brake booster or master cylinder ?

[ted_brah]

how do you know which one needs tending too? i bought my car off craigslist a couple of months ago and has been in my drive way since.
i only drove it once from the guys house i bought from to my house, maybe eight blocks away. he told me it needs a brake booster, but i don't know the first thing about checking to see if the booster is the problem. i've had some opinions, that i don't have to replace the whole thing, but just seals or use a rebuilt kit. other opinions say it's the master cylinder. how do you know? like i said, i've only driven it once (eight blocks) when i did, i had to use a heavy foot for braking. is there a test i can perform?
the master has a full tank of brake fluid....

any help is appreciated

[ballbuster]

if the brake booster is bad or the vacuum line to the booster is faulty the pedal will be very difficult to press, since there will be no power assist

[whunter]

moved for better answer.

[MattBelliveau]

Before counting your booster as the only problem, you will need to buy a mityvac and test your entire vaccuum system. After confirming that the supply vac is correct, THEN check into the actual booster unit.

[dmorrison]

Get a Mityvac or vacuum tester.

Determine if the vacuum pump is working. The large black line going to the booster. Disconnect it at the booster. Remove one of the 2 small lines going to the large line. Using a golf tee block the other small line. Attach the Mityvac to the one small line not blocked. Start the engine. Place your finger or thumb over the large black line connection you took off the booster. You are now reading the output of the vacuum pump. It should be around 20-25 in HG. If not them the vacuum pump is bad. ( I doubt this since you have not complained of engine shutdown or door lock problems.)

Reconnect the large black line. Disconnect the Mityvac to the small line and block it with a golf tee. Keep the other line blocked with the golf tee. Start the car. Are the brakes boosted?? If not then it is your booster. Replace the booster. If it is working fine then you have a vacuum leak somewhere in the other systems that use vacuum and that is another post.

When replacing the booster make sure the master cylinder rear seal is not leaking.

You will need the booster to firewall gasket and the master cylinder to booster flat Oring. As well as a new or rebuilt booster. Oreilly's sells on with a lifetime warranty. I have this one in my daughters car.

The job is a bit of a pain. You have to unbolt the booster from under the drivers dash and a few things are in the way. Not a bad job just upside down and twisting etc.

[kerry]

How do you detect a partially failing booster? My 77 definitely has higher pedal pressure than the 85. It requires probably double or triple the pedal pressure but the booster still seems to be working somewhat according to the test of pumping the vacuum out, holding the pedal down and starting the engine. But something is wrong. I don't see how a failing master cylinder could increase pedal pressure but I also don't see how a booster partly fails.
I have the rest of the vacuum system disconnected and recently rebuilt the vacuum pump.