I'm having issues with my power brakes on my Barracuda. Factory disc in front and drum in back. Everything is new. Bench bled the master dot 5 used. Bled all four corners until brake pedal was hard. Now I start the car and the pedal pressure drops almost 3/4 down. Could that mean I don't have enough vaccum or the booster is bad? Cam is a Lunati 703. The motor has 15-16 of vaccum at idle.

not sure what you mean by pedal pressure drops.

When I depress the pedal when it's running, the pedal goes down 3/4 of the way before the brakes start to work.

DOT 5 might be part of the problem. Better for your paint in case of a spill but not for brake feel. Not enough vacuum shouldn't effect travel I don't believe, just hardness of the brakes because less assist.

"everything new"....did you set the proper air gap between the booster and master, you need to reset or confirm anytime you rebuild/replace these components, this may be your issue? Mopar's need a minimum of .020 to a max of .060 for proper operation....

MikeG

it could also be adjusment of the rear brakes.
if you don't hear dragging when you spin the wheels, the shoes are out of adjustment. which will cause what you describe as well.

If the pedal drops with the engine running, vacuum isn't your problem....the booster is working.

Have you adjusted the rear shoes for a slight drag when turning the wheel? Loose drum brakes will eat up a lot of pedal travel.

I'll double check the rod length and check the rear drums and let you know what happens. Thanks guys.

Every late model power disc brake and 4 wheel drum brake Mopar car I've driven will do what yours is doing
the pedal should be really hard to push with the motor off and no vacuum, easy when running and driving, hence the "power brake" name

Not 3/4" of the way down on my cars.

I appreciate the info guys. I finally got it worked out. I bled all four corners again. Then adjusted the rear drums and the booster rod. After all of that, it was better but not 100%. I decided to take the linkage that attaches to the pivot from the booster to the brake pedal and modify it. I cut it in half and put a threaded stud between the to ends making the length adjustable. By making slightly longer made the brakes work like they should.