Fluid in booster. Replace or clean?

[ByStickel]

My '78 620 had a case of the disappearing brake fluid, even though it had simply been sitting (pointed uphill) I removed the master and booster, draining a lot of fluid from the vacuum fitting on the booster.

 

I've never had the truck on the road. I'm not 100% sure that the booster is still good, but would like to try it before shelling out $90+ on another. Used parts aren't easy to find here in the East.

 

Is there a test I should run to find out if the Booster is good? (The system is already apart)

 

If it's a good idea, what should I use to attempt to clean the inside of the booster, and is there anything I could add to help to put some elasticity back in the rubber?

 

Thanks!

[Draker]

Apply vacuum to the booster and see if it holds. That's about as good as you can test it. I'd just replace the master cylinder and call it good. If the booster goes out later.. you'll notice it but you'll still be able to stop the truck. Although, it will take longer to stop which could be dangerous in the right situation.

[datzenmike]

In the vehicle.... pump brakes to release any residual vacuum. Apply brakes moderatly hard and hold pedal. Start engine. Brake pedal should drop slightly as engine vacuum rises. This is normal operation. If it does not drop then the booster is not helping. 

[ByStickel]

Thanks, guys.  I didn't think I'd notice any 'pedal drop' feel with the master already out.

 

The new master arrives tomorrow morning, so I guess I'll find out, then.

[datzenmike]

lol no you need the system all connected. The booster merely uses vacuum on both sides of a large diaphragm inside. When you push a small air bleed is opened and atmospheric pressure 14 PSI leaks in on the cab side and pushes forward on the diaphragm adding to the push. One hundred and thirty pounds of push on the pedal will give 1,400 PSI of brake fluid pressure