I have experienced what APPEARS to be another master cylinder failure. This is the second new one in less than 2,000 miles(3 months) the original seemed bad when I got it then I bought one and then this second one. Is it possible that the proportioning valve is causing this? My 73 has the disc brake option and I am buying the right ones. I have found many things funky about 73?s so is it possible the master cylinder is that way? They ar elisted as 72-73 only. What am I missing. The pedal fades almost to the floor quickly after applying the brakes. All seemed well immediately after replacing it each time. The front calipers are new and the rear cylinders are not leaking. That leaves only the master cylinder and proportioning valve. What gives. Thanx , crossy
Crossy:
Aside from the proportioning valve, it is >very< important that you properly bench bleed the master cylinder before installation. I speak from experience here, having had GREAT difficulty getting my system up and running as well, even after replacing a proportioning valve as you suggested.
The very same thing happened to me, I had all new wheel cylinders and hoses, and the system was tight otherwise. It would seem fine at first, and then do just what yours did - go to the floor within a short time. VERY frustrating!
The air MUST be out of the system, and this begins at the master cylinder. The bench bleeding is a relatively easy process, and most master cylinders will include instructions on how to perform it. Best wishes on getting up and going!!
Chris
PS - ALSO be sure you buy a NEW master cylinder, not a rebuilt one. Rebuilt ones can be tricky because the reboring "loosens" up the piston and there is less seal integrity. Make sure you buy new - the few extyra $$ are worth it.
Chris
so the one i have IS shot again. the bench bleed will not bring it back?
I would not give up on it yet, though you WILL need to remove it, put it in a vise, get yourself some small hoses, and bleed it manually back into its own reservoir. I hope that does the trick for you.
To bench bleed, you basically:
1) Secure the master cylinder in a vise
2) Run some small hoses from the outlets back up INTO the reservoirs
3) Fill the reservoirs with a small amount of fluid (about 1/2" deep)
4) Using a dowel or screwdriver, gently press the piston at the rear inward. You are trying to force any air out of the piston area, through the outlets, and back into the reservoir, where it will bubble up and out of the fluid. IF you see bubbles, this is good (at first), because you are pulling air out of the piston area.
5) Repeat until no bubbles are left, and pressing the piston only moves fluid, and shows no air. Do it until even the smallest bubbles are gone.
6) Install the master cylinder in the car, keeping hoses in place, and fluid in the reservoirs, DON'T spill it or let it run out.
7) Remove each hose, ONE at a time, and reconnect to the brake lines. Do this quickly as you will lose fluid through the outlets. Do NOT let the reservoirs get low enough again that air is reintroduced into the piston.
8) Once reattached, BLEED the whole system. You should get a firm pedal. If not, you have a bad master cylinder (is allowing air in), bad lines, or leaks at the wheel cylinders.
Best wishes.
Chris
If I might add... DO NOT clamp the M/C in the vise by the body or housing, clamp it by the mounting ear. Alternate between full and half strokes. I changed my '71 to front disc and bleed the M/C just as Chris outlined, then opened all of the bleeders and let it gravity bleed, then did a normal bleed. It's been years and no problems.
i may be getting on here late. when you bench bleed the master cylincer. After the plunger is depressed and released (at a slow rate), wait 10 to 15 seconds or longer before depressing the plunger. Good Luck
Quote from: tbucketjack on September 02, 2009, 12:01:44 AM
i may be getting on here late. when you bench bleed the master cylincer. After the plunger is depressed and released (at a slow rate), wait 10 to 15 seconds or longer before depressing the plunger. Good Luck
Your not late, I'm VERY slow, to get things done that is. I'm looking at another 73 sedan though, thanx, crossy
A man by my own heart - 1st car is not finished and looking at another one. Why do we all do this? ;D
Chris