Power brakes acting silly!

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Sweetjones

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I have a 1975 duster midly built built 340 in it and I've just got it legal and have been driving it around alittle. It has power brakes with discs up front and drums in the back. I'm also running a line lock. Me and dad bled the brakes and they feel great but as soon as I start the car they go to poop. When you hit the peddle it will stop but there's no resistance when pushing the peddle down. Even if you pump them it helps some but still sloppy. My cam is a lunadi voodoo with .513 lift and around 265/275 ish duration. I have a vacuume canister and I'm pulling my vacuume from the intake manifold. The manifold is an edelbrock rpm airgap. I don't have a vacuume gauge so I can't see what it's pulling. So my question is am I not getting enough vacuume? I've looked into a pump from summit. Anyone have any experience with them? I've also been entertaining the idea of switching to manual brakes. Is there a direct bolt on replacement kit for that? Any help or positive comments will be much appreciated!!
 
The vacuum booster is actually a power assist. It is supposed to provide effortless pedal operation. So lets assume with the engine off and residual vacuum used up, you pump the pedal and it sits right at the top and feels fine. Hold it there with medium pressure and wait, feel for a slow give away, pedal sinking. That would be bypassing within the master cylinder. Power assist would help you shove you foot right on through that bypass a lot faster. This is the condition I had in my Camry recently.
If the pedal doesn't move at all, start the engine. the pedal should drop a little, inch or so. That's normal. If it goes to the floor, either the check valve in the vacuum line, or the line connections, or the booster itself is failing.
 
None are the lines are leaking from what I can tell. The peddle is nice and firm when the car is off then I start the car and the pedal loses its firmness. The brake booster and master cylinder was on it when I bought the car. The booster looked pretty new but it did sit for a longtime before I bought it. Maybe I'll change out the booster and see what happens.
 
Just so you know... Many Chrysler products of this era were very little old lady friendly. Power brakes required very little effort. Power steering too. Lots of owners switch both to manual.
Harbor Freight has a cheap little vacuum hand pump that comes in handy for testing servos and such. Comes with a gauge too. :)
 
Another factor I failed to mention is the condition of the brakes. If the rotors are turned thin , pads are worn thin, pistons are out of caliper bores a good distance, you'll have a lot of area to build pressure in. The booster will let you build up to 900 psi in the fluid circuit so sitting still you can make a heck of a lot more braking that is required to stop the car. Amount of area to build pressure in alters the pedals travel though. Add very large bore rear wheel cylinders, all sorts of factors come into play.
Before I bought another booster ( not cheap ), I would dead head the fluid lines at the master cylinder and see what difference it makes.
 
..................Sounds like ur master cylinder is bad....test it like Red Fish says.....or ur rear brakes may need adjusted up, especially if it takes a few pumps to get a solid pedal ........kim............
 
Lots of other stuff could be happening. How far down does the pedal go when car is still and idling? To the floor? Maybe the rears could use a little tightening. Check the flexible lines, two in front, one in rear. When you have the benefit of the booster you may be puffing up the hoses like a balloon instead of operating the brakes.
 
Thanks for the help! I know that the rear brakes are tight because I had to put shoes on them when I got the car and where adjusted. The front brakes looked like they where in good shape but I'm not 100% sure. I know things where built different back in those days my power steering feels like I'm Turing a yacht and it moves with one finger. Lol they just don't feel right to me. anybody know of a manual brake kit I could swap to? No mopar junk yards in my area..
 
kinda thinking about going to manual brakes. Can someone walk me threw the process. I know I need a manual master cylinder and the correct length push rod. I found out the parts store by my house has a adjustable universal push rod I could order. The thing I'm needing help with is the linkage for the pedal. Do I bolt the manual master cylinder to the bracket where the brake booster was mounted? Or do I remove that bracket drill a hole for the push rod to go threw the firewall and mount the manual master cylinder to the firewall? Could anyone post a photo of there conversion?
 
I have the comp vacume motor on mine that fixed your problem, and my problem. Im gonna put manual disc up front this spring if you want to buy a slightly used set up I willtake 200 shipped. Im running power drum right now. Your cam doesnt make enough vacume for your set up you need a brake booster motor or manual brakes.
 
Power to manual master cylinder swap on a 75 duster factory disk and small drums out back. I know someone on here has done the swap before! Help! Anybody! Hello!! Lol
 
Power to manual master cylinder swap on a 75 duster factory disk and small drums out back. I know someone on here has done the swap before! Help! Anybody! Hello!! Lol
 
I haven't done the swap. Have done the opposite ( manual to power brakes ).
Should be as simple as changing the pedal push rod.
 
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