I recently lost breaking power from my power brakes while on the road. Now, before taking action, I have this question: how to find out if it's the power booster or the master cylinder. With the engine off the brake pedal is solid, but with the engine on the pedal goes all the way to the floor with no braking action. The car is a 69 bird; I was sure it's the master cylinder, but now I better ask! Thank you guys.

Do you hear any hissing?

Cracked or loose check valve or vacuum hose?

Check the rubber grommet for the check valve, they can split too.

i think you may have a hydraulic leak. look for fluid.

Sounds more like the MC and down stream then the booster based on your symptoms.

You didn't say how high the pedal is with the engine off. You can pump it several time to bleed off the existing vacuum. Then apply pressure to the pedal and start the motor. The pedal should drop a bit with the power assist (vacuum) but not go to the floor

I'm going to say master is bypassing internally or you have an external leak somewhere in the system. Check the fluid level as that may indicate which 1/2 of the system if it is a leak.
The pedal should not go to the floor unless there is a loss of pressure in the system

X2

My only experience with a booster going bad was with a nearly new Z28 decades ago.
Brakes were normal starting out, failure was kinda sudden; pedal went nearly to the floor and it took A LOT of effort to stop.
Nothing really visible but turned out the canister halves hadn't been properly staked together and one rotated just enough.
That's 1 car out of about 90 that I've had, though maybe half have had manual brakes...

If you can apply a vacuum test to the booster, that'll tell you, but if it's total failure, it's more likely hydraulic.

Ok, guys, thank you for your answers. Let me add few things: there in no leaking fluid in the master cylinder, that is still full. Second the vacuum hose going to the booster is brand new, and with the engine off the brake pedal does not move at all or or at least maybe 1/4 inch. The master cylinder was rebuilt many years ago but never installed until few months ago. So was the booster. Since I started driving (3 months or so) the brakes were very sensitive, to the point I had trouble modulating the pedal without locking the breaks every time, and then....and I repeat, when I start the engine the brake pedal can be pushed all the way to the floor, but no brakes. That' s about it, at least for now.

I am going with the MC is bypassing internally when the additional pressure from the booster is applied

Some more info might be helpful, what type of brake set up, disc and drum, all drum, all disc. OEM or aftermarket. When you say lockup brakes early on was it just front, rear or both? Were brakes bled properly, was MC bled before install. To have no brakes at all and pedal go to floor, both piston seals Front and Rear would have to be leaking by. With lid off MC and pumping brakes can you see any swirl in the fluid coming from the larger fill holes in both chambers?

Well guys, thank you all again for your answers. The car is a 69 Hemi roadrunner with power brakes and front discs. All the components are rebuilt originals with the exception of new discs, pads, and braking lines, with no aftermarket parts. I love the idea of checking engine vacuum ( the engine has a replacement stock-type cam), since the plastic port that connects to the intake manifold has an extra nipple just for that, all I need is to buy a small gauge and see. Meanwhile I will remove the MC and send it for rebuilt, just to be sure, because most of you seem to agree with me that is the problem.

If you have a mitey-vac remove the large line and plug the nipple. Attach the pump to the that check valve to either nipple and see if you can get the booster to hold vacuum. If it does the mc is your issue.

It sounds like the booster is no longer moving the m/c. I believe the booster has fallen apart internally.
You said the brakes were real touchy, that is a telling clue that the booster had a problem from the get-go.
Verify that the booster pushrod is still in contact with the m/c. Paul