[XRacer]

Federal law requires that the various grades of brake fluid be compatible so changing to DOT 4 won't be a problem. This federally mandated compatibility is a good thing since in order to avoid any possibility of contamination from the previously used fluid would require replacing the entire brake system and the hydraulic throw out bearing [ otherwise known as the clutch slave cylinder ] with all new parts. Some amount of old fluid will always be present after flushing. OK, if anyone wants to pick nits, I suppose you could flush the system with alcohol before adding the new and different fluid. Good luck with the throw out bearing. Ford seems to be a bit confused regarding what to use. The master cylinder cap says to use DOT 3, my owners manual calls for DOT 4. The dealers seem to prefer DOT 3. The only way I can see the hydraulic clutch system causing the pedal hang up would be if there is a some sort of restriction in the line or a defective throw out bearing. Pushing the pedal down would generate enough pressure to overcome a minor restriction, but if the fluid can't return unrestricted when the pedal is released it MIGHT cause the pedal to hang up. Generally when a clutch pedal hangs up at high rpm it's due to the pressure plate. Diaphragm type clutches have had issues with this for over half a century. At 73 I am old enough to remember when Chevy guys in the '50's & '60's would replace their Chevy diaphragm clutches [ they really were crap ] with Chrysler Borg & Beck style clutches since they used the same bolt pattern and the Mopar clutches were pretty darn good for a factory clutch. Most modern diaphragm clutches, both factory and aftermarket seem to work pretty well. I use a Sachs diaphragm in my '72 'Cuda and I've never had any problems with it. Of course being an old classic it has a mechanical pedal linkage. The rather stiff clutch assist spring COULD possibly be the culprit or at least contribute to the problem. That said, I never had any problem with mine. The main reason I went with the Steeda spring was for the better feel. The best rear axle ratio depends on how the car is used. For a 1/4 mile drag racer using larger than stock diameter rear tires the 3.73 would be best so long as you can still reach the finish line in 4 th. and avoid the final shift into 5 th. On a tight road course or autocross with short straights, 3.73's would still be good. Longer straights and 3.55's or 3.31's are the way to go. IMO the 3.55's are the "Goldilocks" choice. There is a big difference between 3.73's and 3.31's, not so much when going from one of them to 3.55. I have the standard 3.31's in my '14 GT and I wouldn't bother changing them to 3.55's for the minor difference involved unless I found a complete low mileage rear end somewhere for a killer price.