How Much Vacuum Should a Good Brake Booster Hold?

[1985 Bronco]

How much vacuum should a good brake booster hold? I have a booster from a 1986 F350, it's holding 7 inches of mercury, and not dropping at all, but that number seems kinda low. From reading online, it sounds like I need 14-16 inches?

My brakes suck, but I though a bad booster holds no vacuum at all? My vacuum pump is holding 22-23 inches of mercury.

[viven44]

When I go back home I will need to find out for sure. My bronco's brakes are less than stellar and I have doubts around the brake booster.

[mat in tn]

if your vacuum pump is holding 20 plus while connected to the booster then I think you are good. often when a diaphragm deteriorates it will have a hissing sound near the pedal in the cab and usually changes with range of the pedal. this becomes a vacuum leak to the engine and a controlled one at first then progressively worse until brakes get harder to push and idle suffers. it's a lean condition so pinging is also a symptom.

[Gary Lewis]

I'm confused.  Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"?

If that's what you are saying then the booster is good.  But the problem with your brakes is that you aren't getting enough vacuum to the booster.  So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster.

[mat in tn]

 I just noticed the signature. 12v Cummins. good catch Gary. test the vacuum pump.

[1985 Bronco]

Gary Lewis wrote

I'm confused.  Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"?

If that's what you are saying then the booster is good.  But the problem with your brakes is that you aren't getting enough vacuum to the booster.  So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster.

I disconnected the line going from the vacuum pump to the booster, and inserted a vacuum gauge. With the engine running, I was getting 22-23 inches on the gauge. So, I believe the pump is good.


Then, I connected the mityvac to the check valve on the booster used the mityvac to apply vacuum, and got a reading of 7 inches. The engine was not running, and the only vacuum line was the one going to the mityvac. Did I test this correctly?

[Gary Lewis]

Ok, it does seem like the engine's vacuum pump is working.  But the Mityvac ought to be able to pump up as much as the engine's pump does - unless there's a leak that is killing the vacuum at about the speed you are pumping.  

Can you tee into the vacuum line and see what vacuum you have with the engine running?  Maybe the leak is overwhelming your engine's vacuum pump?

[mat in tn]

does you mity vac while connected to the check valve, not go any higher than seven at all or does it sink down and only hold seven?

[1985 Bronco]

Gary Lewis wrote

Ok, it does seem like the engine's vacuum pump is working.  But the Mityvac ought to be able to pump up as much as the engine's pump does - unless there's a leak that is killing the vacuum at about the speed you are pumping.  

Can you tee into the vacuum line and see what vacuum you have with the engine running?  Maybe the leak is overwhelming your engine's vacuum pump?

The vacuum was holding steady at 7 inches for a good 30 seconds without me pumping, so I don't think that indicate a leak? I guess I could connect it tonight and see if it will hold the same number for 5 minutes.  

[1985 Bronco]

mat in tn wrote

does you mity vac while connected to the check valve, not go any higher than seven at all or does it sink down and only hold seven?

It doesn't go any higher

[Gary Lewis]

Something strange is going on.  You should be able to pump up over 20" of vacuum with the Mityvac if it isn't leaking.  And the fact that it holds for 30 seconds w/o pumping says it isn't leaking.

I think something is going on with the check valve.  But can you show us a pic or a drawing of how you are connecting your pump to the booster?

[1985 Bronco]

Gary Lewis wrote

Something strange is going on.  You should be able to pump up over 20" of vacuum with the Mityvac if it isn't leaking.  And the fact that it holds for 30 seconds w/o pumping says it isn't leaking.

I think something is going on with the check valve.  But can you show us a pic or a drawing of how you are connecting your pump to the booster?

I will tonight after work

[1985 Bronco]

Inserting the tip on the hose into the check valve. Held 7 inches for 5 minutes

[viven44]

OK I tried 3 different trucks

-- Big Blue 2WD -- brakes are pretty good, but could be better. I was able to pump it down to 5" max and it took a while to evacuate the 'chamber' with the handheld pump and I felt some resistance every once in a while.. it maxed out at 5" though.... and it held the vacuum !!! which is crazy.... I suppose an engine can empty that in 1 second... but again the fact that it holds vacuum is probably good. Maybe my pump just cannot evacuate a chamber that big that won't collapse within itself to shrink in volume as I pump down.

-- Parts truck -- 1986 F250 --- same result at Big Blue 2WD.

-- 1978 Bronco --- No response. A lot of resistance to vacuum down and did not move from zero!!! Maybe why my brakes feel useless ? I don't hear that hissing or change in idle while braking though so I had assume everything was as good as it got

I am most interested in more observations of vacuum levels achieved and how good the brakes feel...

Ordered a Duralast brake booster for my Bronco.... We shall see

[Gary Lewis]

I really don't understand, unless Vivek is right that you just can't pump that big of a volume down farther than that.

But the fact that it holds proves there's no problem with the vacuum booster.  So if you have 22" of vacuum on the engine-driven pump then I guess there's no problem.  Still, I can't explain why you can't pump down to less than 7" of vacuum.

The curious side of me would put a tee in the line between the vacuum pump on the engine and the booster and see what you are getting with the engine running.

[1985 Bronco]

I’m going to hook into the booster on my 49 ford (known to be good) tomorrow and measure that

[viven44]

I have a feeling I'm going to get in trouble with another vacuum theory, so I better run the experiments this time so I can keep my strawman theories in the closet

My handheld pump has requested that I bring in its big brother tomorrow - the HVAC pump... We should break the 5 or 7" barrier, maybe?  

If we can break the 7" barrier, and more importantly sustain it via the one-way check valve that means we just need a bigger vacuum pump i.e. engine

[1985 Bronco]

I think this discussion on a Chevelle forum explains what's going on here.

https://www.chevelles.com/threads/bench-testing-a-brake-booster.1097742/

So back to my problem, my brake suck. I need to stand on the peddle hard and it barely stops the truck.

I've only driven this truck about 120 miles since buying it, and the brakes were always like this. I thought it was because the rear wheel cylinders were bad, but I've replaced those and the brakes still act the same.

Front pads have lots of life left, and rear shoes are new and the adjustment seems to be good.

I'm wondering if my master cylinder is undersized? A F250 should have a 1.0625" bore, and a F350 should have a 1.125" bore. My truck has been upgraded to a Dana 60, and F350 booster, but I don't think I can verify that the master was upgraded to the larger bore without pulling it off the truck to measure?

Would the smaller master cylinder even bolt up?  

[Gary Lewis]

That's interesting, but doesn't really explain why you can only get to 7" of vacuum.  However, the internal valving they talked about may be the issue and since I don't understand it I don't understand why you can't pump up more vacuum.  

As for the master cylinder, my master says right on it that it has a 1 5/16" bore.  It is from an F450 and works well on my truck, which also has a D60 up front.  BUT, I'm using hydraulic pressure instead of vacuum for the booster as I have a hydroboost unit from that F450 as well.  And you don't want to touch the brakes if you don't want to stop.  They WORK!

But a bigger bore size doesn't mean that it is easier to stop.  In fact, it is the opposite.  If you put the same pressure on the pedal, let's say 100 lbs of force, and have a leverage factor of 8:1 in the pedal then you are placing 800 lbs of force on the piston in the master cylinder.  So to get pounds per square inch you'd divide by the surface area of the piston:

1.0625 diameter = .89 square inches of area = 899 psi

1.125 diameter = .99 square inches of area = 808 psi

So a smaller piston actually gives you more pressure to the brake pads/shoes.  However, if you go with too small of a piston you may not have enough volume to move the brake pistons as far as they need to go so your pedal would hit the floor before you had full braking.

So check out what master cylinder you have.  It may be embossed on the side of it like mine is.

[1985 Bronco]

Hmm, I'm going to take a look at the master tonight, but now I'm starting to think that it's the correct one for the truck.

My truck was 460 powered originally. I did a little more research and it should be rated for over 8500 GVW, and it would have had the larger master and booster, right? It could have been changed somewhere down the line, but who knows?

Now I'm wondering if my brake pads are glazed from trying to stop the truck with the blown rear wheel cylinders? I have a set of new pads on the shelf, I may replace them and see if it stops better.