back brakes won't bleed on 1992 Jeep Wrangler

Fluid is full and front brakes are working. Pedal is spongy and the back brakes won't bleed at all

Asked by for the 1992 Jeep Wrangler
Check for pressure at the brake master cylinder ports. If the front has pressure and the rear does not, the master cylinder may be bad. IF both have pressure, check the lines at the ABS unit. IF there is pressure in and none out, the unit may be bad.