Rebuilt 4.0 engine doesn't make enough vacuum

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arstandley13

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Hey guys
Looking for some help . I have a 1993 Jeep YJ 4.0 that we had rebuilt a while back, runs great but through some brake issues learned that the engine is not creating enough vacuum to power the master cylinder to the brakes properly. it should be creating 18 to 22 in of vacuum but it only makes 17 in .I've looked for leaks everywhere that I can think of and even had a mechanic try to find a leak but could not find any. So my question to you out there is can there be anything internally that can cause this to happen .If so what ? But also it runs great , timing is spot on , has hydraulic lifters with no external vacuum leaks .
Any help would be most appreciated
 
i may be wrong on this, but vacuum need for power master cylinder is 16-18, and top is 22. could you have a vac leak in master?
 
Is 18-22 in a factory spec for the engine vacuum? I'm not seeing that in the FSM but maybe I'm missing it. I would think if the spec is 18-22 and you are getting 17, that's close enough to be good enough. I would more suspect you have a bad brake booster that needs to be dealt with instead.
 
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arstandley13 said:
Hey guys
Looking for some help . I have a 1993 Jeep YJ 4.0 that we had rebuilt a while back, runs great but through some brake issues learned that the engine is not creating enough vacuum to power the master cylinder to the brakes properly. it should be creating 18 to 22 in of vacuum but it only makes 17 in .I've looked for leaks everywhere that I can think of and even had a mechanic try to find a leak but could not find any. So my question to you out there is can there be anything internally that can cause this to happen .If so what ? But also it runs great , timing is spot on , has hydraulic lifters with no external vacuum leaks .
Any help would be most appreciated
Click to expand...

Thanks for the response
It shouldn't .It's been replaced with new twice. The first time was before the motor was replaced and they worked so well they would try to send you through the windshield. After the rebuilt engine was installed ,we started noticing the brakes getting more and more spongy. after trying multiple remedies we took it took it back to the brake specialist who tried another master just in case the one was internally bad but had the same results . they were the ones that said the master needed 18 minimum to work properly, so I don't know, I will have to look in that direction
Thanks
 
arstandley13 said:
Thanks for the response
It shouldn't .It's been replaced with new twice. The first time was before the motor was replaced and they worked so well they would try to send you through the windshield. After the rebuilt engine was installed ,we started noticing the brakes getting more and more spongy. after trying multiple remedies we took it took it back to the brake specialist who tried another master just in case the one was internally bad but had the same results . they were the ones that said the master needed 18 minimum to work properly, so I don't know, I will have to look in that direction
Thanks
Click to expand...

There might be some confusion between master and booster here. The booster is what receives a vacuum supply and boosts the fluid pressure on the brake system. The master cylinder is what pushes the actual fluid through the system.

The booster is what requires X amount of vacuum to function. Without enough vacuum, the pedal will be rock hard and you will struggle to press the pedal hard enough to stop.

Spongy feel is a bad master cylinder. Vacuum or lack of will not cause a spongy pedal.


Booster:

IMG_0419.jpeg

Master:

IMG_0420.jpeg

Re confirm what part has been replaced twice and the symptoms…it sounds like they are calling the booster a master and don’t realize that they need to change the actual master, rather than the booster that they might think is the master.
 
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Basic questions. Did you thoroughly bleed all four brake cylinders starting with the furthest one (right rear) and working forward? Any leaks?

I've had brake problems and went to a dirt road, built up a little forward speed and stood on the brake pedal to see if all four wheels locked up. Also make sure the rear drums are adjusted properly.

I agree that spongy usually means it's in the hydraulic system.
 
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