How to test brake booster for vacuum leak

[Timmy390]

Short version of the story, my idle was way up like 500 plus RPM's today. It's kind of been creeping up over the past few months.

Found some rubber vacuum caps had cracked (those things are JUNK) so replaced them with some vinyl vacuum caps I had from another project. Think there is another 1 or maybe 2 on the back side of the intake I still need to check and replace.

That got my idle down but not completely.

Found that if I take the check valve out of the brake booster and hold my thumb over it, my RPM's drop 200 or so. I plugged it back in RPM's went back up. I got out the brake cleaner and sprayed it all around the booster and the idle didn't change.

Any suggestions as to how to track this down? I'm pretty sure it's the booster but really expected it to react to the brake cleaner test. Want to be sure before just throwing parts at the issue. I suspected all along since my EFI conversion the booster has been leaking and contributing to my high idle condition.

Tim

 

[pcf_mark]

https://m.roadkillcustoms.com/test-power-brake-booster/

This should help.

 

[Timmy390]

Hummmmm I've done most of what's in the link already

I see it notes "check for a vacuum leak....which is what I'm trying to track down or rather establish to exist or not.

The booster has no residual vacuum in the booster or doesn't seem to. Pull the check valve and air doesn't whoosh like I've heard others do.

Brakes are fine or seem to be, I guess they could have gotten harder as in less vacuum assist over the past 2 years and I've just not noticed.

I guess I'll do more testing.

Tim

 

[pcf_mark]

Based on your description that the idle changes when you plug it I would say it has a leak internally. If you have a vacuum pump (Mighty Vac etc.) you could pump it down then see if it holds vacuum on the gauge. Then you know for sure.

 

[Timmy390]

Based on your description that the idle changes when you plug it I would say it has a leak internally.

That's what I think as well. More testing needed and I'm sure I'll be buying parts. The booster is 12-15 years old.

Put 100 miles on the bronco yesterday on the highway running 65-70mph. Ran great but for the high idle.

Tim

 

[Apogee]

Connecting the booster shouldn't cause the idle to increase, so I think you've already answered your question...bad booster. You could have an internal leak at the air control valve, and I'm not sure the brake cleaner test would yield anything from that type of failure.

 

[DirtDonk]

Unless you spray the brake cleaner up at the backside of the pedal rod in the cab.;D
Along those lines though, you might even try to pull the pedal back up manually to see if that changes the idle at all.

That may still indicate a bad valve, but if it does change the idle, at least you'll know what's going on.

Paul

 

[bam]

I use starting fluid rather than brake cleaner to find vacuum leaks.

 

[Timmy390]

Haven't driven the Bronco for a while but did today. While at a stop light I pressed on the brakes hard and the RPM's dropped. When released RPM's go back up. I could manipulate the RPM's couple hundred with the brake peddle.

Guess I'll be ordering a booster in the next few days.

Tim

 

[DirtDonk]

Did you see what happens when you pull the pedal up hard with your foot too?
Just to see if it's the poppet valve staying open due to pedal pressure.

It's still the back valve no matter what I'd say. The only question remaining is what is actually the problem with it. Since you sprayed fluid around and did not detect any change, that pretty much eliminates all the front stuff and the main seal.
Unless you sprayed your fluid deep into the backside of the booster (hard to do, but probably possible?) you did not eliminate that valve as the possible culprit.
It's also probably the most common failure point on an old booster. Right up there, and maybe above, the main diaphragm on the list-of-things-that-go-bad on a booster.

I can only assume that your idle is high due to the very small leak, but the reason it went down wasn't because you closed the leak, but because you opened it up even more and put the fuel/air mix over that threshold where it's too lean to support the high idle.
Sounds good in my head, but have no real world experience with it.

Did you ever check/change the other vacuum caps you were talking about?

Paul

 

[Timmy390]

Did you see what happens when you pull the pedal up hard with your foot too?
Just to see if it's the poppet valve staying open due to pedal pressure.

It's still the back valve no matter what I'd say. The only question remaining is what is actually the problem with it. Since you sprayed fluid around and did not detect any change, that pretty much eliminates all the front stuff and the main seal.
Unless you sprayed your fluid deep into the backside of the booster (hard to do, but probably possible?) you did not eliminate that valve as the possible culprit.
It's also probably the most common failure point on an old booster. Right up there, and maybe above, the main diaphragm on the list-of-things-that-go-bad on a booster.

I can only assume that your idle is high due to the very small leak, but the reason it went down wasn't because you closed the leak, but because you opened it up even more and put the fuel/air mix over that threshold where it's too lean to support the high idle.
Sounds good in my head, but have no real world experience with it.

Did you ever check/change the other vacuum caps you were talking about?

Paul

Got all the caps changed out. The remaining ones were good....well not good but didn't appear to be leaking but changed them anyway.

Pulling up on the peddle had no impact. I didn't get any spray into the back side of the booster. Can was empty.....I was at the parts store too and forgot to get some. I did swap out the check valves but that changed nothing.

I don't normally throw parts at things but did order a booster yesterday.

Tim

 

[Timmy390]

Got my new booster......now the question is, what was the old booster off of?

Clearly not Bronco. Looks like Bronco but the bolt pattern to mount it to the bracket is not correct.

Bough this off eBay 15 plus years ago.....Attaches to the bracket leaver with a bolt. Note the 2 piece bracket?

Tim

 

[Timmy390]

New booster on. Had to drill a new hole in the bracket and open up the circle in the middle. It's just a bit off level but I think the other one was too.

Idle is down 200 RPM and no change when hitting the brakes. Need to adjust the peddle position. It's just a 1/2 inch too high now. Brakes work SO much better.

Tim

 

[Seventee]

Kind of looks like a 70's Ford truck dual diaphragm booster. IIRC that flat spot at the eyelet is for a cruise control canceling switch.