Did you adjust the rear shoes out tight against the drums, then back off until it just clears and rolls without drag? 

Master cylinder plumbed correctly?  (bowl near firewall goes to front brakes)

What bore size is the master cylinder?

I'm not certain but, I'm guessing the Wilwood valve you are referencing is this one?

http://www.wilwood.com/Images/MasterCylinders/Master%20Cylinder%20Photos-Large/260-11179-lg.jpg


That valve is not the equivalent (in function) of the stock valve you have. It is merely a split manifold with a manually-adjustable proportioning valve for the rear brakes. It does not have a pressure differential valve spool inside nor does it have a metering valve to momentarily hold off the front brakes, before the rears have a chance to start moving the shoes towards the friction surfaces of the  rear drums. 

The switch sticking out the end of the Wilwood valve isn't for the brake warning light on the dash (again, that Wilwood valve doesn't have a pressure differential valve spool inside).  The switch on that valve is for turning on the brake lights --if you don't have a switch located on the brake pedal.

That Wilwood valve might be great for a track car running 4-wheel discs but, personally, I wouldn't run it on my street-driven vehicle.

calipers are correct the fronts lock up when you press the pedal its the rears that dont bite but fliud comes out of bleed nipple

GPatrick wrote:

Have you verified that the pressure differential spool is indeed centered?  If o-rings have deteriorated you may still get some limited flow to the rears but not sufficient for proper bleeding if the spool is off-centered.  A good test might be to use a plug or a short piece of brake line that is blocked off for the rears and check to see if you still have a spongy pedal.  If it firms up, then you still have air in the rears somewhere.  Have you replaced the rubber line for the rears or just the fronts?

all the brake lines are new,i will try that

GPatrick wrote:

Manual or power?  Check to see that the master is being fully released when the brake pedal is retracted.  This often creates a locked brake condition but may also affect the ability to bleed if the piston allowed to move to its retracted postion.  This one is pretty low on the list of possible problems but you have tried most of the other normal checks.

power brakes

Being that your Mustang is a '69 model, it should have a brake warning indicator on the dash to tell you if your pressure differential valve has shifted off-center.

This of course all depends on if the wiring pigtail is connected to the switch on the valve, the switch is good and that the indicator bulb is in working order.

GT350 wrote:

GPatrick wrote:

With power brakes, since you have been changing out so many components, it is possible that you may not have the correct clearance between the actuator rod and the master cylinder piston.  You may want to back of the MS mounting nuts a few turns and see if you are getting contact between the rod and the piston before the MS seats.  Excessive gap usually produces a low pedal and not a soft pedal but your purge when bleeding will be limited due to a low volume of fluid being pumped.  Something to add to the list of potential problems. and one you should not assume is correct until measured.  I have lot of experience with the "it can't be that" being exactly what the problem is.

should the rod make contact when you bolt in the master cyl? obviously not pushing the fluid thru just touching,so if there is a gap between the rod and master cyl when bolted in can also cause these problems?i do have a low pedal,

 

 The rod between the MC and the booster should have a clearance, .005"... also one of the brake lines looks like copper??,if so change it to steel...be sure you have a disc drum MC with the residual pressure valve pressurizing the rear drums Not the front....jj

MarkinSC wrote:

GT350 wrote:

im getting a soft pedal after bleeding the brakes below i alist of new parts fitted.
master cyl,brake hoses,discs,calipers.rear wheel cylinders.
i bench bled the master cyl before fitting into car,im finding the front wheels lock up on braking and the rears dont, even though fluid comes out of the rear bleed nipples,at first i though it was master cyl so i replaced it with another same problem soft pedal no rear brakes locking up any ideas what i check next

Its possible for fluid to come out the rear brake wheel cylinders while bleeding, and not have the rear brake shoes extend out and hit the drums.  if the rear brakes don't lock up at all, and you have fluid geting to the wheel cylinders when you bleed the brakes., then I would check the following:

the shoes are stuck in a groove on the backing plate.  The backing plate has raised "bosses" or ledges where the metal part of the shoes ride on when brakes are applied and then released. It is common for there to be grooves cut into these bosses that prevent the shoes from moving outward to hit the drum.  these bosses are supposed to be lubricated for the shoes to ride on them, but they are usually overlooked.

It is possible that your drums have been cut too much and they are out of wear specs. Drums could have also worn down past their tolerable limits.  The more metal that has been cut or worn from the inside circumference of the drum, the farther the shoes have to travel outward to meet the drum.

It is also possible that the rear brake springs and shoes are not installed properly, or the wrong springs were installed so the fluid pressure can't overcome the spring pressure to push them shoes outward.
 

checked all springs etc all new and correctly fitted ,my dash brake warning light is staying on,can a faulty press diff valve cause the problem im having on rear brakes?

MustangSteve wrote:

The light being ON denotes the valve has shifted and is blocking flow to the rear brakes.  Loosen a front bleeder and stomp on the brake pedal hard a couple of times until the light goes out.  Then tighten the bleeder and all should be well.

thanks steve will attack that issue tomorrow

Last edited by GT350 (7/15/2013 12:55 PM)

update
fitted new master cyl bench bled it then fitted to car i have brakes but a soft pedal bled the car a few times rear brakes will lock when pedal is pushed to floor but when driving its a soft pedal,whats the best brake bleeding  tips

MustangSteve wrote:

If you installed a 7/8" MC, then it is likely too small.  You need 1" bore.  Too small of  mc bore will give a soft, spongy pedal but higher line pressure.

The 67-73 Mustang disc mc is 1.0".

i did order the correct master cyl from cjpony parts http://www.cjponyparts.com/cardone-select-brake-master-cylinder-new-power-disc-1967-1972/p/BMC09/

Last edited by GT350 (7/20/2013 5:52 PM)

I would re-visit post 5 above regarding left and right front calipers and also check your booster rod to master cylinder clearance.  I have often convinced myself that I did something right only to find that the car was smarter than me.  I didn't review all the postings but what is the condition of your booster?  Other than that, you might try to find a local Mustang guy that can give it the once over.  When bleeding did you install the temporary "plug" in the distribution block to keep the spool from going off center?  Are you trying to use any exotic brake fluid that might be incompatible with your system?