Here's one that has stumped every friend i have asked. I'm in the final stages of my truck build. My brakes work without the booster hooked to the vacuum line. But when I attach the vacuum, the pedal pushes to the floor. (when running) Exact opposite of what it's supposed to do. Thoughts? I am running a Holly 650 cfm, and using the correct port per the manual. I also tried an available port on the intake with the same result. Hoping someone has run into this before. Thanks!
Also, i'm running drums all around, front are 46 Frod, rear are Granada. Got a proportioning valve, and I bench bled my MC before install. I have bled the brakes several times now, and i believe i have found all the leaks and bubbles. Engine is a 350 Chevy. The MC/Booster is located under the cab, but i have a buddy with the same setup and no problems. I think it's just more diffucult to bleed vs on the firewall. So any ideas?
That's how a booster is supposed to work. It sounds like you may have a travel issue. With engine off, vacuum boost bled down, and one bleeder on each axle open, firmly push the pedal and verify the master cylinder pistons bottom out before the pedal hits the floor or firewall. Also, the pedal ratio should be in the 3-4:1 range. You still may have air in the system, but check/make sure full travel can be attained first!
I had a Ford do this once. It is an odd problem. The pedal would suck down by itself. The check valve was fine. The booster was bad. Worked fine after I installed another booster.
V8 Bob, the pedal has full travel, and does not hit the firewall. I may have a bubble in the system, and i"ll bleed again, but when running, with the vac line attached - pedal goes to the floor and no brakes.
Try jacking the oppisite end of the truck you are trying to bleed.This will get master cylinder higher than the bleeders
If you're using the stock pedals, your ratio is wrong for power assist. You want 3-4:1, not the near 6:1 for stock old Fords. Make sure your brakes are adjusted properly. The Lockheed fronts adjust much differently than the Bendix rears. What master cylinder are you using? A prop valve is not necessary on drum/drum systems, but with the early less efficient fronts and much better rears you have, the PV isn't a bad idea. Make sure you have a residual valve(s) going to each axle. (An upgrade to Bendix (Lincoln/F-1/F-100) or disc brakes on the front axle should be considered in the furure).
If the brakes are solid and it stops well (as can be) without the booster on, they don't have to be bled. Cars with power boosters don't need to be running in order to bleed them. Being that the pedal drops once the engine is started, the booster is probably okay. I'd look more into the pedal ratio as suggested. Bob
my set-up is like this http://www.ebay.com/itm/UNIVERSAL-F...Parts_Accessories&hash=item48464e1ebe&vxp=mtr i do not have residual valves installed, but I just ordered them. That may have something to do with it.
My '55 Suburban w/disc/drums 8"booster mounted under floor & my '31 Ford Phaeton TCI chassis both have 6:1 pedal ratio.
loosen the bolts holding the m/c to the booster, about 1/8 or 1/4 inch. start it up, see if it still happens. If not, the pedal was engaging the booster at its rest position
A booster that self - applies is likely faulty - valving is faulty. Try a new booster, or, if you are good, you can pull it apart and rebuild it (not usually worth it....)
Wanted to bring this back up. Any solution yet? I have the same problem with a slighly O/T Chevy - drum/drum power brakes - factory set-up.
Gave up for now. Trying to fix a leaky flex line at the cylinder. But, again, when it's running with the vacuum attached, and you push the brake pedal, it's got nothing. Goes to floor. Yet without the vac line to the booster, the brakes work pretty good. Just not assisted.
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