Can’t get a brake pedal when engine is running with vacuum applied to booster. When the engine is off, the pedal feels relatively firm, but as soon as the vacuum is applied the pedal goes to the floor. Everything in the system is brand new, booster, master cyl, comb. valve, all brake lines, calipers, rotors, wheel cyl., drums, shoes and discs. System bled multiple times, appears to be no air present. Master cylinder and booster have been replaced, nothing changed, same problem. Comb. valve brake light warning switch centered and appears to be ok. Any help would be appreciated.
Does this vehicle have ABS?
Tester
And what are you working on? Make, Model, Year, Engine, miles on the odo?
It does not have ABS
@WildVines As already mentioned, for best results, we will be better able to assist you if we knew what you were working on…
It’s a custom 89 Jeep, 350 motor, no ABS, no computer, etc.
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One thing I will say is to make sure all the wheel bearings are tight…
Also, clamp off all the rubber brake hoses, retest the brake pedal, if it is now very firm then the issue is below the clamp off points, if it still drops then it is above the clamped off points…
Sounds like a fun project…
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Thanks for the quick response. The wheel bearings are tight and I can’t clamp the hoses because they are braided steel. Thanks again and let me know if you have any other thoughts.
My diy’er guess is there’s something wrong related to the power booster. Since it is new, maybe that part is faulty right out of the box. Suggest to remove mc from booster and inspect that interface carefully. Before doing that, I’d probably do a quick test w/a hand-held vacuum pump that the booster holds vacuum to 22 inch hg.
Your engine isn’t turbo- or super-charged right?
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I think the problem is that there is no brake pressure from the master cylinder. It’s the second place I’d look. I say that because I put a like on George’s post and then I read it again.
Did you bleed the master cylinder before installing? Or at least after?
Are all the new brake parts from the 89 Jeep? Or is there a little mix and match?
Thanks George, this is the second new booster with same result. I think it’s booster related too. Booster holds pressure, (20 ihg from engine), if I remove the one way valve, air comes from the booster. Engine is carbureted. Thanks again
See if you can find a way to cap off the hard line at the braided lines and cap them off and retest it… I have put nail heads in between the hard brake line and the hose before to block off a blown wheel cylinder to get it homme, so that might work also, just want to block any fluid going past the hoses…
If you can find the right size nail or make some block off caps, then you can also cap off the proportioning valve and even the master itself… again, blocking lines etc off eliminates everything or comfirms something below that blocked part…
And as Mustangman asked, did you bench bleed the master (I’m guess yes but got to ask)??
Two things come to mind. First, did you bench bleed the master cylinder before installing it? Second, make sure the bleed valve on each caliper is above the brake hose.
Did you try to pump up the brakes with the engine running and if you did, did it pump up?
BTW, the master cylinder has to be level when it is bench bleed. On most vehicles, it points up a little when mounted and if you try to bench bleed while mounted, it will trap some air near the front.