Isn't this the problem inherent with drum brakes? They suck in any performance application, and don't give the driver much "metering" ability when pushing them to the limit. Have you tried adjusting them?

I would say you need to go all discs if doing autocross. Also proportioning will be tricky anyway when braking hard because there is very little weight on the rear tires.

 

Maybe the new master cylinder is pushing too much to the rear drums now? I have no idea about the prop valve though.

 

Eesh, Locking rears is not a safe or fast method for rotating the car. You can drive fast in a car with too much front bias, but one with too much rear bias is just plain dangerous.

For 96-00, the prop valve is the same part number for the Si and for the models with 9.5" fronts. The EX is the oddball of the bunch with its own prop valve. So, you already have the "correct" valve on there for a rear disc conversion. I'd start with the rear conversion and go from there. Without knowing anything about the complete rest of your setup it's hard to say what your caliper swap has done to the bias. What calipers did you change to? What is the piston area compared to your stock calipers? Did you change rotor size too? It's a pretty safe bet you went bigger, but there are teeny little setups for dedicated Auto-X setups that fit under 13" wheels... It's highly likely that you are pushing much more piston in the front brakes than you were before, building less pressure for a given amount of pedal travel... Food for thought.

Changing to the aforementioned EX prop valve may be a valid experiment too.

Changing the master cylinder size does not change the bias.

 

Thanks for the clarification. I wasn't sure if the larger bored Master Cylinders effected bias at all.

 

 

I'd find the answers to spAdam's questions before you go through the expense of doing a rear disk conversion. Drum brakes have some unusual behaviors but they don't inherently lock up more easily than disk brakes. In fact they are generally harder to lock than disks, which is why rear drum cars come with brake prop valves with more rear bias (exactly as you originally stated).

If it turns out that you aren't building enough pedal pressure up front due to larger caliper piston area, you may have to go to an aftermarket brake proportioning valve to get the brake bias right. I had to do exactly this to keep the rears from locking in my car. I took out the stock proportioning valve, added tees for the brake lines, and installed a Wilwood pressure-reducing type brake proportioning valve inline with the rear brakes. I have it set 1/2 turn below maximum reduction (brake bias almost fully forward), which lets me lock the fronts just before the rears do under hard braking.