How do you test a brake booster?

[WarBeard]

So my peddle goes halfway down and comes to a sudden stop. A very solid stop as if bottoming out on something.

I want to replace the master cylinder since a new one is pretty cheap and easy to install. I see that on some sites they sell the master with the power booster attached for like $120. The master alone is way cheaper.

So how do I test the booster to see if it needs replacing as well?

1974 C10 with a 350/350 combo.

[STOCKISH]

The pedal should only go about half way down. Do the brakes work ok? If the booster was bad the brakes would be hard as a rock. You would barely be able to stop the truck at all.

[WarBeard]

Ok so it goes halfway down and bottoms out. I can still stop but it is very very weak there is no way I can stop fast. I have a hill leading to my driveway and I hold the peddle as far as I can and just pray I stop. I think the master is messed up on the inside.I can pump the brakes up a little bit and get a better peddle.

Its gonna get replaced either way. Just wondering if I should go all out and do the master/booster combo.

[79k30dually]

With the engine off, depress the pedal 5-6 times. Hold the pedal, start the engine, the pedal should drop 1-2 inches if the booster is good, Ron

[sethv]

I would try checking your check valve (The little plug thing that goes into the booster that has a vacuum line)

[WarBeard]

Ok Ill go check this stuff!

[Goldfinger]

In the mean time when you hit a hill, gear down it will help you stop

[WarBeard]

OK so I did the tests suggested, Booster seems ok So I went to Auto zone to buy a new master and the old guy in there asked to see the truck and I said sure!
So we went outside and he had me pop the hood He checked the master and the fluid level then he went inside and pushed on the brake. He says to me "Before you spend any money here I would check the adjustment of the rear shoes" He says that they may be way out of adjustment causing the pedal to go down so far. He says I should also Bleed the brakes till you see new fluid come out of the bleeder.

I have never been talked out of buying something at a parts store LOL That's a new one for me so I think I will do what he said first and see what results I get.

WHAT DO YOU GUYS THINK OF THIS GUY'S IDEAS?

[Christopher J]

Any luck with this? I'm having a similar problem on my '72 Suburban.

Mechanic just replaced the whole brake booster and master cylinder from a new kit I bought from The Truck & Car shop in Orange, CA and the same problem described above persists that was happening before the booster and master were replaced with a new kit.

I wondering if it was the rear shoes being way out of adjustment, as the AutoZone rep stated.

My mechanic is about to pull the brake cylinder and booster kit so I can drive an hour back down to Orange to try and replace it.

Originally Posted by motomech

OK so I did the tests suggested, Booster seems ok So I went to Auto zone to buy a new master and the old guy in there asked to see the truck and I said sure!
So we went outside and he had me pop the hood He checked the master and the fluid level then he went inside and pushed on the brake. He says to me "Before you spend any money here I would check the adjustment of the rear shoes" He says that they may be way out of adjustment causing the pedal to go down so far. He says I should also Bleed the brakes till you see new fluid come out of the bleeder.

I have never been talked out of buying something at a parts store LOL That's a new one for me so I think I will do what he said first and see what results I get.

WHAT DO YOU GUYS THINK OF THIS GUY'S IDEAS?

[wyattglock]

Sounds like a good plan to me! In all reality changing the fluid every couple of years isnt a bad idea anyway. Also, rear drums are notorious for wearing and going out of adjustment if you don't back up at least 10-15 feet on a regular occasion. The adjusters can freeze up after a while and they wont reseat the shoes against the drum. Don't ask how I know, just take it for a hard lesson on my part!
I personally don't care for Autozone, but it sounds like you found one of the few guys who actually knows what he is talking about.

[andrewmp6]

I was always told let the car/truck idle for 2 mins then pull the hose off the brake booster,If a big rush of air comes shooting out its good,If next to nothing or none comes out then the booster is bad.

[Christopher J]

Thanks Andrew. Going to try exactly that. Watched a series of YouTube videos which confirm that as a good test.

If the new booster is off, that'll prove it. If it lets out a rush of air, then the problems are elsewhere...

Anyone have thoughts what else it might be if the new booster is good?

Thanks!

Originally Posted by andrewmp6

I was always told let the car/truck idle for 2 mins then pull the hose off the brake booster,If a big rush of air comes shooting out its good,If next to nothing or none comes out then the booster is bad.

Posted via Mobile Device

[andrewmp6]

If the booster don't have a rush of air out of it check the vacuum on the line to it before you replace the booster.With anything as old as these trucks you can have a vacuum leak easy,The hoses are rubber and crack with age and most never replace a vacuum line tell they have problems.