Strange problem. I would love to get one of these cars to my house to track it down.
I know of a CR with the 11" front and rear, GM calipers, mustang master and booster (yes, you need the ranger resevior) and a GM proportioning valve and it would through you throw the window. Basically the same set up that folks are having trouble with.
I have got to think that some folks may have booster issues. But according to Don, that has not resolved the issue.
If the condition came on gradually, I could suspect glazing.
If it is like that out of the box then prehaps the valve being used is for a disc/drum setup. I would have to read up again but seems to me they delay the drums for a time until the discs kick in. Like I said, I have to go back and read up on it.
A line restriction or to small of a line could be an issue. Restrictive fittings etc. 3/16" is a good size to use for a brake system.
I am going to keep chewy on this.
For those of you whose four wheel disc systems are working good, I have some ideas for some serious race setups. Of course there are variances.
If you have a good system and just want a bit more, good fluid, bled often, and better pads are usually a good upgrade. A higher temp, higher CF in the front pad is usually the way to go since on the 'model' I am using for CR in a 1G stop the fronts are doing 65% of the work and with the same size rotor the temp rise in the front will just be that much faster. EBC, Hawk, and Porterfield all make good pads that without looking I would think would fit your application.
I don't know if I mentioned this before or not but the base line CR system I am working on is 6to1 peddle, 1" master, 60mm caliper pistons ( I have not been able to confirm if they are the 60mm or 2.5" ones yet), 11" rotors at all four corners. The system on paper seems to have to much rear brake but I am hearing little to support that which to me says there is another issue aside from the basic design. Plus modifications to the system seem to yeild little results for all, but work for some.
By the way, if you are using the GM metric calipers I see that U.S. Brake has a bolt on replacement with an over side caliper that will give you more front brake force. The Metric GM is 60mm where the over size one is 2.75". That may not sound like must but it is about 33% larger than the metric caliper and 21% larger than the 2.5. That is in area by the way. I have no clue if you can bolt on a metric where a standard one is. But if you combine the base system I described above with the oversize caliper and a front pad with a CF of about .1 more than the rear you will have a pretty well balanced system and reduced peddle effort.
If it was my car I would go through it with a fine tooth comb before I changed parts, there is NO reason the 'stock' setup I described above should not work well. Heck I recall one guy with 9" rotors on the front and drums out back and he said his system works fine! 11"ers all the way around has got to work better!
Are the caliper adaptors a stock or custom piece? Not having caliper normal to the rotor is a issue. Deflection is another. Any one getting pad taper? Hard to beleive how much more difficult it is to stop on a set of tapered pads. Measure the thickness front and rear on the pad, if it is not the same, it is tapered.
Any one have a set of hydrualic pressure guages? I would like to take reading at the master cylinder and every junction all the way out to the calipers. This would tell SO much. If that is all good then I would be looking for taper, glazing, deflection, runout, caliper normal to rotor, alignment, suspension condition, pushrod adjustment, peddle geometry, etc, etc, etc.
Doesn't it sound like a fun filled day!! Bring a gallon of brake fluid with you
Rick