no brake pedal when start engine?

[683/4farm]

Did disc brake swap with 84 parts,at first didnt have any pedal no matter what,adjusted the rod where yhe pedal is almost all the way up,then rebled lines incase and now have great pedal until i start the truck then to the floor again,then turn truck off and pump brakes twice and pedal is good again,any ideas?

[old Chevy guy]

Need more info. What did you swap the parts into? Did it have disk brakes before? Did you change the proportioning valve? Did you change the master and or the booster? If you had drum/ drum brakes you have to change more parts than just the spindles and brakes.

[683/4farm]

I changed crossmember rotor to rotor,booster and master all from 84 3/4 ton to my 68 gmc 3/4 ton originally had drum drum ,are new brake calipers ,hoses,and master though ,did reuse whatever prop valve that was on my truck already

[70STOVEBOLT]

That's the problem. Drum/drum system does not use a proportioning valve. You will need the prop valve from the drum/disc system.

[geezer#99]

And you need to change your pedal ratio!
Power needs 4.5 to 1 ratio.
Non power is 6 to 1.
Drill a new hole in the brake pedal to attach the rod lower.
Usually 1 1/2 inches is close.
Measure to be sure.

[683/4farm]

Ok, tuesday i will change those two things thank you guys

[70STOVEBOLT]

Or you can get an adjustable rod, which will be threaded with a jam nut.

[683/4farm]

The rod on mine is adjustable but i only have the one pedal hole so I'll be drilling the other

[ERASER5]

If you used the 67-72 bracket with the pivot bar, no need to drill a new hole. The pivot bar gives the needed reduction. If you just bolted on the '84 M/C, you will need to drill a new hole.

[683/4farm]

I did Just bolt the 84 On

[683/4farm]

Disk brake swap done! Drilled lower hole and moved rod down to it and there ya go brakes have good pedal while running and moving it outside and back in stopped good thank you all for the help