feature will never activate. The line pressures to the front and rear brakes are linear. The only time that the rear line pressure is dropped off would be under a panic stop instance, to reduce the chances of the rear brakes locking up.

An adjustable proportioning valve isn't needed. The only thing you need is to swap out the drum/drum pressure differential valve for a disc/drum combination valve. This will likely mean having to fabricate new brake lines but brake tubing is cheap. Brakes are the most important system of a any vehicle. Short cutting the proper components by not installing them is not a good idea.
.

[70shortwide]

A stock pressure differential valve is designed for an all wheel drum brake system. A stock combination valve is designed for discs front/drums rear.

If you've converted from all wheel drums to disc's in the front, the existing stock drum/drum pressure differential valve isn't going to correctly control the brake functions, regardless if an adjustable proportioning valve is added (inline to the rear brakes, AFTER the pressure differential valve).

Unless you have a set of pressure gauges to be able to read the line pressures, you really have no way of knowing what pressure setting you are setting the manually adjustable proportioning valve to. Even if you did install a gauge set, what pressure rating is the correct pressure point to set the rear brakes to? Various things would have to be known to calculate what this figure would need to be; MC bore diameter, wheel cylinder size, drum brake diameter, friction surfaces, shoe compound coefficient , center of gravity, weight shift of the vehicle, etc.

If your vehicle is never involved in a panic stop, the factory combination valve's proportioning feature will never activate. The line pressures to the front and rear brakes are linear. The only time that the rear line pressure is dropped off would be under a panic stop instance, to reduce the chances of the rear brakes locking up.

An adjustable proportioning valve isn't needed. The only thing you need is to swap out the drum/drum pressure differential valve for a disc/drum combination valve. This will likely mean having to fabricate new brake lines but brake tubing is cheap. Brakes are the most important system of a any vehicle. Short cutting the proper components by not installing them is not a good idea.
.

Does that mean that for 90% of driving around I wont know the difference with the correct proportion valve?

The truck does stop pretty well when I need it to, I just dont like the pedal travel and effort it takes to stop. It feels like it's only activating the rear in the first couple inches of pedal, then the fronts come in kinda weak. If I mash the brakes it stops, I havent ever locked up front or rear accidently. I had the bowls hooked up backwards when I first put this front end in, and it would lock the rears in a panic stop, switched them around and it helped.


I think part of my problem is that Ive been driving newer cars lately and when I hop in the truck the brakes surprise me... I have never been impressed with the brakes since I put the crown vic front end in the truck. maybe the dual piston calipers need more juice? The brakes worked awesome with the 77 f100 brakes...

I think what Im going to do is make a list of parts and change up the whole system, I feel like maybe Im expecting too much from the parts I have. I already have a master cylinder and hydroboost from a superduty, I think Ill get a proportion valve that seems appropriate and go from there... This is a project for when I have more space to work on this thing

[guhfluh]

If the pedal travel is too much, the master bore is too small, or the master rod is connected to the pedal in the incorrect location, and not allowing enough stroke of the master. If the effort is to great, then the master bore is too large, the booster or lack of one has a problem, caliber piston too small, brakes aren't bedded, restriction in a line or hose, etc., etc...

What booster do you have? Can you list out exactly what parts you have in the whole system?

[70shortwide]

If the pedal travel is too much, the master bore is too small, or the master rod is connected to the pedal in the incorrect location, and not allowing enough stroke of the master. If the effort is to great, then the master bore is too large, the booster or lack of one has a problem, caliber piston too small, brakes aren't bedded, restriction in a line or hose, etc., etc...

What booster do you have? Can you list out exactly what parts you have in the whole system?

I have:
Disc brake master cylinder (not sure what I ordered it for, its been a while. Probably '73 w/ disc brakes? I did go look to see if the reservoirs were different sizes, they are)
Brake booster from a '73 f350 (no telling if it was original)
Stock Drum/Drum distribution block
CV police interceptor front discs
Rear brakes are 1975 F100 drums (not sure if they made any changes, but thats what the rearend is from)

My problem with my current setup is that when you first touch the brakes you dont get much, the farther you push the more you get (obviously). To me it feel like the rear brakes are getting fluid sooner than the fronts, which makes for the long pedal travel.

Another thing that is weird, I cant power brake the truck without pushing through the front brakes. I have plenty of power to do it, but the truck will always roll. This is probably the most definitive thing that tells me something is not right. I am pretty sure that if I were to apply the fronts harder I would like it better. I want to do hydroboost anyway, but I am fairly certain that it wont fix the issue I have... I need to balance the braking front/rear.

[ultraranger]

If the pedal travel is too much, the master bore is too small, or the master rod is connected to the pedal in the incorrect location, and not allowing enough stroke of the master. If the effort is to great, then the master bore is too large, the booster or lack of one has a problem, caliber piston too small, brakes aren't bedded, restriction in a line or hose, etc., etc...

What booster do you have? Can you list out exactly what parts you have in the whole system?

I have:
Disc brake master cylinder (not sure what I ordered it for, its been a while. Probably '73 w/ disc brakes? I did go look to see if the reservoirs were different sizes, they are)
Brake booster from a '73 f350 (no telling if it was original)
Stock Drum/Drum distribution block
CV police interceptor front discs
Rear brakes are 1975 F100 drums (not sure if they made any changes, but thats what the rearend is from)

My problem with my current setup is that when you first touch the brakes you dont get much, the farther you push the more you get (obviously). To me it feel like the rear brakes are getting fluid sooner than the fronts, which makes for the long pedal travel.

Another thing that is weird, I cant power brake the truck without pushing through the front brakes. I have plenty of power to do it, but the truck will always roll. This is probably the most definitive thing that tells me something is not right. I am pretty sure that if I were to apply the fronts harder I would like it better. I want to do hydroboost anyway, but I am fairly certain that it wont fix the issue I have... I need to balance the braking front/rear.

A '73 F350 booster is most likely a dual-diaphragm booster. If the MC was ordered for an F350, then the MC would probably have a very large bore --on the order of 1-1/4" diameter. If the MC was ordered for an F100, then the bore diameter is probably 1.00". A small bore MC will give higher output pressure than a large bore MC but, pedal travel will increase as the bore size is reduced. A large bore MC will give a very firm pedal with little travel but it won't put out as much pressure to the brakes as the smaller bore MC will.

http://www.mobiloil.com/USA-English/Mot ... Ratio.aspx

http://www.pirate4x4.com/tech/billavist ... rCylinders

You would need to determine exactly what MC you have to know what to expect from it. I would also bet there's a really good chance you still have some air in the system if it's acting as you describe.

The MC reservoir closest to the firewall is the primary side. The brake line coming out of that port should be going to the primary side of the brake valve and then going to the front brakes. The MC reservoir closest to the radiator is the secondary side. The line coming out of that port should be going to the secondary side of the brake valve and then to the rear brakes.

Basic example of a Ford (drum/drum) MC. Port locations would be the same though for a disc/drum MC (or even an all-wheel disc MC).



Internal view of a Ford drum/drum pressure differential valve. Notice spool inside. The pressure coming out of both the primary & secondary MC ports will be equal on this valve, when everything is 'normal,' to keep the spool centered. If there's a failure in the brake system, the higher hydraulic pressure of the 'good' side will force the spool over to the 'bad' (lower pressure side) to block off the flow of brake fluid to the failed side and to turn on the Brake Warning light on the dash.




This is a 1-1/4" bore MC and the combination valve I pulled from a '75 Ford F350, in order to get the 8-3/4" diameter dual-diaphragm Bendix booster & brackets from, to swap into my F100. --I didn't use the MC, or a replacement like it, because the bore was way too big. I installed a 1.00" bore MC on the booster.




...the '75 F350 brake booster with the 1.00" bore MC installed in my '69 F100.




I believe this cutaway is of a GM disc/drum combination valve but it's very similar to the stock valve from the F350 pictured above.



A drum/drum pressure differential valve only provides one function and that's to sense for a difference in pressure between the primary (front) and secondary (rear) brake circuits. As long as the system is working properly, the valve spool will stay centered. If there's a hard line, flexible line, or wheel cylinder failure, the spool will shift to the low pressure side and the brake warning light come on.

Unlike the drum/drum pressure differential valve, the disc/drum combination valve generally has three main functions: Metering, pressure differential sensing, and proportioning. Discs apply before drums do because the disc pads just basically skim the surface of the rotor even when they are not being applied. They can react almost immediately when you step on the brake pedal. Drum brakes have to, first, overcome the tension of the brake shoe retracting springs and then the wheel cylinders have to fill enough to start pushing the brake shoes towards the inner surface of the drums, to begin making contact.

The disc/drum combination valve's metering circuit is a hydraulic delay that momentarily holds off pressure being applied to the front discs while the rear brakes are given a short amount of time to overcome the rear drum's spring tension and begin putting the shoes in contact with the rear drums. On average, it takes around 30 psi of fluid pressure on the metering valve to overcome the spring tension holding the metering valve closed and then allow brake fluid to begin flowing through the primary side of the combination valve, to reach the front discs. With this small delay, it ends up putting both the front and rear brakes applying at about the same time.

The pressure differential valve spool, inside the disc/drum combination valve, works the same as it does in the drum/drum pressure differential valve.

The proportioning circuit, of the disc/drum combination valve, reduces pressure to the rear drums --but only in a panic stop situation. In all other normal braking situations, the proportioning valve does not function to reduce rear line pressures.

Maybe this will give you a better idea of what the different factory brake valves are and how they work internally. It should also be easy to see that a drum/drum pressure differential valve doesn't work like a disc/drum combination valve and that it would not work correctly if used on a disc/drum system.


The '73-'79 trucks are very similar in size and weight to the '67-'72s of comparable series. If you can get a good (or rebuildable) disc/drum combination valve from a dentside, it would be all you need to correctly control the brake system. Add-on manually-adjustable proportioning valves may look cool when you open the hood but, unless you have a set of pressure gauges and want to go through a lot of math to figure out what the pressure setting of the manual valve needs to be set to, then you have no way of knowing what PSI you are setting it to. The factory combination valve's proportioning circuit is already calibrated to operate at the correct pressure reduction, if you got into a panic stop situation.

[70shortwide]

Maybe my pressure differential valve is tripped and is keeping pressure from going to the front? The brakes bled fine, but the little light has been on for years... I just assumed it was because of having disc brakes...

[ultraranger]

Maybe my pressure differential valve is tripped and is keeping pressure from going to the front? The brakes bled fine, but the little light has been on for years... I just assumed it was because of having disc brakes...

If the warning light is staying on then the valve spool has shifted (or you have a shorted wire). To re-center the valve, you'll have to crack open a bleeder screw on the opposite circuit. If the spool is shifted to the primary side, you would have to open a bleeder on the secondary side, (or vise-versa) and apply the brake pedal slowly until just enough force is built up to turn the warning light off then, close the bleeder. If you apply too much force while trying to center the valve, the problem will switch to the other side.

Not always but most often, the spool shifts to the rear circuit because rear brakes are the first to be bled out after servicing them or from doing a brake swap. The open rear bleeder creates a low pressure situation, on the pressure differential valve , compared to the fronts. The differential valve doesn't know the difference between the system being bled out or if it's a system failure. It just knows there's a pressure differential and the spool shifts accordingly if too much pressure is applied too fast when bleeding out the brakes.

[elgemcdlf]

Do you have the rears adjusted correctly? Rear brakes set pedal height. When I bought my truck it already had the disc swap up front. I couldn't tell what the parts were/are. Did the CV swap and all was fine. I then swapped out the rear for the IRS with drums. Adjusted them and all has been fine. I would start with basics. Adjust the rears and see what that gets you.

[70shortwide]

Rears are adjusted correctly. I took another look at the brake line routing, all is well there. Started bleeding them to center the valve, it went off briefly when I bled the front. Tells me the rears were partially closed off, not the fronts. I ran out of brake fluid, gotta wait til tomorrow to continue. I'll report back.