air in brakes

[jmargush]

I am wondering if I might be drawing air into my brake system somewhere. I have bleed the system atleast 6 times and done all the tricks with the proportioning valve. I got a really good high, firm peadal the last time I bled them. A few days later it wasn't quite as good. then went and autoxed on Sunday and by the end of the day they were about gone unless I pumped them.

Question, if it is drawiing air in somewhere after the master cylinder wouldn't there be fluid leaking? If this is correct and I don't have fluid anywhere, the only thing left is where the lines go into the master cylinder which could draw air in without leaking fluid. Does this sound logical?

[Tidybuoy]

It's possible that the seals in the master cylinder are going bad and so the piston is simply not pushing fluid (i.e., no air in system but no pressure either).

I'm not an expert but I have had this happen to me on aother car of mine.

[914GT]

Ditto what Tidybuoy says. Can't suck air without leaking fluid.

[mikey]

Did you bleed the proportioning valve ???? That thing likes to hide air bubbles.

[Scooter311]

What shape are the calipers in? Mine did the EXACT same thing, and I rebuilt both fronts with a kit from our gracious host, and now they're poifect. New seals were needed - I thought for sure I'd have a fluid leak as well, but it turned out the air was getting 'sucked' in, in a way too complicated for my feeble intelligence to understand.

[jmargush]

CAlipers were all rebuilt completely. New master cylinder and I have pressure bled a number of times whils also working the pedal to help activate the proportioning valve. I guess I will try bleeding again to see what happens.

[jmargush]

I bled them again and the pedal never got really firm. If I push on the pedal really hard and hold it, it eventually moves toward the floor. This does not happen as fast as other bad master cylinders I have had but just wondering if this sounds like a bad master cylinder?

[Tidybuoy]

Unless there is fluid missing - that's exactly what it sounds like.

[mark 74 carrera]

I have a 1974 914 1.8 liter. I have replaced the master cylinder and three hard lines and all flexible lines. I also just rebuilt all four calipers. I have no fluid leaking anywhere.

I have bleed the brakes about 4 times and initially get a hard pedal with almost no play. I have used a pressure bleeder as well as a second person pumping and holding while I bled the brakes starting from furthest to closest. The pedal improved as we went.

When I was satisfied with the pedal, I put the wheels back on it dropped it off the jack stands and went to start it. Brake pedal went to the floor. Pedal improves if I pump it but just like described here in this thread, it is slowly going in with pressure applied. With a new MC, I find it hard to believe its bad.

Is it possible to suck air into the MC if the two reservoir lines are not completely clipped in and sealed? Brakes clamp and release as they should and function properly with a good pedal if only temporarily.

I have searched and that's how I found this thread with my exact problem. Anyone have any ideas.

Mark

[John Rogers]

I would suggest buying a pressure bleeder if you do not have one. Pump it up to about 5# and bleed the calipers: right rear, left rear, right front and finally left front. You can make a huge difference by installing a tee and the weirdness of the prop valve will go away! There will be some who noooooooo, it is dangerous, bad ju ju, etc but those who have done the swap love it!

[mark 74 carrera]

Thanks John. I have read about the "T" replacement. I know exactly where it is on the car. I had to replace the two rear hard lines, so I am intimate with the proportional valve

I have been using a power bleeder and I have ran through a pretty serious volume of brake fluid. This problem still has me perplexed.

Mark

[Mark Henry]

At the P-valve crack the brake line open just as if they were bleeder valves, exact same method as if you were bleeding a caliper manually.
Crack open brake line, pedal down and hold, close (tighten) brake line, then release the pedal. Repeat.
Repeat on all 3 lines.

Yes it will go all over the floor, yes it's a skanky, nasty job, yes it sucks, etc.... but it works every time.

I use a oil change tray to catch the bulk of it and kitty litter to clean up the spilled brake fluid.
Have some soap and water ready to clean up the area on the car.
You might be able to do this with a power bleeder, but to me it's two person job.

[John Rogers]

I would suggest checking the lines and seals that run from the fluid reservoir to the master cylinder as this is about the only place besides the master cylinder that air can be sucked in. On both our 914's the seals were cracked due to old age. If the seals at the calipers were leaking then you would get some fluid leaking out also. If the calipers were recently rebuilt it can take time for the seals to set in due to the pistons moving into place and that can possibly cause the pedal to change position.

[mark 74 carrera]

Thanks Mark. I will try what you have suggested.

At the end of the day, the only thing that makes sense is air in the system and the PV can and will be eliminated as the possible culprit.

I'll update later,

Mark

[mark 74 carrera]

John,

I do think the reservoir lines could be a potential issue. I never felt them "pop" into place. Also when I pump the pedal with the pressure bleeder in place and pressurized, there were always bubbles coming out of the brake fluid reservoir mounted in the car and back into the clear line running to the pressure bleeder. That didn't seem right to me.

I'm going to try all of these suggestions. Thanks guys.

Mark

[beatnavy]

Did you also install the washers that go into the MC underneath the brake feeder lines? I think those are important. I also agree that getting those lines into the MC is both a PITA and critical.

[stinkindiesel]

Whatever you do, don't leave a half a bottle of brake fluid in the trunk and then forget about it for a while. When the bottle gets wedged against that old alternator you throw in there later and splits open, bad things will happen to perfectly good paint. Don't ask me how I know.

Gary

[John Rogers]

Several additional things: do not try to pump the pedal when pressure bleeding the brakes, make sure the pressure bleeder fluid level does not drop too low, keep pressure up to recommended amount of the bleeder (10 to 15#) and use some clear tube on each brake bleeder valve and have it below the top of the fluid in your waste container so you can watch for air bubbles coming out. Generally 1/4 to 1/2 turn of the bleed valve should be enough to get a good flow out. You can also tap the caliper with a rubber mallet to "shock" any air bubbles out.

[jadewombat]

The air is probably sucking up and around the threads of the bleeder valve. Don't crack it open more than 1/4 turn.

Try the two-person method of bleeding, even race teams do this. Fill up with brake fluid, have one person starting out a the right rear wheel, one person in the drivers' seat. String up a bleeder line off the valve that is elevated above the valve so nothing but fluid could be sucked back in. Pump the pedal a few times and HOLD medium pedal pressure on the last pump, have the person at the wheel crack the bleeder valve and don't let the pedal go all the way to the floor. Repeat a few times then go to the next wheel. Now, very important, after you've done all four wheels check how stiff the pedal is and do all four again one more quick time--I've found there is sometimes air still in there that gets pushed from one side to the other through the 'T'. The whole process takes about 20 min. Finish and go drink a cool one.

Never had any luck with those pumps or one-way bleeder valves or bench bleeding or whatever.

[Dave at Pelican Parts]

New parts can also be bad out of the box. This is more common with the bargain-level parts than with OEM-quality replacements, but even the best parts can still be bad.

--DD