I have a 90 DB1. No ABS.

Hit a curb about a month ago and did some pretty serious damage to the driverside front suspension. I'm just finishing repairing that when I go to bleed the brakes and there's almost no pressure in the pedal. Here's what I know right now:

The right front and left rear brakes seem to work fine. The left front and right rear brakes apply almost no clamping force.

The proportioning valve and master cylinder have been replaced. Also the two rear calipers are fairly new.

When the car is off, there is decent pedal pressure but it goes to the floor every time and does not get any firmer. When the car is on, there is almost no pedal pressure and the pedal rises quite slowly.

I deduce from searching that it could be a bad brake booster. But that does not explain why two calipers clamp and two do not, whether the car is off or not. Since the bad brakes are LF and RR I figured it had to be something with the proportioning valve, but that was replaced and changed nothing.

Please offer any advice, I've already bought two new parts in the brake system and can't afford to keep replacing parts without understanding what's gone wrong here...

 

Left-front & right-rear work off the same piston of the MC. They're on the same hydraulic circuit. A really big air bubble in that circuit could explain it. Did you bench-bleed the new MC? Maybe there's air stuck in that circuit at the MC.

Why/when did you replace the MC? Did your brakes act this way before replacing it? I guess it's possible the new MC is defective.

The booster has no control over individual calipers. All it does is push on the MC piston rod.