Odd question I suppose. Just got my 59 olds started after 2 years of rebuilding, etc. Put on new wheel cylinders and brake lines, rebuilt the MC but not the vacuum cylinder or piston as they were working fine. Rebuilt the MC only because I wanted it clean so I could go to Dot 5 fluid. Adjusted the shoes on all four wheels and started to bleed the brakes today but having trouble probably because I didn't bleed the MC first which I will do tomorrow. When I run the car I get good vacuum at the brakes as I can feel the vac assist when I press the pedal. But after turning it off, after a few pumps I deplete the vacuum and start to hear hissing at the carb. I believe I am pumping air back through the system. The check valve is installed between the big vac line at the carb base and the vacuum booster cylinder and reservoir and in the correct direction (99% sure).

Is it normal to "pump air" back through the system in this manner? If not, what might be wrong?

 

The vacuum booster is LOWER than atmospheric pressure, otherwise it would be a pressure booster, not a vacuum booster. The check valve is supposed to retain vacuum in the booster with the engine off so you have power assist for a few stops if the engine stalls. If the check valve were installed the wrong way, you wouldn't have any vacuum for power assist with the engine running. If the check valve is leaking, however, you WILL hear hissing with the engine off, which is air leaking from the intake back into the booster.

 

Thanks Joe. Sorry for the long winded original message. To clarify, I only hear the hissing AFTER I've pumped the brake several times after the engine is turned off, say three or four times. Then I hear the hissing on the fourth or fifth pump. Because I have air in the lines and MC still, the pedal goes to the floor every time thus depleting the vacuum rather quickly with each brake pump. This is air going into the carb base and exiting the top of the carb. Does that make sense? Thanks,
John

 

I've decided that the air being pumped from the vac booster with the engine off is normal. The check valve is designed to remove air from the brake booster and reservoir thus creating a vacuum. Pumping the brakes with the engine not running and vacuum depleted turns the booster into a "pump" and pumps the air through the check valve in the same direction as it does when the engine creates vacuum.

Was having trouble getting the MC to fill with fluid. Disconnected the main brake line from the MC and pumped brake pedal with no fluid appearing. Put the nozzle of my compressor into the narrow neck of the MC reservoir and gave it a shot of air. Made a h*** of a mess but definitely got fluid into the MC. Refilled and pumped the pedal once again and got a mighty squirt as appropriate. Reconnected and bled like normal. Fortunately I'm using Dot-5 fluid so no damage to paint, etc.