Brake pedal goes to floor; pumps up.

[BiancaGT]

I have a '67 GT, all stock. It has a single circuit master and single servo. The brake pedal goes to the floor when I press it down. It pumps up and holds, but when I release it and reapply the brakes, it goes to the floor again. The M/C did not leak but I rebuilt it anyway. When that didn't fix the problem, not trusting my work, I replaced it with a new master. Still have the problem. A couple years and less than 5,000 miles ago, I had the same problem and rebuilt the master and the servo. One or both fixed the problem but now it's back. I also replaced the flexible lines so it seems internal collapse of one of them is the unlikely. The pads have plenty of meat on them and there are no leaks in the calipers. No detectable loss of fluid and no sponginess when the brake is pumped up (i.e., air in the lines unlikely). Could it be the servo (again)? Anyone know what's going on?

Thank you!

[ciscotex]

Did you bench bleed the m/c before installation?

Paul

[BiancaGT]

Yup. Every time. And of course I bled the whole system, back to front.

[ciscotex]

Original style hoses or aeroquip? If original style, how old?

Paul

[BiancaGT]

The hoses are original style, about 2 years old. I checked them again. No cracks, leaks, loose fittings, etc. I read about worn axle brgs causing this problem in other cars because the rear discs would wobble slightly, but I don't think that would be an issue here, even if the brgs were bad because it occurs statically. That is, with the car at a stop, the problem happens with each reapplication of the brakes. But I was wondering if anyone ever had this problem with a sticky caliper or, perhaps one that didn't bleed properly or something.

Thanks for your input...

[ciscotex]

Thanks. There is something compressing/expanding in the system. Only time I ever experienced this was the time I didn't adequately bleed the m/c. I did have an original rear rubber line pull completely out of its ferrule at an indiscreet rate of speed approaching a red light, but that's another story (haven't liked rubber lines ever since).

FWIW, I think you still have air in the system. If the brake pedal is let up too quickly while bleeding, the fluid can boil in the m/c by cavitation. Read Tilton's instuctions with his brake bleed kit or search the bb for different bleeding techniques. I still hate bleeding brakes.

Paul

[BiancaGT]

Thanks for the input, Paul. Bleeding brakes is one of those things that seem far less exact than reason would otherwise dictate. It's like I should do some kind of voodoo incantation, sacrifice a virgin hose clamp, or something else first, so the brake demons will be satisfied. I can't imagine brake fluid boiling from the low pressure cavitation caused by a returning pedal, but stranger things have turned out true. Regardless, I bleed the brakes either with a pressure bleeder (one of those big, expensive, steel tanks you can get off the parts trucks that use compressed air, hold a gallon of fluid, and piss it all over the place because the adaptor never fits properly on the reservoir) or, more recently, a vacuum device I invented that works like one of those you can buy except it includes a glass jar in the bleed line so I can collect the fluid and see what's in it as it comes out. Not a lot of cavitation, either way. But it does seem like there is air trapped in there somewhere, and for some reason, I'm not getting it out. I'll just have to go over this again with a fine-toothed parts brush.

Again, thanks.
Dave

[ciscotex]

LOL, I think I own more tools for bleeding brakes than any other job, and I still hate them all. My favorite is the pressure bleeder that blows brake fluid all over your paint because you didn't say the right prayer to the right deity on the right day. I have hope for the cheap harbor freight "Vacula" clone if I remember to fit o-rings around the bleeder nipples so the damn thing doesn't suck air in around the bleeder while its sucking brake fluid.
BTW, as you know, neither of the approaches you mention would lead to cavitation, only the 2 person pump /squirt pedal dance.

Paul

Paul

[BiancaGT]

Indeed. There's something ominous about brake fluid. Why they couldn't've figured out some other liquid for the purpose is beyond me. It's like the inventor was thinking, 'How can I make this sh*t as absolutely toxic, unpleasant, and obnoxious as possible...'. Anyway, those pressure bleeders are actually nice on the rare occasions that they work. But I think they're mostly good on old Fords or Chevys or something else with integrated cast iron reservoirs - and paint so crappy you don't care if it gets dowsed with this virulent goo. But if you haven't tried this, I have a nice trick for you: Next time you're vacuuming the bleeders on your calipers, slave cylinders, or whatever, take the bleeder screws out completely (you'll only have to do this once) and wrap them in teflon tape. Then, just loosen them slightly when it's time to bleed. The tape will keep air from being drawn past the threads so you won't get misleading bubbles in the evacuated fluid. Also, it keeps the bleeders from rusting frozen in the part so you don't have to loosen them with a vicegrips next time you need to do the job. They seal with less tightening too.

[ciscotex]

Good idea on the tape. I bleed mine often enough that they're not frozen, and the threads are so clean they leak air like mad. Got me thinking (dangerous) as to whether the original non-hardening Permatex might work well, as it stays tacky and doesn't move. It's also the only other sauce I hate almost as much as brake fluid. Couldn't keep it off my hands before we had gloves.

P

[BiancaGT]

I'm afraid of using anything other than USDA/TUV/Brevitatto/eh datsa niceh certified lubes in the presence of hydraulic parts. Permatex might work but I know the tape definitely does. And I bleed brakes with every passing of Hale-Bopp, whether needed or not. So I have a genuine, Rusted In Place Certificate to go with every car.

-d

[kiwi-kid]

Hi Guys,
BiancaGT, did you ever get to the bottom of this?
I have the same problem...
Resleeved master myclinder, new boosters, pressure bled from top, then pressure bled from the bottom, used litres of fluid...still cra*py pedal.
Pump pedal and then it feels fine.
This problem is kind of frustrating, I am running out of ideas!
Cheers
Ian

[BiancaGT]

Hi Ian,

Not sure. I think I might've ended up having a faulty connection in the line running to the rear brakes. It looked a little wet there and I originally thought it was due to a little oil or perhaps grease splatter from the driveshaft. But it could've been brake fluid. If so, it might've been losing some there and drawing in a little air. Anyway, I loosened the fitting, worked it back in and out a few times, and then tightened it back up. I bled the brakes again and some air came out of the rears. After that, the brakes felt as good as they ever have (that is, a tiny, little bit more play in the pedal than I'd like, but otherwise very hard and it no longer pumps up). I drove the car for a couple of days and it remained good but now it's offline again to repair a leaking carb and rebuild the starter.

All I can suggest in your situation is that you check incredibly carefully for leaks, all along the entire system. Even something small enough to barely lose fluid will be big enough to draw air in and that will give you a spongy pedal that pumps up. And it doesn't take much air at all. Another thing that has happened to me was that, after rebuilding the booster and doing both vacuum and pressure bleeds, I found that a little bit of air was still trapped in the booster. If you look at the outlet on the booster, you'll see that it is centered at the end of the cylinder that contains the fluid. Apparently air can remain in that cylinder, floating at the top. What cured it for me was to crack the outlet nipple and spray a bunch of fluid out with a fast, hard stomp on the brake pedal, pushing it right to the floor. Then, I retightened the outlet before letting the pedal back up. Of course, the next thing was to wipe down and hose off that whole area around the car because brake fluid had gotten all over everything.

Hope this helps. Let me know if it does...

-Dave

[ciscotex]

If I understand correctly, the bubble at the booster you're referring to is in the m/c? That is why thorough bench bleeding before mounting is so important, so that the port can be oriented to get rid of it. As you point out, that damn bubble can defy all reasonable attempts at bleeding. Ask me how I know...

Paul

[BiancaGT]

Hi Paul,

No, I mean the booster/servo, up at the top of the engine bay. The m/c has both the in- and outlets on top of the cylinder. So, while I agree with you and do bleed them on the bench, they're actually less likely to retain air for that reason than the servo will, where both the inlets and outlets are on the midpoints of the cylinder diameter. But, yes, I'd like to know what your experience was, indicating the reluctance of that damn bubble to bleed.