No conventional brake prop valves in any 88-91??

[85crownHPP]

Since finding this out on my p72 Ive been curious.. did some looking at the junkyard and at my 91, and this seems to be correct. These are also the years with the modern master cylinder style and what I guess is a restrictor/shutoff valve in the rear line at the MC.
Every 88-89 I looked at has just some sort of junction block for the front lines only down on the frame rail and a dedicated line going directly to the rear. The 90-91's have nothing on the rail with two lines for the front coming off the MC.
This is probably good info for anyone wanting to upgrade the old style MC... I'm still not sure what it means you would need as far as valves - whether you would have to eliminate the frame rail one or not.
Either way, it looks like there will be lots of new plumbing involved.

Attached are pics of the three different scenarios...

And it looks like the 85's header flange ball is leaking a little bit... I thought I felt those threads giving out when i tightened it up. ggrrrr...

[1987cp]

'90 and '91 cars don't have distribution blocks? I like that, provided the proportioning at the master cylinder is accurate. The original proportioning valve on my '87 wagon failed a few years back, and I couldn't get a replacement at all, which is why that car ended up with a tee block and a Wilwood adjustable prop valve.

[pantera77]

No, at least my 91 doesn't. The only thing on it is the little reducer thing on the master cylinder. Made replacing all the brake lines that much easier.

[gadget73]

Hm. Anyone have a factory shop manual for those years to see if there are specs given for a prop valve? There has to be some means to give more fluid to the front than back, otherwise the rear drums would lock up way before the fronts grabbed. A reducer of some means in the MC would do it.

I actually wonder if converting to that setup may have some atvantage for rear disc or big brake cars. I was doing some research into that, and it seems that later models have the same proportioning specs as the box wagons got, but the specifics of the valve seem to be different, something about pressure metering and some other option that I can't recall without looking at the specs.

[1987cp]

Well, it would seem pretty clear that if there is no separate proportioning valve, there must be a pressure-reducing restriction of some sort built into the master cylinder. IIRC, rear disk systems want higher pressure to the back than a drum system does, so I guess that approach may be useful only if one were to obtain a master designed for a rear-disk car. Unless the transition year Panthers, such as a '91 Vic with rear drums versus a '91 Town Car with rear disks, always got the same master cylinder, regardless of what brakes were used?

[85crownHPP]

Could be they just got a different valve for the rear... all the 88-91 box ones I looked at definitely have some sort of valve type thing in the rear line output.

[1987cp]

That's interesting. So this valve type thing does indeed appear to be separate from the master cylinder assembly? If so, that would make it beyond convenient to bolt in one of these masters and add a Summit, Wilwood, or SSBC adjustable prop valve.

[Glen]

I didn't change anything on mine when I put the 97 rear axle with disk's in my 89. All I did was put the axle in. I know it stops a lot better, the pedal is higher and feels a lot better.

[gadget73]

Some research tells me that perhaps that combination valve (thats actually whats on our cars) is not really ideal for a rear disc setup. Whats in the magic block is both a proportioning valve, and a metering valve. The prop valve splits the fluid pressure front and rear so the backs get less pressure. More on this in a minute. The metering valve is a device specifically designed for disc/drum combos. What it does is allow the first 100 psi to go to the rear brakes to get the shoes into position, then the front brakes get pressure once you exceed 100 psi system pressure. With 4 wheel disc, you don't need or even want a metering valve. If you look in the shop manual, it tells you that the stock valve is a pressure metering type, and later models that came with 4 wheel disc are not pressure metering. I really need to look into switching to a later model prop valve to see if it helps.

Another thing that drums apparently have is a residual valve. This keeps 10 psi in the rear brake line to keep the shoes closer to the drum. Don't need or want that with disc brakes. I have no idea where this part is hidden though, but when I find out I'll pass it on. I'm told that its often in the master cylinder itself, but I've never looked.


The more on the prop valve that I promised. Disc brakes have a linear pressure to torque relationship. In other words, double the fluid pressure, and you double your braking power. Drum brakes don't have a linear relationship. This has to do with the mechanical atvantage that drum brakes get. Basically they self-apply to a point. If you double the pressure on a drum, you may get 125% more brake effort for example. The prop valve is intended to help even out the curves so you get more consistant braking. With 4 wheel discs, this may not be needed, or may not be needed in the specific proportioning that the original valve was designed with. It also depends what front brakes you have. Big front brakes need to reduce pressure to the rears less than stock front brakes will because the larger front brakes will have more braking capability.

some more reading, with probably better explanations of the above. pictures too if you're of the graph type.

http://www.thebrakeman.com/valvetechi

So basically to make a long story long, removing the stock magic block for an adjustable prop valve on a totally stock braking system is probably a bad idea. Retaining the stock magic block on modified brake systems is probably also not a spectacular idea. When I have a chance I'm going to play with that prop valve and see what I can get. I've always kind of felt there should be a little more braking capabilty in my car and I'm wondering if that valve is where its hidden.

[pantera77]

So what would you recommend for those without the magic block? I was thinking of converting to disc in the not so distant future, but now I don't know...

[gabegt90]

feels fine with my big brake swap good info guys I wonder where that valve is hidden lol

[gadget73]

So what would you recommend for those without the magic block? I was thinking of converting to disc in the not so distant future, but now I don't know...

Don't know. There must be some means of changing the pressure so the rear drums work. Might be in the MC or elsewhere, but if it was just run with nothing at all, the rear drums would lock up all the time.

[gadget73]

Stuff I uploaded elsewhere, just to muddy the waters a bit:

I'm off to go scan the 86 manual for the same data.

Does anyone have a shop manual for a 98-2002 model? I could really use the master cyl and valve specs for something in that model year range.

[gadget73]

More reading. This data is specific to a drum/drum to disc/drum conversion on Fords. The valve looks the same, or damn close to what boxes got. Its posted more for reference to how the stock disc/drum setup with it's valves and master cylinder operate than anything else. Note the mention of the residual valve living inside the master cylinder. One more piece of the puzzle solved.

http://www.cometeastcarclub.org/PDFs/BrakePlumbing.pdf

[gadget73]

The 1986 brake system info. Note the "pressure metering" box is checked. This means it puts pressure to the rear brakes first. Pressure differential means theres an idiot light switch in there. Basically its a slidey piston thats supposed to sit centered when things are working. If it shoves one way or the other, it means there is a leak in either the front or rear brake system, and it trips that brake light on the dash. I don't know how later models get this done, but I assume there is either a seperate differential valve somewhere, or its in the abs magic block.