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Old 01/10/12, 07:56 PM   #1
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Power Brakes not getting enough vacuum

I installed all new power brake booster, master cylinder and have new brake lines/brakes. Master cylinder was bench bled. I've bled brakes many times, did it, and still didn't feel right. Ended up bringing truck to a excellent mechanic i've used before for a couple things and told him to check brakes. He said engine wasn't producing enough vacuum for brakes. He dialed in engine, runs good, was rebuilt not too long ago. It has a lumpy cam which he said could have something to do with it. The hose coming from the booster going into carb goes into a tee with vacuum line from tranny. There is another port I could go into in back of carb. If I go straight into carb will that make much difference? Anyone else had this problem? When I bought the truck it had hydroboost on it which I took for my own truck,wondering if that's why maybe it had it.
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Old 01/10/12, 08:08 PM   #2
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on the stock 2barrel cast iron Intake there is a plug you can remove from behind the base of the carb. I put a barb fitting there to get my vacuum
another option is a vac tank
I have a new crane cam on that I could let go

Its a Bronco thing ...only you would know
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Old 01/10/12, 08:18 PM   #3
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How many inches of vacuum do you have at idle? If you 17+ you should not have any issues. Less than 14" I have had them work sporadically - good when you left off the gas but not so good at idle.

The hose going to the booster should be 3/8" diameter and stiff or thick walled. I had a customer car with "bad" power assist but the vacuum hose 3/8" thin walled. Under high vacuum (like if you rev it at idle and slam the throttle shut) the hose was actually flat in the middle. The factory Ford hose is really stiff.

Hydroboost is the best $200 project on my rig.

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3G, PS, Hydroboost, front discs, 4R70W/NP205/MicroSquirt, forged 5.0 roller w/EDIS/serp/Mustang EFI/MegaSquirt controlling it - all tuned by ME, Hedman headers, Extreme Fab Family cage, H-of-K paint stock body and suspension.
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Old 01/10/12, 08:19 PM   #4
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How much vacuum do you have? Never mind, that was just ask.
Most of the SBF I have checked only have 14-15 and the brakes work fine. Why do you think they don't work. When I get involved in posts like this it always reminds me of the customer that called and said I had sent him the wrong windshield gasket... I thought, there is only 2, 1 with the grove for chrome and 1 that did not, he did not have/want the chrome groove so how could he have gotton the wrong gasket? I finally got a picture, the dam thing only had 3 sides. So with that in mine more info here might be helpful

Last edited by chuck; 01/10/12 at 08:31 PM..

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Old 01/10/12, 08:58 PM   #5
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I can't check vacuum until later tomorrow or thursday. I've had many broncos with power brakes and they've all stopped wayyyyy better. I have another bronco at my place right now with power brakes and stops excellent. I did go with the 8" diaphragm as well. When you're driving it and hit the brakes, it's hard to stop, pedal does not go down and is not soft.
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Old 01/10/12, 10:29 PM   #6
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More ?? what brake setup? what MC? what sized tires? What type of brake pads?
Many times a MC with to large of a piston bore is used and the brake pedal becomes harder and doesnt go down much. Thats why many people convert to H boost.
Brake pad choice can also effect braking some of the newer high dollar setups dont work well until they are hot and even then dont work that great on systems that are not correctly setup.

73 Ranger 2 1/2 lift 302 TFS heads NP435,PS,PDB, tierod over, 33x10.50x15 BFG M/T
71 Sport 63,000 miles all orginal except for cut fenders 302 3sp 4.11's D44/BBTrac lok rear
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Old 01/11/12, 01:54 PM   #7
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I'm glad you posted, because I have been thinking that something isn't right. This summer, I installed the "major four wheel power disc brake conversion". The install went well, no real hiccups, I had to bleed the rear calipers upside down if I recall, but the pedal is hard as a rock, no air in there at all, the pedal has maybe 2 inches of travel, tops, but they feel as if there is no vacuum assist at all. If you come up with anything, please post it. Otherwise hydro boost is getting added my to do list.

1967 LUBR, 302 4V, NV3550, 4:11 gears, power discs all around, Full Cage, Dynamat, Custom Carpet
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Old 01/11/12, 03:51 PM   #8
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Make sure your rear breaks are adjusted correctly. I know my rears are not engaging at all, but only found out when getting the tires rotated...
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Old 01/11/12, 06:22 PM   #9
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san jose ca.
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There should be an a 3 tube outlet in the intake manifold behind the carb with 2 small outlets and one larger outlet. If no outlet is there then there should be a plug in the manifoldwhre the oulet goes. I could send a picture of the outlet but it's a pretty common part found on all Ford V8 blocks. You get better vacume from the maniford than from the carb.
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Old 01/11/12, 07:05 PM   #10
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I have a crane cams vacuum resevoir with the stock pb setup and works great!!!

68 Sport-automatic undercarrage lubricaton system, anti stop rear brakes,flow thru fresh air ventlation system.
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Old 01/11/12, 07:09 PM   #11
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At the brake vacuum booster there is a fitting that is a check valve that the vacuum hose connects to. Is there any chance in hell that it's back wards? I would measure vacuum at the booster hose fitting with my thumb to beguine with. Then go from there
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Old 01/11/12, 07:16 PM   #12
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Like av said, if your vacuum signal is too low, then a reservoir can may be your only hope for the short term quick fix version.
If you have that 17" or more of vacuum, then low vacuum at the engine is not really your issue I wouldn't think.
Personally, I wouldn't "T" both of those into the base of the carb either, but I can't see how that would negatively effect your braking unless there is a vacuum leak at the transmission end that's reducing the available vacuum to the booster.

If this is all at the carb, where is your PCV valve plumbed to? Do you have two large vacuum ports on the carb available for these then? If not, then you may have to re-route some things.

Agree too on the good firm hose suggestion. That factory stuff was some nice rubber I must say. When I re-do one, I like to use 3/8" steel line all bent up nice and out of the way, with rubber at just the ends. It's not for everybody, but I replace rubber with steel wherever I can.

Good luck. Let us know what you find regarding your vacuum. Did the mechanic at least tell you how much he was reading?


'71 Wagon, 3.5" WH, F150 disc brakes and steering, 4.11 33x11.50 Thornbirds, Kayline soft top, Hanson bumpers. "Built, not bought"

'68 LUBR, 2.5 + 1 WH, 31x10.50, 4.56, Explorer and 4R70w by EFI Guy, WH disc brakes, Hanson front bumper, Warn winch. "Bought, not built"
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Old 01/11/12, 08:20 PM   #13
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Thanks for all the replies. I'm not going to get to this until the weekend. I will check things that you guys have mentioned. I will check vacuum and all lines. I'm wondering if master cylinder is the wrong one. I've had this problem before. If vacuum looks good, I may switch it. I have a brand new one here from Toms that is for 76-77 with disc brakes.
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Old 01/12/12, 10:04 PM   #14
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Did the EB stop right with the H-boost that you took off? What kind of brakes do you have front and rear?

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