Should Brake Booster Hold Vac. W/Mightyvac?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

mwebfj60

mwebfj60

10 Year 15 Year
Joined
Sep 17, 2010
Threads
76
Messages
1,740
Location
Central Valley, CA
Can you test the brake booster using a Mightvac or is the air volume too much? I tried sucking on the hose and I can hear the diaphragm move and it feels like it builds up a little pressure but I can keep pulling a little air till I run out of ..."breath''.."suction"?? I will try the FSM procedure when I put the carb back on...just wondering if the Mightyvac should work. Thanks.
 
Last edited:
Thanks...I am getting zero vacuum with the Mightyvac. The strange thing is, my vacuum reading from the three prong fitting on the manifold doesn't change if I have the brake booster hooked up or if I have the port it goes to capped. The reason for all this is I am only getting 16in of vacuum at idle.
 
It takes about 15 minutes of pumping to get any vacuum with the mity-small-vac- but you can do it...

t
 
mwebfj60 said:
Thanks...I am getting zero vacuum with the Mightyvac. The strange thing is, my vacuum reading from the three prong fitting on the manifold doesn't change if I have the brake booster hooked up or if I have the port it goes to capped. The reason for all this is I am only getting 16in of vacuum at idle.
Click to expand...

Do you mean that the mighty vac wont pull dowm a vacum on the booster?

Do you have any symptoms or suspext that the booster is leaking? or are you systematicaly checking potential vac leaks?

i dont tink that you would see a change if the booster is OK.

swamp60
 
I am systematically checking for vacuum leaks. The brake booster is really the last piece I am not sure about. I pumped the mighty vac for about fifteen minutes and couldn't get any vacuum at all to hold. I pulled my carb today to check the manifold for a crack and all is well there. To date, I have found the following leaks...VCV#2, primary advance, three prong fitting, I had an intake/exhaust leak so I pulled the manifold and installed an OEM gasket..good to go. All other diaphragms, VCVs, BVSV,...etc are air tight. Valve lash is spot on, timing is dialed in, carb is set...thing runs pretty much perfect...I just think I should be getting over 16in at idle. If you haven't realized it from my other posts....I am ultra picky when it comes to my cruiser.
 
Step on the brakes, start the engine. What happens

Release the brakes and then pump them. What happens?
 
To add with what Mace posted .

Step on the brakes, start the engine. What should happen is pedal drops and holds firm .

Holding brake pedal in that spot with all the same pressures applied with foot and having engine run for 30 seconds to stabilize itself and your foot/leg shut off engine and now don't move anything , You should feel through your leg/foot no movement of rise in 1 minute keeping the same pressure (just like sitting at a red light pressure) . If it raises as soon as you shut off the engine , Id check the one way booster check valve first .

This would be the first checking as Mace posted.
On conclusion of what you find with these quick tests , is what way to proceed to check further,

If the Booster passes the drop and hold test , then alls fine there.

VT
 
Test, I pushed the brake and held constant pressure, started the engine, the pedal dropped maybe an inch and then held steady. I then pumped the brakes and the pedal returned to the same position with the same amount of pressure. I held this position for over 30 seconds then turned the motor off. I can feel a little bit of pressure pushing back up but not a whole lot. I also notice that on initial startup if I have the break pedal held down, the idle is rough until I let off the brake for about three seconds and then it is back to a smooth 650. If I press the brake and hold again, idle isn't affected. If I pump the brakes, at about the fourth pump, the idle dips again and If I hold the brake pedal down at that time, the idle still stumbles. Release, and after three seconds idle returns to normal. So this leads me to believe that the diaphragm is ok but when I am using the pressure it is leaking? Check Valve? Thanks for the help.
 
well, the easy test's just to unhook it and plug the intake port. Then see what happens.

But huh, the mityvac SHOULD have eventually pulled SOMETHING.

It is possible that the valve's messed up in the booster. It should be able
to hold vacuum almost indefinately, and not need any unless you actuate the brakes.

Oh, hey, here's a test- start your truck, and let it pull full vacuum on the booster.
Unhook it before the check valve, and then have at it with the mity- vac. It should
then pump down pretty quickly, as all you'd be evacuating would be everything before the valve.

You might have to rig something up temporarily to get this to work. But it'd prove that the
booster's not leaking air into your intake.

hth

t
 
I can feel a little bit of pressure pushing back up but not a whole lot. , TIME WAS ? 30 seconds to a minute then for an old truck , passable .

Pumping the brake pedal will let the vacuum / air into the intake , and cause it to stumble. Thats normal.
Stumbling after one brake application warm engine and holding the brake pedal down , and then shutting off engine and the pedal rises against foot (just like sitting at a red light pressure) . If it raises as soon as you shut off the engine , booster is failing.

After that , your done.

Do the testing slow and note the times.

EZ and most techs do this without really knowing there testing it , till they "feel" the pedal / or something thats failing.

A mityvac is a dashpot low volume tester only. I've never seen it used to test boosters (It takes about 15 minutes of pumping to get any vacuum with the mity-small-vac- but you can do it..) Not in the shops I visit.

VT
 
yep, it does. I don't often visit shops. But I DID use a mity- vac to test a booster once.

Long ago.

I was young, and loved my mity- vac.

That was long ago...

t
 
The brake pedal starts pushing back up immediately after shutting the engine off. I think the booster is going bad. Just going to live with it for now and call my vacuum leak search complete. Thanks for all the posts.
 
Very old thread but wanted to post what happened. About 4 months after this, I was running down the highway around 65 and had to stop quickly for a yellow changing to red stoplight. There was a loud pop and power brakes were gone. Had to use the hand brake as an assist not to blow through the light. I am guessing the diaphragm gave.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom