I was cruising along quite nicely when someone decided to back out of their driveway in front of me. I slammed on the brakes of my non ABS 85 and proceeded to slide on by the culprit just missing them amongst a cloud of tire smoke. I could tell I only locked up the fronts and I was on the pedal hard. I checked at home and all 4 wheels have brake function (wheels turn, pedal pressed, wheels stop). When I went back to the scene of the almost accident, I saw only 2 skid marks. I did some wet road testing and I have hardly any rear braking power even when the pedal is really pressed. I'm familiar w/ the whole "almost all braking is done w/ the fronts", but this is waaayy beyond that. I have changed the proportioning valve w/no results. The pedal feels perfect, brake fluids good, no air in the system, no kinked lines, calipers don't bind, pads are good, rotors good. The only thing left is the Master Cylinder but I figured I'd see if anyone else has a thought before I do that. I actually felt a little foolish when a simple slam on the brakes and stop turned into a smoky squealing mess.

 

I'd revisit the pads and rotors. Thick pads and smooth rotors might not be developing enough friction. Especially if you don't brake hard often enough. (Glazing) Maybe the fronts and backs are different brands?

Then look at the brake bias valve.

 

I have a 33 BAR regulator on my '91 and the 55 BAR on my '89. I have yet to experience a rear lock-up (except on the track under extreme trail-braking.) But, if you're sure - and it sounds like you are - and it isn't glazing as Glen suggested then the next step might be to check the hydraulic pressure at the calipers with a brake pressure gauge. You're looking for 260 to 700+ PSI depending upon the proportioning valve you've installed. You might have a block in the system. A pressure test would at least let you know if that's the case or not.

 

Thanks Glen and Dave. Those are 2 things I have not done yet. Looks like I'll have all winter to do it.

Randy