Brake booster vacuum

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martmanmcfly

martmanmcfly

Joined
Jan 13, 2026
Threads
2
Messages
7
Location
Oregon
I recently purchased a ‘78 all stock fj40. I had a ‘77 that I regrettably sold about 20 years ago. So, I’m going through the rig, getting to know it. I noticed the vacuum wasn’t hooked up to the brake booster and the holes were capped. The brakes work but they’re a little stiff.
So I ran a vac hose from the booster to the intake manifold thinking it would improve the stiffness at the peddle. Pulling out of my drive way for a test, I barely touched the peddle, and the brakes slammed hard full stop lockup. It released and so I tried again…full lockup.
So, I took the vacuum hose off and they went back to normal, alittle stiff, braking. I’m not sure what the vacuum is at the manifold but I’m going to find out. Is there some kind of PRV that should be on there? I have no Idea whats going on with that. I see pictures of other rigs with the hose hooked up so it must be a normal thing to have vac on the booster can. Why does mine slam tight?
 
Maybe the booster is leaking and they unhooked it. I'm sure the manual has a test procedure to check that. If it leaks #5 will run lean and burn up.
 
The po obviously had an issue and he let you try to figure out what’s wrong lol

Imo
Sounds like brake pedal adjustment issue… do you have a return spring on brake pedal?

Had you jacked up the truck and checked the drums for correct adjustment?

Had you done any brake work to it?
 
JohnnyC said:
The po obviously had an issue and he let you try to figure out what’s wrong lol

Imo
Sounds like brake pedal adjustment issue… do you have a return spring on brake pedal?

Had you jacked up the truck and checked the drums for correct adjustment?

Had you done any brake work to it?
Click to expand...
 
No I haven’t looked at the drum and rotors. I just got this rig. The brakes seem to be working fine just a little stiff without assist. Hook up the vac and then they slam tight when i press the pedal. They release when I take my foot off the pedal. It seems like its not so much a leak but too much vacuum. But its weird, I see others in pics with the hose connected to the booster can.
 
Obviously the vacuum should be connected, otherwise it's pointless having a booster.
Maybe post a photo of your master cylinder and the disk setup - is it original, or converted to 60 series / mini truck / random other manufacturer disk kit???
Seems like there's either a mis-match between the booster and disk, or else is badly adjusted somewhere in the MC.
 
If the pushrod in the booster is not set properly, it could be engaging the master where the pressure won’t release. But without vacuum, the piston in the master may be able to release. IOW, the ‘neutral’ position in the booster is off.

You can test this theory by finding a long flat stretch of road that you can get to without the booster hooked up. Then hook it up there so you can drive without touching the brakes. If the brakes start dragging on their own, you will have confirmed that the pushrod is engaging the master.

Literally a 1/16” of pushrod adjustment can make all the difference.
 
65swb45 said:
If the pushrod in the booster is not set properly, it could be engaging the master where the pressure won’t release. But without vacuum, the piston in the master may be able to release. IOW, the ‘neutral’ position in the booster is off.

You can test this theory by finding a long flat stretch of road that you can get to without the booster hooked up. Then hook it up there so you can drive without touching the brakes. If the brakes start dragging on their own, you will have confirmed that the pushrod is engaging the master.

Literally a 1/16” of pushrod adjustment can make all the difference.
Click to expand...
I’m going to try that thank you 👍
 
This is a tool I made to check the adjustment.

IMG_0679.webp
 
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