I'm trying to get my 30 coupe finished but dealing with "bugs." I have a dual master brake cyl. under the floor with a P&J single power booster. I'm running the vacuum line from the rear of the carb and getting vacuum. With the motor off (355 sbc), I have a good brake pedal (discs front, drums rear). When I start the engine, my pedal drops to the floor and I have no brakes. I've bled the brakes and its good. I exchanged the new booster for another and it does the same thing. I have an RV cam (no lumpy one). I "should" have no issues but can't seem to get an answer for the power assist. Anyone run into this or thoughts?
If your sure you got all of the air out of the system then you most likely have an issue with the master cylinder. It's sometimes difficult to bleed the air out of the system with an under the floor master. Do a search on here about brake bleeding.... there are a number of posts on the subject.If your positive that you have bleed all air out then try another master
Had that issue with an old Mustang PB setup. The rod between the MC and booster was adjusted incorrectly.
Thanks. My kid said he could borrow a power bleeder. I will try that. I set the heim joint on rod to where the pedal is at full up position and no free play in rod between pedal and master cyl. I guess I will keep playing with it. Thanks for the input. Keep ya posted! My vac line measures 5/8" with 3/8" insid hole so I have good sidewalls that aren't collapsing.
I added a power booster to two of my cars that are not traditional hot rods and learned on both, that I needed a longer stroke. On the 60 Ford pickup this was done by drilling a hole lower on the brake arm. On the 66 Chrysler, I got a power brake pedal off a power brake car and the pivot point for the brake arm was higher. this gave the same result as using a lower hole on the 60. Two ways of doing the same thing. If your pushrod at idle has the booster in too far, its like having the pedal partly pushed in, and its asking the booster to take over
I added a power booster to two of my cars that are not traditional hot rods and learned on both, that I needed a longer stroke. On the 60 Ford pickup this was done by drilling a hole lower on the brake arm. On the 66 Chrysler, I got a power brake pedal off a power brake car and the pivot point for the brake arm was higher. This gave the same result as using a lower hole on the 60. Two ways of doing the same thing. If your pushrod at idle has the booster in too far, its like having the pedal partly pushed in, and its asking the booster to take over I had a bad booster once that would pull the brake pedal to the floor as soon as I cranked the engine. You could take the pushrod off the pedal to test
That says the booster is good. Now plug the outputs of the master cylinder and see if that fixes it. If so you have not gotten all the air out of the lines or the master cylinder has an internal leak. It can be a bitch to bleed a new system. I've had to bleed many times on a new system with air everywhere. Make sure the bleeders are at the top or you'll never get all the air out. You may have to jack the car on either end to make sure there are no traps for air in the lines at the bends or junctions.
Or be traditional (like me) and get rid of the booster. Cosmo Seriously, those little boosters are crap.
I believe you need a little free play between the pedal and the booster. Otherwise the booster thinks you are applying the brakes a little. Do the brakes work OK without the booster connected to vacuum? Plug the vacuum line and go for a careful ride. If the brakes work OK then the master is OK and bleed out correctly.
Thanks for the great ideas. I got stuck having to fix the wife's car but will get back on this asap. Damn how priorities change. some people just don't understand the important stuff!
Quick update. Might be gaining on it. I hadn't noticed that there is free play in the black plastic housing surrounding the threaded rod when the brake pedal is depressed. Thought only the rod had play. I threaded out the heim joint to where I had a really stiff pedal with engine on. It turned out to be too much that the brakes were locked up shifting into gear. I backed off several turns and had brakes! Inaugural run around the block. However, the pedal doesn't want to return fully up so I had to manually pull it up with my foot to release any drag on the brakes. So I think it has to be a fine adjustment on the pedal play. I don't know how one might atach a return spring to a floor pedal? Hope to get further along here and be running soon. Will have to post updates and pics! Really appreciate the comments!
I'm currently working through setting up my under floor booster/master, too; got it on the road for the first timebrakes feel solid but the braking power is weak, I can't get the tires to lock up at all even with full force. I'm thinking maybe the booster is bad or there is an issue with the check valve, anyone have any thoughts? I have vacuum, just seems like no power.
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