Brakes problem

[Cheyenne1010]

What's up guys,I got a 1973 chevy C10 cheyenne.I'm having brake problems but first I'll tell you the parts I've replaced with new ones. brake master cylinder,brake power booster, front brake calipers, rear brake cylinders, front rubber brake line and rear rubber brake line,front brake pads,rear brake shoes and proportioning valve. I bled out all the old brake fluid,bled the brakes in the right order.I adjusted the rear shoes to where they are barely rubbing the drums and I benched bled the master. so the problem I'm having is when I go to press on the brake pedal it'll be firm and the pedal will be at the top of its travel,then I'll come to the next stop and press the pedal down again and it goes to the floor,about a couple of inches from the floor board.if I step on the brakes real quick the pedal will go to the floor.sometimes the pedal will be good and then other times it'll go to the floor.I've bought 3 different master cylinders thinking that maybe it might be defective. this problem I'm having with my truck is really stressing me out,so if anybody has experience this kind of problem with their truck I'd appreciate your help very much.

[Henry]

Hi Cheyenne:
When you bled your brakes, did you make sure the combination valve was held in the "open" position before you started the bleeding?
Regards,
Henry

[Cheyenne1010]

How is that procedure done?the combination valve is new on my truck

[Henry]

Hi Cheyenne:
The combination valve should have a pin underneath a rubber boot at one end. The factory used to recommend using a special tool to press the pin in while bleeding. This tool was essentially a piece of sheet metal with a right angle bend in it...it had holes to mount it to the valve...you loosened the bolts holding the valve and mounted the tool under the bolts and positioned the right angle bend pressing down on the pin...then you re-bolted the valve down with the tool in place to hold the pin. Since no shade tree mechanics have this tool you need to use a clamp of some sort to hold the pin in while you bleed. Depending on the clamp you use, it is probably not necessary to remove the boot over the pin...just press against the boot and the pin should go down.
Regards,
Henry

[Fast68Chevy]

persoanlyl ive never had to mess with the centering pin when bleeding on these trucks etc..  they alwasy center themselves when front and rear system is good shape all good fine.    i think something else is going on.   sounds like air in system but it should be same always every time hmm..    i always bleed by pushin pedal down all way with bleeder screw open farthest one  away form m/c first.   then close screw and pick up pedal.    then push all way down again and open screw same time.   then close it and lift up pedal again..  then move to driver rear and do same a coupel few times..  then go to front. pass side and do same.  then to driver side..   can start in front first though it doesnt matter on dual system  front and rear cant mix air they ar separate hydraulically.  are you somehow re sucking air bakc up into bleeder when bleeding?    do you use the big sysring when bleeding the m/c ?  is the best way and so easy and pushes ALL of the air out of it real quick easy.  go to home and tools section of walmart and get one of the syringes it comes with a nice clear bleeder hose for a couple bucks.. fill it up and push brake fluid through outlet port of mc  just unscrew each line and leave mc on the truck.  easy. wont be any air left at all in one syringe full push.  just jam the clear bleeder hose that comes with syringe in the port real firmly,.