brake booster on a 74 C20 350.

[Sniper X]

Where is the brake booster on a 74 with a 350 in it supposed to be hooked up?

[Jonboy]

there should be a large port at the rear of the carb towards the mounting flange at the manifold. Depending on the carb, it will be able to accept a hose (Edelbrock) or have a threaded fitting (Q-jet).

[Sniper X]

So it doesn't go streight into the manifold behind the carb.

[Jonboy]

nope. There should be a fitting at the rear of the carb. The port on the intake is usually used for vacuum to the trans. I get the definitions for ported and manifold vacuum mixed up, so I don't have the "textbook" answer.

[78 Chevyrado]

Yeah, it goes on the one big port at the back of the carb like jonboy said.

[Sniper X]

Could that be why my brakes wont work right..one of the reasons? Too much or too little vacum?

[78 Chevyrado]

You can have too little vaccum, but you can't have too much. Also is the check-valve still good? it's the fitting on the booster that the vaccum hose connect to.

What do you mean by work right? I've had one truck that the brake booster went bad and it made the brakes real grabby. otherwise it may be a hydraulic problem.

[Sniper X]

Originally Posted by 78 Chevyrado

You can have too little vaccum, but you can't have too much. Also is the check-valve still good? it's the fitting on the booster that the vaccum hose connect to.

What do you mean by work right? I've had one truck that the brake booster went bad and it made the brakes real grabby. otherwise it may be a hydraulic problem.

Here's the weird thing, when the truck is not running I have a very firm pedal. When it is running it is intermittent, sometimes so spongy it has to go almost all the way to the floor, and sometimes it works just OK. I do have a bad as in stuck adjuster on the back right drum, so need to replace it, but the brakes are very spongy at times. Sometimes they do work OK but still not as well as they did on the 77 or 78. On those they work like brand new factory brakes. My booster hose is now run to a fitting on the center back of the intake manifold.

[ToughBowtieTruck]

Originally Posted by Sniper X

Here's the weird thing, when the truck is not running I have a very firm pedal. When it is running it is intermittent, sometimes so spongy it has to go almost all the way to the floor, and sometimes it works just OK. I do have a bad as in stuck adjuster on the back right drum, so need to replace it, but the brakes are very spongy at times. Sometimes they do work OK but still not as well as they did on the 77 or 78. On those they work like brand new factory brakes. My booster hose is now run to a fitting on the center back of the intake manifold.

That is not the booster. That is in the brake line system.

Pedal is supposed to be firm with the truck off. Without the engine running you do not have your vacuum-assisted brakes working (making it easier to apply pressure to the brakes).

The only thing a bad brake booster will do is suck brake fluid from the master cylinder into the engine or cause a very firm pedal to stop the truck if it is not working.

Even with the assist working, you should have a good firm pedal, just an easier time pushing the pedal down to apply the brakes until the vehicle stops.


Boosters are originally hooked up on the back of the carburetor, but can be hooked up anywhere with full time vacuum, as you need full time intake vacuum anyway.

Brake spongyness could be a bad master cylinder, check that for leaks. Otherwise, your rear brake shoes may not be adjusted properly and require more fluid to push the shoes against the drums or you could have a leak somewhere, like a line, wheel cylinder, etc.

[Sniper X]

I think you are right on the rear brakes. I just did new calipers, pads and rotos on the front so that isn't it. When I had a guy try to adjust and bleed the rears for me he said one of the adjusters was stuck and wouldn't adjust out enough to take up slack on that set of shoes so I would bet that is it. Isn't it true that if you have the drums adjusted incorrectly this would be the result?

[Damien]

Yes. You need to adjust the rear brakes. Then if you still have the same problem, you need to either bleed the brakes or you may have a bad master cyl.

[ToughBowtieTruck]

Originally Posted by Sniper X

I think you are right on the rear brakes. I just did new calipers, pads and rotos on the front so that isn't it. When I had a guy try to adjust and bleed the rears for me he said one of the adjusters was stuck and wouldn't adjust out enough to take up slack on that set of shoes so I would bet that is it. Isn't it true that if you have the drums adjusted incorrectly this would be the result?

Yeah. When you lose a pair of brakes (either front or rear) you have to push the pedal down more to get the remaining brakes to apply. If you lose the front, you have to put it to the floor to get the rear to work. If you lose the rear, there is a pedal difference also.

If you have shoes that are not properly adjusted against the drum, you are having to use more fluid and push the top of the shoes out more to apply the brakes.

Now take what I said above about losing the brakes, and there is your weak pedal. The front brakes aren't applying well because no pressure is being built in the rear of the system until the pedal is down a ways. The master cylinder and proportioning valve need pressure on both sides to give you a strong feedback at the pedal. If one end of the system is low on pressure, the other system is affected making the entire truck's brakes spongy.

The times the brakes work okay could be due to the shoes sticking against the backing plate so less fluid is needed for the next stop. Then the times they don't stick and return fully, you need all the fluid again.