Master Cylinder kaput?
 

Here's the deal. 1974 911 with (dual?) master cylinder. Press brake pedal and it goes to the floor and does not return. No visible leaks in the brake lines (fairly new by eyeballing them) plenty of fluid in the resevoir. Hayne's says master cylinder seal failure. My question is, if the master cylinder seals fail would I see leakage out of the master cylinder into the front axle pan? Does the fluid stay in the system when the seals fail with no discernable leakage at the master cylinder? I know these are pretty basic questions but please bear with me.

Thanks,

Pete

Pete if I remember correctly, there does not necessarily have to be a leak...the master cylinder could just be bad.
Also could be air in the system. Check the level of the fluid, try to bleed it first, before replacing the MC.
From what I remember my buddy's '75 (? don't remember now...) took well over a galon of brake fluid, after we replaced his lines, brakes, calipers,etc...

If the master cylinder is bad it will leak, either air and/or fluid. If you're not already doing so, hook up a Motive pressure bleeder and give it a whirl.

I've seen many cars leak fluid into the vacuum resevoir not onto the floor when the master cylinder is bad. Does the 74 have power brakes?

Mastr cylinder 68-77 $172 or kit $51.95 Performance 800 423-3173

I'm with Chris on this. When mine failed, I found fluid on the floor. On most cars the back side is in the engine bay but, on ours it is in the cockpit, especially on a manual brake car like yours. A motive will help heighten the issue. Don't assume anything, though. Check before you buy the parts. A repair kit should be available if you are adventuresome.

In general, a master cylinder failure will involve one of the two (68 and later) cup seals.
For a high mileage car moisture collects in the bottom of the master cylinder bore and creates rust pits. The front inner cup seal rides over the pits and the rubber get abraided. Eventually a groove is worn into the cup seal and its sealing function fails and pedal goes almost to the floor.
The rear cup seal is at the end of the cylinder bore near the pushrod (at the mounting flange). Every time the pedal is depressed the rear of the bore wall is exposed to the atmosphere. Eventually rust develops here. Every time the brakes are released the rear cup seal rests in the rusted area. The cup seal erodes and fails. This is the most common failure of the master cylinder. On the 911's in the pre failure mode you will notice paint corroding on the booster under the attachment flange of the master cylinder. For the 928's and 44's you will notice fluid leaking down the firewall underneath the carpeting at the master cylinder mounting flange. Another symptom of rear cup seal failure is that in manual bleeding the rear brakes will not completely bleed out. There will be little air bubbles that will not go away. However pressure bleeding will remove all the air bubbles because the brake cylinder is not actuated. After some miles of usage the bubbles will return.
Yes, the master cylinder can be rebuilt providing the bore is not too big in diameter after honing. For cylinder bores 19mm or larger the bore tolerance is at least .004" (a little bigger for larger bores--listed in the instructions in kit). High mileage (100K or more) cars usually will exceed the limit without honing. Honing out rust pits is guaranteed to exceed this spec.

Great tip. I have come across this before and it was only cured by replacing the MC. We didn't know the exact cause but your explanation sheds some light on the matter :D

Jason

This is why I like this board so much. A new lesson every day. :D

With many coming from Matt. Thanks for all the good advice Matt!

Thanks guys, everyone here is so well informed that I feel kind of dumb asking basic questions.

I haven't noticed any fluid on the floor or in the front pan. When I was replacing the tie rods with Dan M we unbolted the MC to see if it would help with elbow room and when we were reinstalling it there was some fluid when it was actuated.

I have no problem replacing the MC (money wise that is :) ) it's just I want to make sure I'm not overlooking anything. Not sure how much fluid I have poured into the system, I have run one liter of ATE Super Blue and a good quantity of Castrol "less expensive to watch flow from the resevoir to the blled nipple" brake fluid.

I did notice a lot of the tiny bubbles from the rear calipers that Matt was describing but the fronts, once I put the calipers on the correct sides, seemed to be free of bubbles.

I'm really anxious to drive this thing, I want to be able to participate in the Halloween Rally the car should win a prize for "scariest paint job"!

The last major things I really need done are to get the brakes to work and get a 4 wheel alignment so I can get it to Karosserie for the front pan fix.

This board is the best Help Desk I've ever used.

Thanks,

Pete