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Converting to disc brake...proportioning valve question.

3K views 29 replies 10 participants last post by  farna 
#1 ·
Ok, we are putting the 78 Granada disc brakes on the son's 63 Falcon. We are going to run a 78 Granada manual disc brake master cylinder. I am getting confusing info from the million sites where this swap is discussed and wanted to get some of your opinions. I see it at simply putting an adjustable proportioning valve at the MC going to the rear brakes, the front gets plumbed right from the master to the discs. Seams easy peasy to me.

Is this pretty much it? I see where people use both the original proportioning valve AND the adjustable, one guy even gutted it and used the Adjustable too! I am thinking the only reason is because the brake light switch is on original proportioning valve!

I could just get a stock valve, or put the switch mounted mechanical style like I did in my Rambler.

What do you all think?

Brian
 
#2 ·
Since you had drums, thats not a prop valve, just a distribution block with the switch in it. I think you leave that and just add the aftermarket valve in the rear line wherever its easy to mount. Keep the setup just add the valve. Of course you know I'm no engineer so take it with a grain of salt. And be careful when dialing in the new valve.
 
#4 ·
Since you had drums, thats not a prop valve, just a distribution block with the switch in it.
The original drum brakes for a 1963 would have been single circuit. There was no distribution block and no differential pressure switch.

To the OP: your original plan is correct. There's a lot of incorrect info on the web. The adjustable prop valve is plumbed in the rear brakes. You want the adjustable because your 63 does not have the same front/back braking needs as the 78. The differential pressure switch is worthless anyway, as it turns on the BRAKE light on the dash just about the time that your foot has gone clear to the floor. Your 63 probably doesn't even have the idiot light anyway.
 
#8 ·
CRAP, I went to check the Granada MS and it would bolt in but the shaft that hooks to the brake pedal is longer. Now, the piston is not quite as deep as the original MS too. The shaft from the original MS could work but the tip isn't round, it's flat. If I were to grind it round then it ends up a lot narrower than the fatter "ball" like end on the Granada one. Other than that it looks like it would work.

Is this a common mod?

Brian
 
#12 ·
Where is the existing brake light switch on the '63 Falcon? Hydraulic ones are okay, you can also use one that goes on the brake pedal from a '65-ish Falcon or Mustang.


I also agree on a simple valve to dial-in the rear brakes if needed. I look for a master with the same rod as the old one. One from a 1972-ish Mustang usually works for me with the Fords.


I did my '59 with a 5-port proportioning valve and eliminated the old brass block. This one has a hydraulic switch on it.


A little hard to see on my car, I bolted the proportioning valve to the inner fender and plumbed it. Adjustment is the dial towards the back.
 

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#18 ·
Ok, in going with many web sites on the swap I picked up a 68 Mustang disc brake master cyl it will bolt in, same rod length all that, exactly the same. The one thing is that it has a 15/16" bore where as the Granada master that went with these calipers has a 1" bore.

Now again, everyone seems to do this but what is the difference, more pressure but less volume?

Brian
 
#19 ·
Now again, everyone seems to do this but what is the difference, more pressure but less volume?

Brian
Exactly. You'll have lower pedal pressure for the same braking, but longer pedal travel will be required to move the same amount of fluid into the calipers. This is only a problem if the pedal or M/C bottom out before the brakes fully engage.
 
#20 ·
Now you are scaring me! LOL How much difference can a 16th inch make? People do use these masters on these cars. But that was with the stock discs, maybe I need to know how big the pistons are in the calipers?

Brian
 
#22 ·
If you really want to get nervous, consider this. A 1" master cylinder at full stroke (1") will move a pair of 2 3/8" (GM mid-size caliper) pistons 0.088" each. That's not even 1/8"! Drop that master to 7/8" and the piston moves a whopping 0.068", a tick larger than a 1/16" drill bit. Of course, if the system is set up properly, with the pads in light contact, with everything solid, very little travel is needed...

Russ
 
#23 ·
Correct, but don't overlook the fact that the rear brakes are still drum brakes. The M/C needs to be able to move enough fluid for both the front calipers and the rear drums, including situations where the rear shoes may be worn and the adjusters not fully adjusted. And yes, it's a dual circuit M/C, but that still requires the brake system to equally pressurize both circuits.
 
#25 ·
Brian---for what it is worth

on my 65 Ranchero

I put a scarebird disc kit on it
using mid 80s buick/caddy calipers and toyota rotors.
(stock spindle-14inch wheels)

used a master from a 76ish mustang II (disc/drum) (using 65 pedal rod)

stock rear drums and wheel cylinders

No prop valve or hold off valve at all-----(come to think about it, the 66 Elky does not have one either)

No power assist (on the Ranchero).

recently had a 'panic stop' episode.

This little truck stopped dead straight and all 4 wheels seemed to lock up at the same time.

Personally, I would install without the valve and install an adjustable valve if needed----easy to make a connection just under the MC or on the firewall if using new lines.
 
#26 ·
No prop valve or hold off valve at all-----(come to think about it, the 66 Elky does not have one either)
Use of a prop valve is not mandatory, as you've proven. If the M/C bore and caliper/wheel cylinder sizes, brake swept area, and vehicle weight distribution are all correctly matched, the brakes will stop just fine without one. In may cases, automakers used them to allow the use of common brake hardware on a wide range of vehicles. Of course, in those cases, the prop valves were tailored to the application.

The problem is maintaining brake balance under a wide range of vehicle load conditions and road traction conditions. Even a prop valve won't guarantee this, but an adjustable one will allow you to compensate. Prior to the advent of electronic antilock brakes, many tucks came with a prop valve that adjusted based on load (actually ride height, but the two are related). Of course, ABS performs the same function electronically now.
 
#28 ·
Did not modify the rod, but if you were to desire to do so, it probably would not take much effort to round it off.

Mine slipped into the piston just fine and the pedal height change I did not notice.

Might also look into changing the brake switch to a pedal mount.
Not sure on the earlier cars but my 65 uses that common pedal/rod switch.
I have a set of 63 pedals outside and they look very similar to the 65.
I'm sure a little research on part numbers would bring you a good answer.

I mention this as the 64/5 pedals are the same in the Falcons and a different set is used in the 64/5 Mustangs
pre 65 used the master cylinder mounted switch and after 65 used the pedal mount.

Might be worth looking into.
 
#30 ·
I've been over this many time on the AMC forum. You may not need a proportioning valve at all. Put the car together without one and take it to a wet (not necessarily standing water wet, but more than just damp) or sand/dirt/loose gravel road. Get up to about 45 then stop hard. Not a panic stop, just a hard stop. If the rears don't lock you should be fine without a valve. Just for kicks get up to 45-50 and lay on the brakes as hard as you can. All four should lock (on the wet/dirt road, might not on dry pavement). If it slides straight you're still good to go, no valve needed. If you have room you might try turning the steering wheel slightly to one side as you hit the brakes in both scenarios above. The rear end should slide out a little if/when it locks, so under the first test it shouldn't slide out, the second it might. If all four lock at the same time it may just slide in a crab position, which would be ideal, but the rear usually locks and continues kicking out since the rear wheels are sliding.

Some cars come with proportioning valves, some don't. If the brakes are balanced right they aren't needed. That can be done with the right combo of brake surface area and wheel cylinder sizing. Hard for a rodder to do it that way, and it's easier for the manufacturer to use a prop valve instead in many cases. The valve can be adjusted to a setup cheaper than getting different size brakes for each combo. The AMC Concord didn't use a valve in the 80s, but he AMC Spirit (short, like a Gremlin) always did. Both used the same rear brakes though, in addition to the short wheelbase making it harder to keep the rear brakes from locking on the Spirit. I ran my 63 American with late 70s Concord front disc brakes and the stock rear brakes, no proportioning valve. Never had an issue -- it stopped perfectly straight every time. I had to make a panic stop (well, didn't quite make it...) once and it tracked perfectly straight on wet pavement down hill... would only have been worse in ice and snow! All four were locked -- I had both feet on the pedal and pulling back on the steering wheel! Idiot taxi driver suddenly slammed on brakes right in front of me at about 45 in Chicago, coming off a bridge in light rain!
 
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