79 Elco soft pedal to the floor.

[ratengine]

Sons car 79 with power disc brakes all stock. Mild 350. I have 19 LBS of vacuum @ idle. We have replaced the front calipers rotors and pads. New brake hoses new master cylinder, new power booster. Rear shoes and hardware kit. Have bled the system. No air in lines or leaks any where.I have bench bled the M\C and adjusted rear brakes with slight drag. The pedal stays firm with with the car off. As soon as he drives 1\2 block brake light comes on and brake pedal goes to the floor, it may work well a couple of times but then pedal gets soft again. I am at a lost here. All I have left is the rear wheel cylinders. I also tried a new M\C in case the I got crapped out. I don't think it's the distribution valve. So any help or ideals welcome.
Thanks Dave

 

[LS6 Tommy]

Almost sounds like the pressure differential warning switch in the combo valve is not resetting. Did you step on the pedal with any part ot the brake system open? If so, you just need to bleed & add fluid until you get the line pressure up to 450 for it to unlatch. If that doesn't do it, replace the combo valve. If that's not the problem, it may be a long shot, but is one of the caliper hoses twisted? I ran into that once a billion years ago.

Tommy

 

[ratengine]

Tommy, Thanks for the tip. Well it's worth a try anyway. Didn't think it could be the dist. block and that there is a pressure valve in there. I may convert to manual brakes if it don't work out.