Fired Car up for first road test and we have a brake issue.. essentially pedal pushes to floor when booster is under vacuum with essentially no pedal pressure. Essentially no brakes. Lines are bled well. Guessing bad boaster but I expected pedal to be hard as a rock with bad booster??? Really need some thoughts as I hate brake work!! We have a booster ordered but won’t be here till Tuesday. Pedal has plenty of pressure when vacuum hose removed. 70 Cuda with factory front disks.

if you disconnect the vacuum line to the booster, will it stop ? like a manual brake car?

Yes

Go on youtube and look for the Ramman, he has a video that tells which part is bad.

(1) too much booster threaded rod clearance (but I am assuming it was good before & there was no MC change I am assuming). (2) Air (3) bad MC (4) linings spooned way loose. I'm thinking (2) or (3)

Is this a sudden event, or have you changed/adjusted any components then have experienced an issue?

sorry, a bit of what you said is confusing.
if it is sucking the pedal to the floor, then the brakes should be locked on.
but you said no brakes unless you unplug the booster.

do you mean no pedal instead?

there is an internal valve that goes bad causing the vacuum to apply the brakes.

since there is still a physical connection you could just plug the vacuum line to the booster, but pedal effort will be very hard compared to with boost.

Do a search on "ramman" on this site. Read the posts.

Got to FABO, do the same search, read the posts.

Then decide what you want to do in regard to anything from the Ramman, even if it's free.

I know what you are saying about the ramman and I agree, but the one video does show how to diagnose the problem. I didn't say buy anything from him. I'm not endorsing his parts, just how to diagnose the problem.

sorry just getting back to post. The car was just restored never running. Brakes bled fine while not running. Fire it up and pedal can be pressed to floor with no resistance and does not make brakes do anything. I can press it right to the floor. Unplug booster while running and pedal feels-like manual brakes.

I did not check booster prior to install. and it was used. I’ve never had an issue with them before. I put a vacuum gauge on 5 of them at my house and can’t get any to hold vacuum tor even read on vacuum gauge....

feels like sure, but are they actually working?
because that would be odd.

I might agree, the rod adjustment might be out of whack and you might check that before getting a new booster.

how are you checking if the brakes are actually working?

Checked on the lift when bled. All four tires were locked up when brake depressed. Once we start car pedal pushes to the floor and you can here air loudly under the dash when pedal is depressed to the floor

My experience with a hard pedal and air noise is a broken diaphragm in the power brake booster or your vacuum hose is not connected or has a hole.

Pedal is not really hard which is what I was expecting. Pedal depressed easily to the floor with a wooosh of air sound down under dash by booster...
also I just tried driving with booster vacuum unhooked in garage and you are correct pedal is hard to push and barely stops. It’s not air in lines as we’ve bled them into oblivion. Just looking for someone to corfirm they think it may be the booster over a master cylinder issue..

the question is did you try the brakes with vacuum hooked up.

the whosh of air may be a seal issue in the booster, but it may be working just fine otherwise and still be drivable.

pedal going almost to the floor but still stopping is fine.

I just want to confirm you tried that too.

Confusion is I’m answering different peoples questions I think. Yes the issue is 0 brakes when car running and booster hooked up. Pedal goes to floor with loud air woosh sound with little to no brakes. Restored numerous cars but never had this issues but also have grabbed many boosters off my shelf, painted them and put them on a car with no issues..

so have you checked the push rod in the booster to master cylinder gap?

No but will check that in the AM.

I hate to say it but it sure sounds like air (unless the rod is screwed in way too short). What I would do is cap the MC with a pair of brass plugs from your parts house (make sure you get the right thread pitch, iirc one is an oddball but I might be wrong) & cap the MC & with the vacuum hose hooked up/eng idling the pedal should be hard with virtually no travel (applies to manual brakes also). Do keep us updated, we love the toughies! EDIT I've had good boosters that I could hear a decent whoosh of air when pedal pressed. MORE EDIT I'm thinking the booster rod is screwed in too short