Lousy Pedal after Rear Disc Swap

[psquare75]

So I got this finished last weekend.
http://www.v6z24.com/jbodyforum/files/000_0174_162.jpg

Parts are as follows.
Master Cylinder - Lincoln Mark V 77-79 with 4 wheel discs (1.125 bore)
Proportioning valve - Lincoln Mark V 77-79 with 4 wheel discs.. I gutted it and used the ford prop valve brass plug.
Power Booster.. Stock Cougar
Front calipers - Stock Cougar (midsized ford)
Rear calipers - Ford Explorer (unsure of years.. that same kit Ford sells).
I also have an adjustable valve in line with the rear.

My problem is two fold.
1. The pedal is REALLY easy to push. It has no firmness, however the front brakes work well. So well in fact, the car stops great locking up the front wheels, nosediving the whole car. Just the pedal has 'feedback'. It goes right to the floor with little effort. This leads to #2.

#2. The rear calipers aren't doing anything worth noting. I've had line locks on both front hoses. The rears barely hold the car still, and forget about locking up the rear tires.. at all. I have a hard time believing that 150 wheel HP is too much, hah.

I've bled the system a few times.
I've bled the rear calipers "off" the car, so the bleeder was in the actual position it should be, as I had to rotate the brackets slightly. (I had the rotor between the caliper)
I've changed the master 1x, still the same thing.
I've got an SSBC adjustable inline with rear, after the proportioning valve... it's all the way clockwise/increase. Turning it reallly didn't make that much of a difference.

I would think that with the larger bore size the pedal would be even harder to press, vs the 1" stock Explorer unit. I can deal with problem #1... but, is it at all possible that both rear calipers are bad.. (they DO grab the rotor).

Any ideas?

[hotroddr]

I didnt see anything in your post about an emergency brake setup. I know with my rear disc brakes the play between the pad and the rotor is taken out by applying the e brake and releasing it. Thats how a lot of rear discs self adjust. If you hooked up the e brake pull and release it a few times, if not, get under the car and do it manually...this might fix it.

[gmachinz]

Aside from using an adj. prop. valve, why gut the OE one? How off is the weight bias f/r of your car compared to the Mark V? For example, I use a stock type prop. valve from a 4-wh. disc brake 3rd gen. Camaro along with it's booster and mc and I don't have a lazy pedal. How big are your lines going to the rear? If volume goes up, pressure comes down-ie anything bigger than one 1/4" line split to two 3/16" lines can cause your rears to hardly grab. -Jabin

[psquare75]

Hotroddr: These are not like the GM Caddilac calipers. The Explorer rear disc setup has a separate Ebrake which is actually under the hat of the rotor. The rear caliper looks identical to a front caliper (except with regards to size, orientation, etc).

Gmachinz: The weight bias is/was probably close, but most everyone and their brother (literally), including Ford Tech, said because of height differences, tire size differences, and brake capability differences (Explorer vs stock Mark V), that I'd be better off with the inline valve only. For what it's worth, it's open all the way. As for rear line size, it's actually 3/16" the whole way to the rear, which is what was there when the car had rear drums.

[gmachinz]

It seems that maybe two things are happening here. First, even though it's a sealed hydraulic system, since you bypassed the rear prop. valve (essentially anyway by gutting it...) and have gone instead with a universal type plumbed somewhere further back I presume, that the brake pressure being applied is reaching the front calipers much faster than the rears. Your front brakes are working as designed but the rears are late to the party so to speak. You have 3/16" lines going to the back so I would start there. I wonder if the pressure is good but not enough volume? I would try running a single 1/4" line to the rear (if you can plumb it this way of course...) and splitting off to two 3/16" lines to each side. This would give you volume and pressure. Or, the other issue might be too big of a primary bore in your mc? I wouldn't think so but can you swap the mc out for one with a 1" bore? A smaller bore would yield a little more pressure but the more I think about it, it seems as though the rears are not getting enough fluid volume. Lastly, what is your vacuum reading at your intake before and after applying your brakes? I'm trying to visualize your problems and offer suggestions that I would try. -Jabin

[psquare75]

I should look up the size of the Explorer rear brake line to see what diameter it is, however, it would still bottleneck at the smallest point, that being the hoses.

In a continuing quest. I've tried the following
Removed adjustable valve
Changed factory prop valve to a non gutted 4 wheel disc unit.

I checked all the fittings, each one is dry.

With the front brakes blocked off, the rears barely even coast the car to a stop. They won't even hold the back wheels during a holeshot (you're telling me that this 351w has that much power?) Even on dirt, the back wheels simply won't lock.. and the pedal is lousy. I'm thinking either.
A. both calipers in the rear are bad (I doubt that, as they bleed fine, and move fine/no hangups).
B. I need to try that other dual diaphragm vacuum booster.. But I feel like that would only apply if my pedal was too hard, and I couldn't press it.
C. Both rear hoses are collapsed.. But again..
D. The master doesn't put out enough pressure for the rear discs. ** This one I'm not sure of.

I'm stumped, and about to put these parts on eBay and try something else.

As for vacuum, the engine is a totally stock 351W. The disc/drum combo worked fine (points to me for changing something that worked, hah)

[psquare75]

Tonight I tried my original 1" disc/drum master (only difference should be diameter and the reservoir size)..

Again, same deal. In fact, I had all the wheels clamped off except the left rear... pedal felt fine.. but, it just doesnt stop, or do anything. You'd figure one wheel would spike pressure to lockup.. Nope.. it just barely grabs, and still rolls along. The Ebrake in the center of the hub stops the car better and locks up the rears better.. I'm almost tempted to put the drums back and say forget it. https://static1.pt-content.com/images/pt/2006/01/frown-1.gif

It's at the point where I'm about to drop the $499 for the 11.5" cobra rear disc kit I found, but I'm VERY afraid that I'll still have the same problem.