I Believe I have a booster/check valve issue.

[Dsertdog]

Once again, our '66 Polara convertible seems to have an issue.
The brake pedal is very hard. And I can step on it until I feel I could bend it but I can't lock the brakes up.
Pumping doesn't help, so I don't believe there's air in the lines. Everything seems adjusted correctly.
Is there a way to test the booster or check valve?

 

[Davea Lux]

The check valve won't usually cause a hard pedal with the engine running. Check the vacuum supply to the booster to be sure the port coming out of the manifold is not carboned up and the the internal membrane for the hose has not collapsed and blocked the hose. If the check valve has the auxiliary port that is capped off, attach a vacuum gauge to that port to see if vacuum is getting to the booster. Was this booster rebuilt? Hard pedal is also a classic sign of a ruptured vacuum diaphragm in the booster.

Dave

 

[65pacecar]

I agree with the above, likely a bad booster diaphragm. Check the other items first but be prepared to change the booster or get a rebuild on yours. Brake Booster Dewey is the man in those.

 

[Dsertdog]

I think I found the problem...

I could hear a hiss around the vacuum booster after driving the car. So I sprayed around the booster with carb cleaner and got a hit when I sprayed around the gasket between the booster and the master cylinder.
The FSM shows some kind of spacer/retainer with a filter in it. And it looks like there's one in place but perhaps the spacer has degraded enough that it's causing a vacuum leak. This would explain the driveability issues and hard brake pedal I have.
Pictures below...

 

[Davea Lux]

You have a leaking diaphragm.

Dave

 

[Turboomni]

Can you hear any hissing sound when the brakes are depressed inside or outside the car? I heard a hiss inside very faintly. In my case last year I was having some symptoms and spun the check valve and that leak subsided but I didn't realise the over all lack of brakes till this year when I had to really press the pedal. I checked again the grommet and check valve and sadly no luck. I replaced my booster last month. If it goes too long the vacuum will pull brake fluid out the back of the master cylinder or the leak in the back of the master takes out the diaphragm. Anyway after replacing the booster [no master rear leak] I can't believe how good 4 way drums can be.
Do not throw out your booster. Have it rebuilt as off the shelf reman boosters are made of unobtainium it seems.

 

[Dsertdog]

Thanks @Turboomni and @Davea Lux ! I will be pulling the entire booster then.Time to update to a dual master cylinder as well!

I didn't hear any hissing inside the car. It was all on the outside under the hood and quite loud, like not running an air cleaner at idle loud. My brake light switch stays on all the time now, it never did that before either.

How do I find Brake booster Dewey???

 

[65pacecar]

Power Brake Booster Exchange | Providing Parts for Classic Cars

 

[Dsertdog]

Thanks @65pacecar, and I just found it myself too!