Vehicle: '66 F100 CC/CS with 390 and C6. I did a complete Power Disc Brake conversion using a '73 F100 donor truck a couple of years ago - been driving the truck since end of December. All new/remanufactured calipers, master cylinder, power booster, rear cylinders, new flex lines, new steel lines. Up until last week the brakes have been excellent.

Then, coming home from work I pressed on the brakes to pull into a parking lot and the pedal just about went to the floor. I can still feel the brakes come on as soon as I start pressing the brake pedal, it just has no firmness. I can push the pedal to the floor. When I turn off the truck the brakes pumped up firm.... There is no brake fluid loss anywhere.

After reading thru many threads here, it pointed to an internal failure of the master cylinder as the likely culprit. So this weekend I had the pleasure of R&R'g a re-manufactured MC and all the bleeding, and NO DIFFERENCE!?!? In fact, I'd say it is a little more mushy when the engine is off now. Yes, I spent a good 20 minutes bench bleeding the MS and sequencing thru the brake cylinders/calipers....

So, need some guidance on what I missed, messed up or need to do next. Did I leave a bunch of air in the lines somehow - possible, but to have the same symptoms as before on a system that had no air in it seems to lessen this possibility? Could it be a bad booster (I put a vacuum gauge on the booster line and it would only take 5" of vacuum, but held it with no leaks; that and the boost function is definitely working as when the engine is on the pedal goes to the floor..)? Bad proportioning valve (though I would target this when brakes lock up or aren't even)? And did I mention I hate working with this caustic brake fluid!? The truck drives and stops - but praying I don't need to slam on the brakes!

Thanks in advance,
daggerNC

 

Discussed this with a recommended mechanic/brake specialist. Based on everything I've done, and how this new reman. MC is behaving, he said it is highly likely I have another faulty MC. He noted that the reman. quality from the Fenco (?) company that does the work for AutoZone is not the best....

I've ordered a new (not reman.) MC, so we'll see how that goes later this week. One other piece of advice given to me from this brake mechanic is that after a bench-bled MC is installed in the vehicle you should do an additional 4 to 5 bleeds per port using the brake pedal pressure (pump it up, hold, then bleed). He said it is very hard to get 100% of the air bubbles out of the MC via bench bleeding. Just make sure you get good rag coverage over those fittings so the caustic brake fluid doesn't spray all over the place.

Cheers,
daggerNC