Brake booster vacuum leak?

[chaseincats]

Hi guys,

My brake booster definitely has a small but increasing vacuum leak.  I know it's leaking because if i hit the brake pedal the engine rpm slightly rises.  I noticed yesterday the brakes really arent what they used to be when going at speed (new pads were put on about 8 months ago and the car has probably been driven fewer than a few thousand miles so that isn't it).  Also it wouldn't be the master cylinder as a new one was installed last July.

My question is, I'd like to get a larger non-vacuum leaky booster on there and I hear the 79-81 ZX's 10'' booster is the way to go.  I can't seem to find a definitive yes or no as to if that will bolt right onto my '78 without needing to modify the piston length or requiring a new master cylinder which which works well currently.

Any ideas?

-chase

[Patcon]

I do not believe it is a bolt in mod

Have you looked on hybrid Z? They are very good at researching mods and all the options

[Yarb]

I recently purchased a 79 280zx booster from Power brake exchange. I’m in the early stages of the installation. The booster does fit without modifications at the firewall. Leaves just enough room for the clutch master. I’m using the same 15/16 master cylinder as well. Working on other areas of the car right now and have not gone any further with the install.

[chaseincats]

So I put a vacuum gauge on the booster and pumped it up all the way to 22 lbs without a problem (for at least 10 minutes until I disconnected the line).  When I push the pedal, there is a hiss and vacuum pressure drops to zero.

Is that how brake boosters are supposed to work (holding vacuum until the pedal is pressed)?

[Patcon]

11 minutes ago, chaseincats said:

So I put a vacuum gauge on the booster and pumped it up all the way to 22 lbs without a problem (for at least 10 minutes until I disconnected the line).  When I push the pedal, there is a hiss and vacuum pressure drops to zero.

Is that how brake boosters are supposed to work (holding vacuum until the pedal is pressed)?

Yes

[chaseincats]

29 minutes ago, Patcon said:

Yes

Awesome, glad I don't need a new booster

[Captain Obvious]

The power brake booster basically works like this...

1)  With no foot on the pedal, apply equal vacuum to both sides of a the big diaphragm.
2)  Then when the pedal is pressed, allow some atmospheric air (from the car's interior) into the cavity on the rear side of the diaphragm (the side you sit on).
3)  Allowing some atmospheric air into the rear side reduces the vacuum on rear cavity and allows the vacuum on the FRONT side to help pull the diaphragm towards the front of the car.
4)  The further down you press the pedal, the more atmospheric air is allowed into the rear side and the more assistance you get from the vacuum on the front side
5) Then when you let off the brake, the vacuums equalize between both sides of the diaphragm again.

Not that anyone asked....

 

* I know that it's pushed by the higher pressure and not "pulled" by the lower pressure, but I think it's easier to describe as being pulled.

[Captain Obvious]

Oh yeah, I got caught up and forgot to tie that back to your question...      

The POINT is that every time you actuate the booster, you'll be "using up" some of the vacuum on the back side of the booster. And then when you stop actuating it and everything equalizes, it won't return to the same vacuum because you've vented some of it off. Every time you actuate it, you'll lose some of that vacuum.

[chaseincats]

Just now, Captain Obvious said:

Oh yeah, I got caught up and forgot to tie that back to your question...      

The POINT is that every time you actuate the booster, you'll be "using up" some of the vacuum on the back side of the booster. And then when you stop actuating it and everything equalizes, it won't return to the same vacuum because you've vented some of it off. Every time you actuate it, you'll lose some of that vacuum.

Thanks for both posts' explanations, that makes total sense.

So that means that in order for a brake booster to be bad - it would need to not be able to hold a vacuum with the pedal not pressed and me pumping the vacuum gauge pump?

[Captain Obvious]

Well there's lots of ways that a booster could go bad, but that's one of them.

If you vacuum the booster down and then stop pulling it down, it should hold that vacuum. I've gone to junkyards and pressed the brakes on a car that's been there for who knows how long, and sometimes you'll still get a whoosh or two because the booster still had vacuum in it. Check valve still worked and the valve built into the booster didn't leak.

That said, another common way for a booster to go bad is for it to leak through the big diaphragm (like there's a crack in it or something). The reason I bring that up is because it will act almost like a working one on the bench with one detail...

If you pump down a working booster and put a gauge on the engine side (the front side) it should hold that vacuum while not pressed. It sounds like yours does that.

But as you actuate the booster, it should not lose much vacuum on the front side. It should stay pretty much the same. Then when you release the booster rod, it should bleed from the front side to the back side, and that's when you should see the vacuum change on the front side. Not on "pedal down", but on "pedal up".

If you see all of your vacuum disappear immediately on "pedal down", then you may have a hole in the diaphragm.

 

[chaseincats]

24 minutes ago, Captain Obvious said:

Well there's lots of ways that a booster could go bad, but that's one of them.

If you vacuum the booster down and then stop pulling it down, it should hold that vacuum. I've gone to junkyards and pressed the brakes on a car that's been there for who knows how long, and sometimes you'll still get a whoosh or two because the booster still had vacuum in it. Check valve still worked and the valve built into the booster didn't leak.

That said, another common way for a booster to go bad is for it to leak through the big diaphragm (like there's a crack in it or something). The reason I bring that up is because it will act almost like a working one on the bench with one detail...

If you pump down a working booster and put a gauge on the engine side (the front side) it should hold that vacuum while not pressed. It sounds like yours does that.

But as you actuate the booster, it should not lose much vacuum on the front side. It should stay pretty much the same. Then when you release the booster rod, it should bleed from the front side to the back side, and that's when you should see the vacuum change on the front side. Not on "pedal down", but on "pedal up".

If you see all of your vacuum disappear immediately on "pedal down", then you may have a hole in the diaphragm.

 

So I just went downstairs to test it and here is what I found:

• The booster will hold vacuum indefinitely if vacuum pumped and the pedal isnt pushed
• Vacuum pressure is lost when the pedal is being depressed and when returning to the non-pressed stopper.
• If vacuum pounds >5lbs, holding the pedal in will only deplete a set amount of vacuum silently
• If vacuum pounds <5, holding the pedal will cause a hiss until the booster his 0 lbs of vacuum

Any ideas on if this is dead or not?

[Captain Obvious]

I think as long as it depletes a set amount, the diaphragm is intact. The "set amount" thing is promising.  From what I can tell (from a distance and all that) it sounds like a good booster.

How much vacuum are you able to draw down on it? Hand pump, or you got something powered?