Just to recap so far you have replaced/done (correct me if I’m wrong here):
Master cylinder
Rear wheel cylinders (both right?)
Front calipers converted to Isuzu
Front rubber flex lines
Rear rubber flex line (this one I’m not sure on, I seem to remember but I’m not sure)
One thing that stands out to me as a possibility is the proportioning valve. I’ve never heard of one of these going out in a Chevette but I have seen it in other vehicles. With the age of these cars now, perhaps this will start cropping up.
I’ve also been doing some thinking and I may have had a thought. You main complaint with the brakes is that the pedal is soft/ long travel. How well does it stop though? Can you lock the tires up? I ask because the master cylinder bore size changed over the years. According to Rockauto’s catalog 76-82 is a 3/4” bore and 83-87 is a 7/8” bore. The master cylinder size increased with the improved brake calipers in 83. I also looked up the caliper piston diameter, 80-82 49mm and 83-87 52mm. I looked up the Impulse master cylinder its 7/8” and the caliper diameter is 54mm. So to recap in chart form from Rocks catalog:
Year_____Master Bore___Caliper Diameter
1980-82___3/4”__________49mm
1983-87___7/8"__________52mm
Impulse___7/8"__________54mm
I’ve never sat down and looked at these numbers before and I’ve never seen anyone else pull these numbers before. Seeing this and thinking of what I think I remember your pedal issues to be and that your car is an 80? I think it may be that the Impulse brakes will need a 83+ 7/8” master cylinder. Yes, I may be over simplifying as I don’t know the pedal ratio and bore stroke of the Chevette and Impulse master cylinders. I do think that the telling sign is the step up in Chevette master with 83 caliper. Chevy made that master cylinder change over a 3mm change in caliper diameter, the Impulse is an additional 2mm on the 83+.
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