Hard Brake pedal w/engine off, Too the floor while running

[Mudbug1979]

Ok, swapped in a dana 60 bled system. With the engine off I can get a hard pedal, but as soon as I start the truck when I mash on the pedal it will go to the floor and barely stop the truck. Sounds like a bad MC to me, but I always welcome opinions. Thanks.

 

[Mudbug1979]

ttt, gotta go 2 work tommorow.

 

[73k5blazer]

Are you sure it's bled all the way. The manual method, on a dry system, can take many many round to get all the air out.
How was your master before the swap? If it was fine, I'd think perhaps mabey it just needs more bleeding.
I did my k5 recently, all new lines, new rear calipers, new master, new front calpers. Bone dry system.
I let it gravity bleed overnight, then went around at least 15 times. Then gave my wife a break (she was pumping the pedal), then went around another 10 times. Felt tight, but not perfect. Gravity bled overnight again, went around again a final 5 times.

 

[bear76]

Remove the booster vaccum hose and try pressing the brake. If the pedal remains hard then its the booster.

 

[bear76]

Sorry, I ment while driving SLOWLY. If the pedal stays hard but you can still brake then It might be the booster.

 

[Mudbug1979]

brakes

Brakes were great before, all I changed was new lines on the front and new calipers. The MC almost went completely dry during first bleeding, so I'm wondering if I do still have air in it. Gonna go open the bleeders and take the top off and see what happens.

 

[Thunder]

Bleed your MC. If your MC went dry you will need to bleed it.

 

[Mudbug1979]

So, I need to pull it off and bench bleed it or will the gravity bleed do the trick?

 

[Thunder]

Bench bleed is the best way. Gravity bleed wont work for a mc because it is the highest point in the brake system and all the air goes up\ and collects in the cylinder.
You can try bleeding it on the vehicle. Pump up the brakes and crack the nuts on the lines where they connect to the MC. Just be sure not to let any air back in. I have done that method and it has worked for me. Just be careful not to get brake fluid on your paint.

 

[73k5blazer]

Yep, I agree, I think your MC needs bleed, then re-bleed your brakes.
The last master I bought had neat little plastic screw-ins with tubes, you just screwed them into the master outlets and bent the tubes back into the reservoir submerged.
I've used old (or new) short pieces of brake line and did the same thing.
You can do it on the truck, but you'll need to remove the lines going to it.