Occasionally my brake pedal goes hard and I really have to push hard on the pedal to stop. As if I am not getting any help from the vacuum booster. Most of the time the brakes are fine.
What could be the trouble?
Mike

 

check the rubber hose in between the booster and steel line on the firewall.
There is a one way check valve in there,and if it sticks you get no vacuum to the booster.
Spray some silicone lube in there and that should be able to free it up.
NOTE: that hose only goes on ONE way,so do not install backwards or you will get NO vacuum and hard brakes.

 

I'm not sure the silicone spray will do any good.
If the valve is (potentially) bad, they are simple to replace with any of a million generic check valves.

 

Wouldn't silicone spray ruin the O2 sensor?

We replaced that hose with a silicone one not realizing there was a checkvalve embedded in there. We picked up a W0133-1637053 ATE Power Brake Booster Check Valve for a 1985 BMW 528e from AutoPartsNetwork.com for $11.50. They are used in a lot of German cars and are made in Germany. Worked perfectly where the one we picked up from Grainger didnt.

Good luck.

 

Ruin the O2 sensor?
Not unless you were filling the engine with about 30 cans of it.
Take hose off,spray the hose in the direction of the booster.shake out excess(it does evaporate) ,put air to the hose,(blow through the hose,Come on Big Boy..lol!)..and try the hose out..simple.
You can get another check valve,IF it doesn't work,but it is worth a try.

 

A long time ago I'd wondered why some of the Permatex FIPG variants said "sensor safe". It's because of the silicone. I recently saw an episode of MotorWeek (which lately seems more like a propaganda engine for the Sierra Club than a car enthusiast show) where Goss warned about it.

Berryman's - who makes a lot of car chemicals - has a warning that Silicone Spray in "minuscule" amounts will destroy O2 sensors.

This article says the w/b sensors from Bosch and NTK are vulnerable to silicon contamination as well.