Any advice on this puzzling brake problem?
 

So my brakes rather suddenly went downhill last week, all of a sudden I could put the pedal on the floor! If I pumped them up I could brake, but it took two or three pumps before I got some real stopping power, and this made driving the car an unsafe thing to do. So I figured that maybe I had a bad master cylinder. So I bought a new one, and installed it this afternoon and bled the system.....absolutely no improvement, symptoms are the same, I can put the brake pedal on the floor with very little reistance and almost no braking power, but the brakes firm up if I pump them. What on earth is happening here? Could this somehow be the booster? the brakes are very solid when the engine isn't running, but when I put pressure on the pedal and then start the engine, the pedal travels to the floor as soon as the engine catches. I know just about nothing when it comes to power brakes, but I've rebuilt entire brake systems on antique cars, and so maybe my suspicion of the booster is just based on the fact that I don't really know how it works. Anyway, I really want to get my 944 back on the road, and need some advice on what I should be looking at to fix this problem. Any and all advice appreciated, thanks all!

Might have gotten a bad mstr cylinder,is it rebuilt or new? Are you losing fluid anywhere? Check all your calipers. If the booster was bad you SHOULD have a very stiff brake pedal.

Master was new, no fluid loss that I have found. Of course, I haven't been driving the car much since this brake problem occured, so I'm not cycling the brakes enough to lose a noticable amount from the resorvoir unless I had a really huge leak. But anyway, no obvious fluid dripping off of the new master or any other brake components.

Try bleeding again,and bleed the clutch too. (if its a standard) The mstr cylinder could still be bad,call the people you got it from and ask them.

Already bled the system once with no improvement. So I bled it again, this time with the clutch as well for good measure. Still no improvement. Bled it a third time with my dad, since we both agreed that it seemed like a classic "air in the lines" symptom, and still no improvement. I was alternating between regular (I think I used STP?) and superblue brake fluid, so it was obvious when the entire system had been flushed. Is there any weird spot (the ABS system comes to mind) where the brake system could have some sort of difficult to bleed air pocket?

Umm, stupid question, did you bench bleed the master before you installed it?

could be a leaky brake line. had a car with similar symptoms. new brake line fixed everything

Are you sure the fluids are compatible. I encountered a BAD OR MIS-LABELED CAN OF BRAKE FLUID AND HAD TO CHANGE OUT ALL THE BRAKE SYSTEM SEALS ON AN S-10 PICKUP.

Andnew-Yep, bled it before installation as described on clarks garage.
Cebu-the first time I bled the entire system I replaced the old fluid (Previous owners stuff) with superblue, which was definitely blue and came in the correct can. I'd be surprised if there was something amiss with the fluid.

First off, check your rubber brake hoses for soft spots, swelling\blistering.

Then make sure all of your brake lines are tight, as well as the connection between reservoir and the master cylinder.

Thirdly, function check your brake booster - pump up the pedal with the car turned off - the pedal should get very stiff. Then, when you start the car, the pedal should drop slightly.

If this does not happen - check vacuum lines and check valve in brake booster.

If vacuum lines and check valve are good, replace brake booster.

AFJuvat

same thing happened to me last week, pedal went to floor MC is relativly new so I was pricing out boosters and by chance while trying to fix a different problem (stalling) I inspected and jiggled all the vaccum hoses under the hood including the booster hoses and the related ones above and near the charcoal canister...problem solved.
new boosters go for just under $400.