1990 bronco II engine revs higher when brakes applied

mandrake001x · Feb 4, 2014

I have a 1990 bronco II 2.9L 4wd 5 speed. I have recently noticed that when I apply the brakes with the clutch in, the engine revs about 500 rpm higher. New vacuum assist installed, several new brake lines installed... it appears to be intermittent... any suggestions, maybe the proportioning valve in the brake lines ????? any help would be appreciated....

ab_slack · Feb 4, 2014

Do you actually hear the engine rev higher (and see on the tach) or do you only observe this on the tachometer?

If it is the former I would expect maybe something with the vacuum. If the latter it still could be the engine changing but it could just be the indication which points in the direction of something electrical.

mandrake001x · Feb 4, 2014

engine revs and tach increases.... also requires more pressure on brake pedal.. sorry about the multiple responses... still new on this forum

ab_slack · Feb 4, 2014

I was hoping maybe someone has some more specific answer as I can only make some more general comments, but I'll post my thoughts to bump it.

I am doubtful it is proportioning valve or any element in the hydraulic part of the system. I can't think of any connection to the engine unless perhaps there is a fluid leak that allows the power assist to move so much it affects the engine.

The only connection I see is the vacuum assist.

You mention when the clutch is pushed in this happens, does it also happen in neutral? I suspect it does. It makes sense that it won't when in gear because engine RPM is pretty much locked to wheel speed and there probably isn't enough energy behind the RPM increase.

I am assuming that you are able to replicate this when sitting still, clutch depressed (or neutral) engine idling and push the brake pedal then see the RPM increase. Such would eliminate a number of other variables

This being the case I am thinking it has something to do with the vacuum assist. Maybe some air is leaking around the diaphragm or air leaking thru the valve that allows vacuum on both sides of the diaphragm that is supposed to close when brake pedal pushed. Either of these would cause a loss in brake force and at the same time dump air into the engine's vacuum system changing how it runs. This seems like a good fit with the symptoms you experience

Is the RPM increase solid for as long as the brake is pressed or is it just a one time surge when the brakes are first pressed?

Did this problem exist before the vacuum assist was replaced? Was it replaced trying to resolve this issue?

Keep in mind that I am just making guesses based on my understanding of the system. I've never had to deal with a vacuum assist issue so can't give the sorts of insights that someone who works on these things knows. I could be entirely wrong too.

adsm08 · Feb 4, 2014

It sounds like an air leak through the booster. The proportioning valve certainly cannot cause it.

I would take the hose out of the booster, wrap it up good and tight in a plastic bag, and try again, see if it still does it. If that fixes it then you have a defective booster.

wildbill23c · Feb 5, 2014

Its definitely the brake booster leaking from a vacuum leak or the booster itself has failed. My 84 B2 does the opposite though it idles really low when I mash down on the brake pedal LOL.

mandrake001x · Feb 8, 2014

Thanks guys.... kinda what I figured.... the rubber grommet on the booster vacuum inlet was split, so I replaced the check valve, grommet and vacuum line, however it still isn't right.... been super cold, so wondered if some moisture got inside the booster and froze... just put this one on about 2 months ago after the oem failed. been working fine until the below zero weather set in. This one is under warranty, so I may wind up replacing it (again)... cruse worked before this replacement but doesn't work now, so I probably did something wrong attaching to the brake pedal...

RonD · Feb 8, 2014

Cruise control also has a vacuum line at the brake pedal, if brake is applied it opens vacuum to disable cruise control, it is a failsafe device.

Look at this diagram, lower left, "vacuum dump valve":
http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Cruisecontrol_1of2.JPG

If that is broken then you could get a vacuum leak when brake was pushed in, this could raise RPM.

1990 bronco II engine revs higher when brakes applied

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