No pedal when truck running - Fustrated!

[nhnozzleman]

I have a 67 short bed, with 460. Dream beams, with 76' spindles and disc in the front, stock 9" with drum in rear. New and correct portioning valve from Summit, 1" bore new master on a booster. All 4 corners are bleed correctly. Decent pedal with the truck off, start the truck and pedal goes to the floor. Pumping does nothing. Spaced the master off the booster with washers, no change while running, worse with engine off. Stroke of the pedal and the master cylinder bore is of proper size. At a complete loss!!!

[ButtaBean]

The master cylinder isn't bled yet. It'll fight you forever trying to bleed it out the lines, put the bleed plugs into the cylinder and pump have someone pump it real slow, no more than 1/2 pedal. I had best luck on mine just patting the pedal with my hand, maybe 1/2 inch travel.
I've used a vaccumn style pump with good success as well, but always master cylinder first or it just won't work.

[cep62]

Welcome to our site.
I'm no brake expert , but spacing the master cylinder out will make it need more travel.

Some brake pedal to booster bolts have an eccentric (egg shape ) bolt shoulder to adjust the pedal travel.

Is the rod that comes out of the booster able to be turned out for more stroke ?

Do you have the back shoes adjusted up so they drag the drums ? they'll wear in soon.

Is the proper port of the master going to the front brakes? the big reservoir or maybe the rear most should go to the front.

Check back with an update.

[Busboy]

I say there is some compatibility issue with the booster and master cylinder.

[Ranchero50]

You'll have some pedal resistance until the engine vacuum helps. Put a flat bar across the booster flange and measure the depth of the diaphragm socket. Make sure the MC piston is @ 1/16-1/8" shorter. Fill the MC with fluid and crack all of the caliper and wheel cylinder bleeders. Then let it sit overnight to self bleed. Often forcing the issue just makes it worse.

[nhnozzleman]

Thanks guys!
I have bleed the master several times in a vise, level, tip down slightly, up slightly. I started with gravity bleeding all 4 corners, tried with vacuum as well as manually pumping with sucess, no bubbles!
I was worried about the pedal rod being to long, hence the washers to space the master out case the rod was to long. I will try measuring it though. It is adjustable, could be to short I guess!!
I have double and triple checked the proper line placement on the master and portioning valve. The shoes are providing drag on the drums, should be good there!
I have consulted with a brake company, the pedal geometery and bore of the new master should be compatible with my set up

[Ranchero50]

If the rod is too long the piston won't retract far enough to pick up the next squirt of juice. It'll also hold the brake engaged once they bleed. BTDT and they get pretty hot pretty quickly when driven. Usually I'll fill the reservoir and just wiggle the pedal and watch the bubbles come out of the intake ports in the bore. Once the bubbles stop the MC is pretty much bled and you can move the aerated fluid towards the wheels.

[nhnozzleman]

Update ...... the rod measured out to far, when the master was bolted tight the rod from booster was definately pushing in on the master. Also, tried a new vacuum “back-flow” off the booster and either or both pretty much solved the issue. The pedal holds the truck, seems slightly spongy. I will adjust the rear shoes again, let the new front calipers and pads wear a bit and see what happens. Thanks for all the idea’s!!