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Brake pedal feels ok until I start the engine, then it hits the floor. Why?

1.8K views 13 replies 10 participants last post by  70sportsroof  
#1 ·
Redoing the colors in my engine bay, everything has been out. Now, when all is back in, I got brake problems. Pedal feels ok until I fire the engine. Then the pedal gets very loose and hits the floor and the rear brakes locks very easy. I'm tempted to install a new Booster/Master Cylinder, but is there anything I should check before spending the $$?

Also my Tilt Steering Wheel has change. Now it only tilts, with the engine running. Before I could stop the engine - open the door - and it tilted away. It seems it doesn't build up vacuum in the tank.

Just a thought - when both brakes and Tilt uses vacuum, could there be a connection?
Is it possible to reinstall any of the hoses wrong with this result?

The car is a -67 w/ disc brakes.

Thanks in advance.


Paul
 
#2 ·
On the brakes my guess is either you still have air in the lines or your master cylinder has gone bad. And on the tilt steering sounds like you are not storing vacuum. What you might try is have the car running and shut it off. Then unhook the line in the storage tank for the tilt and listen to hear if vacuum escapes. If not you have a leak in the system. All you have to do is find it.
 
#5 ·
I've bleeded the system three times without any luck.

Check valve?? Not sure which one you're referring to here?

Thanks.

Paul
 
#6 ·
Then unhook the line in the storage tank for the tilt and listen to hear if vacuum escapes.
My engineering background shuddered when I read of a vacuum escaping. The absence of air cannot escape; you can have a vacuum leak, which allows air to rush in. What you'll hear is air filling the vacuum tank by whistling through the small line of opening.

I guess everything depends upon your perspective.
 
#9 ·
Technically speaking, Randy is correct. The term vacuum is very misused in the automotive world, as you really don't have vacuum anywhere. Vaccum is the lack of everything. What we truely have in our engines (intake manifold and "vacuum lines") is low pressure, which cause the higher atmospheric pressure to rush in to equalize it.

The term vacuum, however, has changed over the last several decades to now also include the a new meaning "pressure significantly lower than atmospheric pressure", but I think most engineers still consider the term to mean the lack of all matter.

So the "vacuum" in our cars can never "escape". Instead what happens if you have, say a hole in a vaccum line, is that atmospheric pressure goes into it and raises the pressure back up at least closer to atmospheric pressure. The "hissing" sound you'll hear on a "vacuum leak" is air going in, not coming out.
 
#10 ·
I'll pass that to my car, but I doubt it'll fix my problems ::

Good infomation, though. Thanks.

I'll examine the hoses again.

Paul
 
#11 ·
Perhaps your check valve on the line to the booster is bad.
Found the leak in the tilt system :D

Now it's "just" the brakes. Could someone please explain where the check valve, mentioned in the quote, should be or show me a pic. of it? I have no valve between the booster and the brass attachment.
http://i48.photobucket.com/albums/f207/pahawi/Brace014.jpg

Thanks again.
Paul
 
#12 ·
Check the booster rod length. I just completed a power disc conversion using parts from CSRP and Mustang Steve. Highly recommended by the way. The MC and booster were not mated exactly and I had full peddle travel with the engine on. I adjusted the booster rod out about 1/4" to obtain proper peddle travel. This document gives a pretty clear explanation of the proper measurement. http://www.maximummotorsports.com/instructions/MMBAK-3r2.pdf

Good luck!
 
#13 ·
By the way, your picture is the vacuum tree. THe check valve is on the booster and is designed to prevent back flow of pressure. As I understand it this is a safety feature so that you have sufficient vacuum to brake if your engine dies. The check valve is connected to the vacuum tree pictured. Follow the hose to the booster and that's it! (Probably the larger hose.)