Explorer rear Discs no work!

[psquare75]

Hey guys.

I have a 78 Mercury Cougar XR7. 523" 460 based engine.

Dyno'd 376 HP and 420 ft lbs

http://i9.photobucket.com/albums/a72/psquare75/460/100_1848.jpg

http://i9.photobucket.com/albums/a72/psquare75/dynoday/100_1881.jpg

Still a work in progress..

My issue with the car is the braking system.

Here's what I have.. chuckles allowed.

Factory Ford Calipers single piston (72-79 style).
12" Rotors from a 72Tbird (upgrade from 11" stock).
proper bracketry to line this all up.
Pads are Brutestops by Raybestos

Rear is stock Ford Explorer Calipers and rotors
Pads are Wagner Thermoquiets

Master is the Lincoln 1 1/8" disc brake master cylinder

Brake booster is a dual diaphragm booster from a 77-79 Ford Ranchero.

1 1/8" was used in both the Explorer and the Lincoln, which used these calipers in front stock.

I also have a wilwood adjustable prop valve, which is inline for the rear ,after a gutted 4 wheel disc prop valve (incidentally, I found a website detailing how to convert a mid 70s Ford disc/drum prop into disc/disc, I'll have to find it and post it, might be helpful to some).


My issue is... the rear brakes are, well, underwhelming to say the least. Rear pressure is 1100 psi... However.. They really don't "DO" anything.

On a dyno, they couldn't lock up the wheels, at all. Slowly dragged the wheels to a stop. They simply cannot lock the rear wheels at all. In fact the car nose dives quite a bit with application of the brakes.

vacuum bled, no leaks. Other things I've tried.
I've tried:
Another lincoln master
My original disc/drum master (smaller bore)
removing the adjustable prop valve
going with a non gutted lincoln disc/disc prop valve
bleeding many different ways (bleeders are at 12"oclock)


I'm about to go back to drum, as the trouble isn't worth the end result. However.. I have one last thought..

Are Explorer calipers "quick take up" type calipers, like GM went to in the early 80s?

My last idea is to try an Explorer disc/drum master cylinder, flip the lines so the 'disc' portion is on the Explorer rear disc, and the drum portion (non quick take up side) is on the Cougar front discs?

I am at my wits end.

[silver69camaro]

At what location was the rear fluid pressure verified?

[psquare75]

The gauge was screwed into the bleeder port of one of the rear calipers.

[silver69camaro]

Good, that eliminates some variables.

Remember that disc brakes are very simple. Fluid pressure moves the piston, which then forces the pad against the disc. Questions to ask:

1. Is the piston seized?
2. Is the caliper able to slide?
3. Are the pads glazed; proper break-in perfomed?

[psquare75]

1. No. If the vehicle is up in the air, both calipers will stop the wheels from spinning.
2. See above. glides are greased/not worn.
3. I've had the same *exact* results with the used pads that came with the calipers, and with the new pads I purchased.

The brake pedal is rather soft (think 1 ton GM van with hydroboost... like a Budget rental van). The fronts work amazingly well considering what the parts are. I can modulate the pedal nicely to just before lockup/etc.

However.. if I clamped off both front calipers, and tried to stop using the rear brakes only.. No lockup.. no matter the brake pressure it'll slowly glide to a stop. I do have the in-rotor-hat emergency brake setup hooked up and working properly.. if I activate the ebrake while rolling, the rear wheels will 9 times out of 10 lock and skid.

I figure the calipers should be able to do the same?

One person locally suggested that maybe the rear tires are too large. However, I highly doubt a 275 50 17 on a steel rim has that much more rotational inertia than say, a 31" tire on a mildly lifted Explorer.

I always hear people crying about how the Explorer rear discs will simply lock up before the front creating a death spiral... I almost wish I had that problem, at least that could be dialed out with an adjustable valve.

Even with a full poly setup, and police -Crown Vic Bilstein shocks, the car immediately nose dives with the discs.. it didn't with the drums.
*edit* I should update my sig.. been awhile, lol.

[silver69camaro]

If sufficient pressure is being applied to the piston, and the piston is indeed free to move, then they should stop the wheel quickly. Simple as that.

But there is a couple things left: pad friction and clamping force. Do you have any way of validating those? Obviously, pad friction is something that probably wouldn't change...even steel blocks placed in the caliper should provide enough friction to fuction.

But...pressure multiplied by area results in a force. Do you have any method to check that?

[psquare75]

Like I said.. up in the air, the wheel locks immediately.

On the ground.. forget it. I have never gotten these to lock at all. Not even briskly stop. I might as well drag my foot out the door, it'd have more of an effect. Not that locking is a sign of good brakes.. but, I'd rather have them lock and back off on their available pressure.

As for the piston, I am not speaking about just one wheel, but both rear wheels calipers.

Pad friction.. I've tried 3 sets of pads.

1. whatever came with the calipers.. 1/2 worn nameless pads.
2. Performance friction carbon metallics.
3. Raybestos, not BruteStop, but I think one below that.

There was no appreciable difference.. cold or hot with any of them.
The calipers do indeed clamp around the rotor.. no abnormal clearance or anything like that.

I will note that on a drive, the rotors DO show that the brakes have been working (no rust/stains on the rotor), and the caliper/rotor does get hot, as if it's been used.

As you can imagine, I'm really frustated. I am well aware that the piston size ratio front to rear isn't ideal. However, they should do more than they do I would think.

I'm *very* tempted to pull it all and just put drums back, but it's more the fact that I don't want to give up yet.

I also saw the 14" rotor post using magnum rotors front and rear, and I am competent enough to make up brackets for calipers, I just seem to have a great deal of trouble making it all work together.

[MonzaRacer]

OK now what you havent figured out is weather the actual pressure is actually doing the appropriate amount of work.
While you have pressure is it able to multiiply into clamping force to actually "stop " the rotor.
I have seen several setups that had too much flow and not enough "pressure rise".
If you have to slow a pressure rise till it peaks you will not get stopping action.
For one I would lookaround for a smaller bore disc/disc master cylinder OR have it sleeved down to say the 7/8 or 15/16 internal diameter to increase the pressure rise curve.
You might even lookat the steel lines to the rear also.
I have seen that 1/4 in rear lines on certain cars have too much volume to let the "rise" happen quick enough.
See if you have 1100 pounders per square inch in a 100 square inch volume of space its still making 1100 pounds of force on EACH SQUARE INCH of that area. Now if one area of that space moves then it will force that part out.
Now then if you lessen the volume the fluid pressure will rise faster to the 1100 PSI and the piston will move out faster.
Now first off i would install a 2psi brake check valve in the rear brakes and then put a smaller bore master cylinder to achieve a better pressure rise curve.
I would give a few things a try before you scrap the whole mess.
Try the check valves and simply buy a couple pieces of 3/16 brake linerun it from front to back as simply as possible andsee what happens, and also you might even try to adapt up a smaller bore master cylinder.
Then I think you will need to figure out where to install the adjustable valve or even install one in both sides of the brake system.
And you may even DUE TO THE COMBINATION OF PARTS may need to try the 10 psi check valves (sometimes used by drum brake systems) to actually get all parts working together properly.

[Apogee]

...friction to fuction.

Not to hijack, but that mispelling just might be worth my signature. Thanks for the chuckle.

As for the Explorer rear discs, there are a ton of guys running these on Early Broncos with up to 42" tires that I've seen (and probably bigger) and loving them...so I can fairly confidently say that it's not your tire size.

Do the Explorer pad abutment brackets (PABs) use shims where the pad backing plates contact? I've seen similar symptoms on the 94-96 Impala SS rear brakes where the pads have worn grooves into the PABs because they weren't designed with shims like the more performance oriented C5/C6 stuff have. The pads hang up and a portion of the clamping force from the caliper goes into the abutment instead of the rotor. It's something the check anyway. Another thing to check would be your hoses, just make sure they're in good shape and not restricting flow. Just because they look good on the outside doesn't mean they are on the inside.

[silver69camaro]

Not to hijack, but that mispelling just might be worth my signature. Thanks for the chuckle.


Heh, I didn't notice the misspell! It sounds borderline inappropriate! Ha!

[psquare75]

I checked the pads I bought. Wagner Thermoquiets. A pretty good street pad, so I thought. These have shims, and I actually added extra shims in a vain attempt to get these to work.

I did some reading and learned something.

Explorer swaps need some sort of spacer? I have no idea what this is all about. Anyone know?

It almost feels like the caliper just stops moving.

:thinking aloud: I wonder if I need to space out the rotor so that it's closer to the outboard caliper pad? Maybe that would help?

[terryr]

It should be centered so both sides have an equal chance to squeeze. Do the pads have an even wear pattern? [Those aren't really a performance pad.]

Does playing with the prop valve change anything?

The front calipers -are they the 60mm or the 75mm? You might be getting a lot more pressure to the front, which would make the rears do less of the work.

Trolls. Any evil trolls around?

[did you find that article on converting prop valve?]

[MonzaRacer]

Actually the prop valve controls the amount of work done.
As for shims thoise are only for silencing pad squeal not anything else.
If he has all mounting hardware and caliper brackets and its properly mounted then thats not an issue.
your problem is the staging of the brakes.
One thiong I thought of have you tried moving the lines on the master cylinder,,honestly that should not be the issue.
Again you may need residual pressure check valves in the real ines, alsi if the lines are 1/4 in this may be causeing some issues but you should still be able to stop the wheels.
Remember the Disc brake doesnt use volume to move the pistons it uses pressure rise. Disc are only supposed to move a few thousands of an inch to clamp the pads.
Use the pressure regulator to rebias the fronts not the rears(most race setups have 2 adjustables).
I still feel that you MAY need the check in the rear line, then start adjusting the adj valve for the front and also make sure you have clean fluid front to back.
As long as the pads arent hanging super hard the brakes are not dragging.
As for the Thermoquiets, well they wont take a lot of track time but they will brake good a not dust as bad as some.
If your getting pressure but its not working the brakes then the pressure has to be against something but not the rotor/pads.
Good Luck

[terryr]

Actually the prop valve controls the amount of work done.


How do you know what his prop valve is doing?